MariaDB: Difference between revisions

Revision as of 16:04, 13 August 2022

For emergencies, check the subpage about MariaDB troubleshooting.

MariaDB is the main database management system used to run the Wikimedia sites.

For a general overview, check the MySQL@Wikipedia (2015) slides (MariaDB is a drop-in replacement for MySQL, which we used to use).

Sections and shards

For a full (?) list, look at db.php.

Core MediaWiki databases

s1: English Wikipedia
s2: 17 large wikis, including Chinese, Italian, Dutch, Polish, and Portuguese Wikipedias and English Wiktionary
s3: Most small wikis (~800)
s4: Commons
s5: German Wikipedia, some other large wikis, and most new wikis since mid-2020 (T259438)
s6: French, Japanese, and Russian Wikipedias, and Wikitech
s7: CentralAuth, Meta, 10 large Wikipedias, and French Wiktionary
s8: Wikidata

External storage

The external storage servers host the full text of all page revisions in a compressed format.

es1: read-only cluster
es2: read-only cluster
es3: read-only cluster
es4 and es5: read/write cluster

Parsercaches

pc1-pc3: parser caches

Extension storage

x1

The x1 cluster is used by MediaWiki at WMF for databases that are "global" or "cross-wiki" in nature, and are typically associated with a MediaWiki extension. Apart from being used across wikis, it is otherwise configured, replicated, and queries in the same way as "core" databases.


Feature	Database	Phabricator project	Steward
BounceHandler	`wikishared.bounce_*`	#MediaWiki-extensions-BounceHandler
Cognate	`cognate_wiktionary.*`	#Cognate
ContentTranslation	`wikishared.cx_*`	#ContentTranslation	Language Team
Echo	`wikishared.echo_`, and `[wiki].echo_`	#Notifications	Growth Team
Flow	`flowdb.*`	#StructuredDiscussions	Growth Team
GrowthExperiments	`[wiki].growthexperiments_*`	#MediaWiki-extensions-GrowthExperiments	Growth Team
ReadingLists	`wikishared.reading_list_*`	#Reading List Service
UrlShortener	`wikishared.urlshortcodes`	#MediaWiki-extensions-UrlShortener
WikimediaEditorTasks	`wikishared.wikimedia_editor_tasks_*`	#Product-Infrastructure-Team-Backlog

x2

This cluster is reserved for use by the MainStash. It is read-write in all data centers and replicated in both directions.

As of June 2021, the x2 cluster is not yet in use and MainStash is still backed by Redis.

Launch task: T212129.

Miscellaneous

The miscellaneous servers host databases for various auxiliary services.

m1: Basic ops utilities: Bacula, Etherpad, LibreNMS, Racktables, others
m2: OTRS, Gerrit, DebMonitor, XHGui, and others
m3: Phabricator
~~m4: EventLogging~~ No longer a MariaDB database.
m5: testreduce, WMCS-related dbs, Toolsadmin.wikimedia.org, Toolhub.wikimedia.org
tendril: Tendril (including dbtree backend) and Zarcillo

Database creation template

If you need to request a new database please create a task with the DBA tag and filling out the following fields if you know them (or can estimate them)

QPS:
Size: (An estimation is fine)
DB Name:
User:
Accessed from server (s):
Backup Policy: Needed? Frequency?
Grants needed:

Cloud Services (WMCS) Wiki replicas

clouddb1013-1020: Wiki replicas

Analytics

clouddb1021: Analytics dedicated replica. Used the first days of the month.
db1108: Eventlogging hosts
dbstore1003-1005: Multi-instance analytics hosts

Replicas

The master database is the primary server for a given shard. If the master database server fails, then a replica can be promoted to be a new master. See Master switch for more information.

A database replica (or slave database) is another MariaDB or MySQL instance that replicates data from the master database. Most often, we add replicas in production for the purpose of load-balancing read queries, and for backup/failover purposes.

Cloud Services Wiki Replicas

Copies of the core databases with private data removed are part of the Data Services hosted by Wikimedia Cloud Services for bot, tool, and analysis use. The redaction is done during an intermediate stop on Sanitarium. As of Feb 2021, wikireplicas run in multiple hosts, with multiple processes, see new wiki replicas setup for more details.

To see how to operate (pool/depool) the new multi-instances check the new wiki replicas setup

Recloning a Wiki replica

This is an old procedure for the multi-source hosts, which are deprecated. This needs updating.

In case a Wiki Replica needs to be recloned from another one, there are several steps that needs to be done as these hosts are running multi-source.

Stop all slaves on the source and gather its replication coordinates
Stop MySQL on the source
Stop MySQL on the target host
Remove /srv/sqldata on the target host
Transfer the data from the source to the target:

transfer.py --no-encrypt --no-checksum SOURCE.eqiad.wmnet:/srv/sqldata TARGET.eqiad.wmnet:/srv

Make sure replication won't start automatically on neither of the hosts:

systemctl set-environment MYSQLD_OPTS="--skip-slave-start"

Once everything is transferred: on the target host remove the following files

relay-log-sX.info
multi-master.info
master-sX.info

Start MySQL on both hosts
If necessary run

mysql_upgrade

On the target issue:

reset slave all;

On the target configure replication using the coordinates from the source that were gathered previously and start replication

Analytics MariaDB cluster

The Analytics MariaDB cluster contains full replicas of the core and extension storage databases.

Database backups

Main article: MariaDB/Backups

Replication protects (in general) against hardware issues, as well as physical corruption. However, it is not enough to prevent application or operation errors. Also many people (e.g. developers and service maintainers) sometimes require observing past state of the database to solve data application issues.

In the past, 2 boxes replicate all shards using MariaDB 10 multi-source replication: dbstore1001 and dbstore1002 (as well as dbstore2001 and dbstore2002 on codfw). dbstore1001 and dbstore2001 also used to be delayed 24 hours. Multisource was abandoned for several reasons (task T159423): TokuDB (used for compression) bugs, difficult to recover and maintain, as well as difficulty for scalability as data grew more and more, MariaDB bugs and GTID limitations, among others.

In addition to the delayed slave, a bacula-orchestrated dumpdump was done weekly from dbstore1001 on misc and metadata core databases, and stored syncronously on bacula.

In 2017, multiple hardware and scalability issues lead to a focus and complete redesign of the database backup architecture, leading to the current one explained at MariaDB/Backups, with focus on full coverage, validation and testing and full automation recovery.

Start / stop

See MariaDB/Start_and_stop

Packages

Asher started us using stock upstream packages from mariadb.org with the debs "de-debianized" each time to have fewer hooks, allowing puppet to handle stuff. Simply:

apt install wmf-mariadb101

We currently have wmf-mariadb10, wmf-mariadb101 (yes, I know, the names are horrible) and wmf-mysql80 built from source, with no patches except forcing the usage of openssl instead of yassl, that install to /opt.

Puppet controls manually the rc.d script and the package uses update-alternatives to update the symlinks in /usr/local/bin. Unlike the upstream versions our packages are fine to install alongside one another.

Installation

See mariadb roles in puppet.

Hardware installation checklist

Once the physical racking and OS installation has been, normally by DCOPs, the following items must be checked before considering the host ready to receive data.

Amount of memory showing up on the OS matches the expected

RAID setup
- RAID level (typically 10)
- RAID stripe size (256K)
- BBU in place and policy set to WriteBack

Amount of disk space available for srv matches the expected

Number of CPUs matches the expected value

Setting up a fresh server

To initialize the database: /opt/wmf-mariadb104/scripts/mysql_install_db --basedir=/opt/wmf-mariadb104/
- If this is a multi-instance db server, you'll need to provide --datadir=/srv/sqldata.<SECTION> as well.
Start the service: systemctl start mariadb
- For multi-instance, the service is mariadb@<SECTION>
Set up defaults: /opt/wmf-mariadb104/bin/mysql_secure_installation --basedir=/opt/wmf-mariadb104/ -S /run/mysqld/mysqld.sock
- For multi-instance, change it to -S /run/mysqld/mysqld.<SECTION>.sock
- Don't set root password, accept defaults for everything else.

Loading Data / provisioning

Setting up a new replica (or repairing a current one) is done through the recovery/provisioning system: MariaDB/Backups

However, one can do still manual hot or cold copies of running or stopped servers through transfer.py script on the cluster management hosts (cumin1001.eqiad.wmnet, cumin2002.codfw.wmnet).

Schema Changes

See Auto schema

First decide if the schema change can be done online. This only works if:

The table has a PRIMARY KEY or UNIQUE KEY on NOT NULL field(s)
MediaWiki can ignore the change behind a feature flag
Table metadata locking has been considered

Offline Schema Changes

Schedule downtime
See operations/mediawiki-config/db-eqiad.php
May need a master rotation?

Online Schema Changes

Use pt-online-schema-change or the ops wrapper script operations/software/dbtools/osc_host.sh.

$ osc_host.sh --host=dbstore1001 --dblist=/a/common/s2.dblist --table=page_props \
  --method=percona "add pp_sortkey float default null"

--method=percona uses pt-online-schema-change and automatically checks slave lag on the host itself and its slaves.
--method=ddl uses normal ALTER TABLE which will probably cause replication lag.

The wrapper script prompts for confirmation of settings first and also does a dry run for each wiki. It generally requires little supervision but should never run unmonitored.

If modifying indexes, or adding new fields that are nullable and/or have a default value, the change can be applied to each slave first and their masters last, using --no-replicate; ie, alter the replication tree bottom-to-top so that no slave ever tries to replicate a non-existant field from a master. This:

Avoids risking the entire shard all at once; one can only break a single slave at a time :-)
Allows the slave to be depooled if something goes wrong or load is too high.

$ osc_host.sh --host=dbstore1001 --dblist=/a/common/s2.dblist --table=page_props \
  --no-replicate --method=percona "add pp_sortkey float default null"

If the change simply must go to masters first and propagate to slaves through replication then the wrapper script may still be appropriate to use but such cases deserve careful consideration from a DBA. This especially includes any changes to primary keys!

Sanitarium and Labsdbs

See more: Sanitarium and labsdb

The sanitarium hosts have triggers defined on some tables which will clash with pt-online-schema-change. For small tables which should be fast to alter simply use --method=ddl. For large tables where DDL would cause unacceptable lag just schedule downtime.

Table Metadata Locking

Both ALTER TABLE and pt-online-schema-change need to hold the table metadata lock, even just briefly for the latter. They must wait for open transactions to close the table and also block new transactions opening the table. On a busy server and a hot table like page or revision this can easily result in many seconds of delay which is more than enough time for connections to pile up and hit max_connections.

Consider reducing load or depooling the box.

Replication Lag Trick

Issuing STOP SLAVE SQL_THREAD on a slave before starting pt_online_schema_change will cause immediate replication lag, which will in turn make MediaWiki load balancing code reduce traffic on the slave until it catches up. Just a few seconds of leeway is enough to allow the tool to create triggers, leading to far less interruption than waiting out the metadata locks traffic jam.

Monitoring

See MariaDB/monitoring.

Replication lag

Replication lag can be checked on https://noc.wikimedia.org/dbtree/.

See MariaDB/troubleshooting#Replication lag and MySQL#Replication lag on how to handle replication lag.

Manipulating the Replication Tree

Tendril displays the full replication tree.

The following is interesting info, but thanks to gtid replication, implemented almost everywhere in production, except multi-source replicated slaves, you can move slaves just by executing:

STOP SLAVE; CHANGE MASTER TO MASTER_HOST='<new master fqdn>'; START SLAVE;

A common task is manipulating slaves within the replication hierarchy. A script exists to help out: operations/software/dbtools/repl.pl. It allows controlling two slaves in a parent/child or sibling/sibling relationship and will do sanity checks plus confirmation prompts. It has nothing to do with masters so don't try to use it for those :-)

Child to Sibling

Move a slave one layer upward in the hierarchy:

./repl.pl --switch-child-to-sibling --parent=db1007.eqiad.wmnet --child=db1035.eqiad.wmnet

Child must be replicating directly from parent.

Sibling to Child

Move a slave one layer downward in the hierarchy:

./repl.pl --switch-sibling-to-child --parent=db1007.eqiad.wmnet --child=db1035.eqiad.wmnet

Both slaves must be replicating from the same master.

Stop Siblings in Sync

Stop two slaves on the same layer in the hierarchy at a common binlog position:

./repl.pl --stop-siblings-in-sync --host1=db1007.eqiad.wmnet --host2=db1035.eqiad.wmnet

Both slaves must be replicating from the same master.

Multisource Slaves

A MariaDB 10 slave can have multiple masters. Have to set the default_master_connection session variable to indicate which stream is to be maniulated by subsequent commands:

./repl.pl --switch-child-to-sibling --parent=db1007.eqiad.wmnet --child=dbstore1001.eqiad.wmnet --child-set=default_master_connection=s7

Partitioned Tables

Links Tables

The *links tables tend to have data skewed toward certain namespaces depending on the wiki. In most cases this doesn't matter and the MariaDB optimizer always chooses a fast execution plan. However some edge cases on heavily used namespaces can cause massive filesorts. Historically MediaWiki used STRAIGHT_JOIN however that blunt instrument only introduced a different set of edge cases.

The *links tables respond well to range partitioning on namespace with ranges chosen appropriately on a case-by-case basis. Eg, commonswiki:

CREATE TABLE templatelinks (
  tl_from int(8) unsigned NOT NULL DEFAULT '0',
  tl_namespace int(11) NOT NULL DEFAULT '0',
  tl_title varbinary(255) NOT NULL DEFAULT ,
  UNIQUE KEY tl_from (tl_from,tl_namespace,tl_title),
  KEY tl_namespace (tl_namespace,tl_title,tl_from)
) ENGINE=InnoDB DEFAULT CHARSET=binary
PARTITION BY RANGE (tl_namespace) (
PARTITION p_9 VALUES LESS THAN (10),
PARTITION p_10 VALUES LESS THAN (11),
PARTITION p_827 VALUES LESS THAN (828),
PARTITION p_828 VALUES LESS THAN (829),
PARTITION p_max VALUES LESS THAN MAXVALUE)

Special Slaves

Shards S1-S7 have one slave each with table partitioning in place for revision and logging based on user id. These boxes handle special MediaWiki query groups like recentchangeslinked, contributions, and logpager.

Eg, from eswiki:

CREATE TABLE logging (
  log_id int(10) unsigned NOT NULL AUTO_INCREMENT,
  log_type varbinary(32) NOT NULL DEFAULT ,
  log_action varbinary(32) NOT NULL DEFAULT ,
  log_timestamp varbinary(14) NOT NULL DEFAULT '19700101000000',
  log_user int(10) unsigned NOT NULL DEFAULT '0',
  log_namespace int(11) NOT NULL DEFAULT '0',
  log_title varbinary(255) NOT NULL DEFAULT ,
  log_comment varbinary(255) NOT NULL DEFAULT ,
  log_params blob NOT NULL,
  log_deleted tinyint(3) unsigned NOT NULL DEFAULT '0',
  log_user_text varbinary(255) NOT NULL DEFAULT ,
  log_page int(10) unsigned DEFAULT NULL,
  PRIMARY KEY (log_id,log_user),
  KEY type_time (log_type,log_timestamp),
  KEY type_action (log_type,log_action,log_timestamp),
  KEY user_time (log_user,log_timestamp),
  KEY page_time (log_namespace,log_title,log_timestamp),
  KEY times (log_timestamp)
) ENGINE=InnoDB DEFAULT CHARSET=binary
PARTITION BY RANGE (log_user)
(PARTITION p00 VALUES LESS THAN (100000),
 PARTITION p01 VALUES LESS THAN (200000),
 PARTITION p02 VALUES LESS THAN (300000),
 PARTITION p03 VALUES LESS THAN (400000),
 PARTITION p04 VALUES LESS THAN (500000),
 PARTITION p05 VALUES LESS THAN (1000000),
 PARTITION p06 VALUES LESS THAN (1500000),
 PARTITION p07 VALUES LESS THAN (2000000),
 PARTITION p08 VALUES LESS THAN (2500000),
 PARTITION p09 VALUES LESS THAN MAXVALUE);

CREATE TABLE revision (
  rev_id int(8) unsigned NOT NULL AUTO_INCREMENT,
  rev_page int(8) unsigned NOT NULL DEFAULT '0',
  rev_text_id int(8) unsigned NOT NULL DEFAULT '0',
  rev_comment varbinary(255) NOT NULL,
  rev_user int(5) unsigned NOT NULL DEFAULT '0',
  rev_user_text varbinary(255) NOT NULL DEFAULT ,
  rev_timestamp varbinary(14) NOT NULL DEFAULT ,
  rev_minor_edit tinyint(1) unsigned NOT NULL DEFAULT '0',
  rev_deleted tinyint(1) unsigned NOT NULL DEFAULT '0',
  rev_len int(8) unsigned DEFAULT NULL,
  rev_parent_id int(8) unsigned DEFAULT NULL,
  rev_sha1 varbinary(32) NOT NULL DEFAULT ,
  PRIMARY KEY (rev_id,rev_user),
  KEY rev_timestamp (rev_timestamp),
  KEY page_timestamp (rev_page,rev_timestamp),
  KEY user_timestamp (rev_user,rev_timestamp),
  KEY usertext_timestamp (rev_user_text,rev_timestamp)
) ENGINE=InnoDB AUTO_INCREMENT=74016150 DEFAULT CHARSET=binary
PARTITION BY RANGE (rev_user)
(PARTITION p00 VALUES LESS THAN (10000),
 PARTITION p01 VALUES LESS THAN (20000),
 PARTITION p02 VALUES LESS THAN (30000),
 PARTITION p03 VALUES LESS THAN (40000),
 PARTITION p04 VALUES LESS THAN (50000),
 PARTITION p05 VALUES LESS THAN (60000),
 PARTITION p06 VALUES LESS THAN (70000),
 PARTITION p07 VALUES LESS THAN (80000),
 PARTITION p08 VALUES LESS THAN (90000),
 PARTITION p09 VALUES LESS THAN (100000),
 PARTITION p10 VALUES LESS THAN (200000),
 PARTITION p11 VALUES LESS THAN (300000),
 PARTITION p12 VALUES LESS THAN (400000),
 PARTITION p13 VALUES LESS THAN (500000),
 PARTITION p14 VALUES LESS THAN (600000),
 PARTITION p15 VALUES LESS THAN (700000),
 PARTITION p16 VALUES LESS THAN (800000),
 PARTITION p17 VALUES LESS THAN (900000),
 PARTITION p18 VALUES LESS THAN (1000000),
 PARTITION p19 VALUES LESS THAN (2000000);

Queries that filter on log_user by equality or range run faster. Same for rev_user. The partition sizes are chosen on a per-wiki basis because one size does not fit all. Tendril has a report for choosing sizes based on row distribution.

Note the modified PRIMARY KEY definition that includes log_user. This is relatively safe for a slave but not appropriate for a master, so a partitioned slave should never be eligible for promotion. See coredb::$topology in puppet, or mediawiki-config/db-eqiad.php to identify them.

Extra notes: Having only 1 special node is a "Single Point of Slowdown". I am currently making sure that we have at least 2 nodes load-balancing this kind of traffic, which can be significant for uncached traffic, and allows to properly perform maintenance. Be careful, running the ALTER TABLE, as is, takes Query OK, 629385781 rows affected (5 days 54 min 6.51 sec) for the English wikipedia, and as much amount of space as the original table, so probably pt-online-schema-change or other methods can be considered to avoid failure and painful rollbacks.

HAProxy

Main page containing : HAProxy

dbproxy1XXX boxes run HAProxy. Besides making for easier Ops server rotations simply by having a proxy IP in the mix, there are two modes for other stuff: load balancing for slaves and failover for masters. So far, only misc shards masters and the latest labsdbs replicas uses it via dbproxy1XXX.

Failover

See role::mariadb::proxy::master

Load Balancing

See role::mariadb::proxy::slaves

Puppet

The main module for DBAs on the operations/puppet tree is "mariadb", which is in its own repo operations/puppet/mariadb. Remember to update the subrepo when committing changes to the mariadb module, otherwise it will not be caught by palladium, strontium, CI, etc.

There used to be a class coredb_mysql, used from 5.5 nodes. Right now it is obsolete, but it is still in use by nodes that have not been updated to mariadb10, in particular, the masters. When all nodes are updated to 5.6/10, we will discontinue it, but right now it is essential for everything to work correctly.

Despite all the previous, there are mariadb-related files on the main repo- shared among the clases. Those are the puppet/templates/mariadb/ files, where the main configuration and grants lie.

There is a forth place where you will find mariadb-related files, and that is the private repository, but I suppose that requires no explanation (passwords, etc.).

Other DBA related repos are:

operations/software/dbtools: for scripts used for maintenance
operations/software/redactatron: labs filtering
operations/software/dbtree: DBtree
operations/software/tendril: DB monitoring

Long running queries

Main article: /troubleshooting#Overload_due_to_long_running_queries

You can identify long running queries on logstash. The query killer should stop read queries running longer than 60 seconds, but may not function properly under high load. db-kill can be used in emergencies to kill any query running for longer than 10 seconds.

Storage engines

TokuDB

In 2015, TokuDB was trialled but was found to be buggy. We're now back to using InnoDB.

Importing table spaces from other hosts with multi source replication

Transportable tablespaces

Since MySQL 5.6 it is posible to use advantage of transportable table spaces to move around .ibd files from one server to another (if file per table is enabled.

This feature provides a fast way of copying data from one host to another over the network using compression and nc for instance. To learn more about to copy .ibd files from one host to another, please check out MariaDB/ImportTableSpace.

Account handling

root@localhost account no longer uses passwords for authentication (task T150446), but the UNIX socket authentication plugin. This allows us to stop using passwords (that can be copied, exposed and compromised), and allows the root system user to login from localhost by running:

sudo mysql

or

sudo mysql --skip-ssl

Because mariadb uses the uid of the linux user, there is no need to write passwords to the filesystem anymore, and in the event of a host compromise, only localhost is affected, and not any other host sharing the same password. This could later be extended to other system accounts that only connect from localhost, such as icinga or prometheus. Note that if a superuser is compromised on a host, not having a password is not a further barrier, as root has the ability to manage, read and modify at will MySQL files.

How unix_socket authentication works

All mysql servers should have been configured to load the auth_socket plugin (https://gerrit.wikimedia.org/r/320822):

plugin-load = unix_socket=auth_socket.so

It can also be enabled at runtime with:

INSTALL PLUGIN unix_socket SONAME 'auth_socket';

That only enables to plugin, to use it, we have to alter the user we want to authenticate:

GRANT USAGE ON *.* TO root@localhost IDENTIFIED VIA unix_socket;

This will delete the user password, but will indicate it does not uses the mysql native password authentication, but unix socket.

MariaDB [(none)]> SELECT user,host,password,plugin FROM mysql.user WHERE user='root';
+------+--------------+-------------------------------------------+-------------+
| user | host         | password                                  | plugin      |
+------+--------------+-------------------------------------------+-------------+
| root | localhost    |                                           | unix_socket |

By the way, note that the authentication name is unix_socket, but the plugin loaded is auth_socket.so. Do not use auth_socket on the GRANT/CREATE USER statements.

Also, if for some reason, you revert this change, make sure you put a password back:

GRANT USAGE ON *.* TO root@localhost IDENTIFIED BY '<super_secure_password>';

Dumping tables with mydumper

What is mydumper?

From the documentation:

* Parallelism (hence, speed) and performance (avoids expensive character set conversion routines, efficient code overall)
* Easier to manage output (separate files for tables, dump metadata, etc, easy to view/parse data)
* Consistency - maintains snapshot across all threads, provides accurate master and slave log positions, etc
* Manageability - supports PCRE for specifying database and tables inclusions and exclusions

Why are we starting to consider it?

Its speed and parallelism makes it perfect to save time and load data a lot faster.

It is easier to recover single tables or rows.

It compress nicely

From the tests we have seen

Taking a full dump of s3 takes 1:15h - 88GB
Taking a full dump of s1 takes 53 minutes - 93GB
Taking a full dump of s5 takes 1:20h - 93G

Quick cheatsheet

Dump data

Right now and in order to start it - as it doesn't accept any flag as no to read the default file, the following section needs to be commented out on the host my.cnf

# ssl-ca=/etc/ssl/certs/Puppet_Internal_CA.pem
# ssl-cert=/etc/mysql/ssl/cert.pem
# ssl-key=/etc/mysql/ssl/server.key
# ssl-verify-server-cert

Once that is done, a typical way to dump a database:

mydumper -c -h localhost -t 8 -u root -r 100000000 -B wikidatawiki -S /tmp/mysql.sock -o output_directory_name

-c To compress the data files

-h The host

-t How many parallel threads you want dumping at the same time

-u The user

-r Try to split tables into chunks of this many rows

-B Database you want to dump

-S Specify the socket to connect to

"-o" Specify the output directory where you want to save the files to. If not specified, it will generate a directory under your current path called: export-2017xxxxx-xxxxx with all the files.

Load data

Please check the dump data section to make sure you are able to connect (comment out the SSL options).

By default it disables the logging into the binlog

Once that is done, typically you want to run:

myloader -h localhost -t 8 -u root -S /tmp/mysql.sock -d export-xxxxx

-h The host

-t How many parallel threads you want loading the data at the same time

-u The user

-S Specify the socket to connect to

-d The directory you want to import the files from

Production section failover checklist

It has been suggested that this page should be merged with MariaDB/troubleshooting#Depooling_a_master_(a.k.a._promoting_a_new_slave_to_master)

Note there is a script. db-switchover that automates most of these steps.

In order to generate the task you can use the script switchover-tmpl.py that lives on operations/software/dbtools/ and will generate the task content. As an example, generate a task for the s1 switchover where db1163 will be the new master.

python3 switchover-tmpl.py s1 db1163

If you want to generate the task manually, use the following checklist:

NEW master: OLD master:

Check configuration differences between new and old master

pt-config-diff h=NEW.eqiad.wmnet,F=/root/.my.cnf h=OLD.eqiad.wmnet,F=/root/.my.cnf

Silence alerts on all hosts
Set NEW master with weight 0 and depool it from its section

dbctl instance NEW edit
dbctl config commit -m "Set NEW with weight 0 TXXXXXX"

Topology changes, connect everything to NEW

db-switchover --timeout=15 --only-slave-move OLD.eqiad.wmnet NEW.eqiad.wmnet

Disable puppet @NEW and @OLD

 puppet agent --disable "switchover to NEW"

Merge gerrit puppet change to promote NEW to master (Example): https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/538747/

Start the failover

!log Starting sX failover from OLD to NEW - TXXXXXX
dbctl --scope eqiad section sX ro "Maintenance till 05:30AM UTC TXXXXXX" && dbctl config commit -m "Set sX as read-only for maintenance TXXXXXX"

Check that sX is indeed on read-only
run switchover script from one of the cluster management hosts (cumin1001.eqiad.wmnet, cumin2002.codfw.wmnet):

root@cumin1001:~/wmfmariadbpy/wmfmariadbpy# db-switchover --skip-slave-move OLD NEW ; echo OLD; mysql.py -hOLD -e "show slave status\G" ; echo NEW ; mysql.py -hNEW -e "show slave status\G"

Promote NEW to master and remove read-only, leave OLD (old master) with weight 0 for now

dbctl --scope eqiad section sX set-master NEW && dbctl --scope eqiad section sX rw && dbctl config commit -m "Promote NEW to sX master and remove read-only from sX TXXXXXX"

Restart puppet on old and new masters (for heartbeat): OLD and NEW

 run-puppet-agent -e "switchover to NEW"

(Equivalent manual steps)

Set old master in read only: mysql.py -hOLD -e "set global read_only=ON;"
Disable heartbeat @OLD

 killall perl

Confirm new master has catched up

 host=OLD; echo "== $host =="; mysql.py -h $host -e "show master status\G show slave status\G" | grep ' File:\| Position:\|Relay_Master_Log_File:\|Exec\_Master\_Log\_Pos:'; host=NEW; echo "== $host =="; mysql.py -h $host -e "select @@hostname; show master status\G show slave status\G" | grep ' File:\| Position:\|Relay_Master_Log_File:\|Exec\_Master\_Log\_Pos:'

Stop slave on new master

 mysql.py -h NEW -e "STOP SLAVE;"

Confirm pt-hearbeat has been started on NEW

 pgrep perl  /  mysql.py -h NEW -e "SELECT * FROM heartbeat.heartbeat ORDER BY ts DESC LIMIT 1\G"

Remove read-only from NEW: mysql.py -hNEW -e "set global read_only=OFF;"
RESET SLAVE ALL on new master

 mysql.py -h NEW.eqiad.wmnet -e "reset slave all;"

Change old master to replicate from new master

 mysql.py -hOLD: change master to master_host='NEW.eqiad.wmnet', master_user='repl', master_password=, master_port=3306, master_log_file=, master_log_pos=, master_ssl=1;

if all looks good: start slave; Enable GTID on old master (OLD)

(End of equivalent manual steps)

Clean up tasks:

change events for query killer:

   events_coredb_master.sql on the new master NEW
   events_coredb_slave.sql on the new slave OLD

Update DNS (example): https://gerrit.wikimedia.org/r/#/c/operations/dns/+/538748/
Clean up lag in orchestrator. Typically you need to go to the new master's heartbeat database and clean up the old master server_id from that table (with replication enabled):

select * from heartbeat; -- Get the server_id from the old master
delete from heartbeat where server_id=171970778; -- example id for db1183

Give weight to OLD if needed
If the old master will be decommissioned or will go under maintenance: depool it from dbctl: dbctl instance HOSTNAME depool
Change candidate master note (generally remove it from NEW and add it to OLD)

dbctl instance OLD set-candidate-master --section sX true
dbctl instance NEW set-candidate-master --section sX false

Update/resolve phabricator ticket about failover

External store section failover checklist

External store failovers differ a bit from current sX ones, as we need to disable writes on the given section first. RO external stores (as of today, es1, es2 and es3) do not need this, as those hosts are stand alone and only read only.

As of today, es4 and es5 are RW, so they have a normal replication topology.

Disabling writes on a section can be done safely, so the failover needs no rush. Even if it can be done hours ahead, disabling writes should be done just a few minutes the actual failover time, to avoid un-balancing of both sections.

When one section has writes disabled, reads are still happening without any issues.

If the idea is to depool the old master, make sure to leave the new master with a bit of weight, to avoid having just one slave serving traffic. Usually leaving the master with weight 50 and the slave with weight 100 is enough.

Check list for failing over an es section with es1023 and es1024 as examples:

NEW master: es1024
OLD master: es1023
# Check configuration differences between new and old master
$ pt-config-diff h=es1023.eqiad.wmnet,F=/root/.my.cnf h=es1024.eqiad.wmnet,F=/root/.my.cnf

# Silence alerts on all hosts
# Set NEW master with weight 50
dbctl instance es1024 edit
dbctl config commit -m "Set es1024 with weight 50 T255755"
# Topology changes, connect everything to es1024
db-switchover --timeout=15 --only-slave-move es1023.eqiad.wmnet es1024.eqiad.wmnet
# Disable puppet @es1023 and @es1024
 puppet agent --disable "switchover to es1024"
# Merge gerrit puppet change to promote es1024
es1024: https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/607236/


# Start the failover  
!log "Starting es failover from es1023 to es1024 - https://phabricator.wikimedia.org/T255755"
Disable es5 writes https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/606663/

# Check that es5 is indeed on read-only
# run switchover script from one of the cluster management hosts (cumin1001.eqiad.wmnet, cumin2002.codfw.wmnet):
root@cumin1001:~/wmfmariadbpy/wmfmariadbpy# db-switchover --skip-slave-move es1023 es1024 ; echo es1023; mysql.py -hes1023 -e "show slave status\G" ; echo es1024 ; mysql.py -hes1024 -e "show slave status\G"
# Promote es1024 to master and remove read-only, leave es1023 (old master) with weight 0  
dbctl --scope eqiad section es5 set-master es1024 && dbctl config commit -m "Promote es1024 to es5 master T255755"
# Restart puppet on old and new masters (for heartbeat): es1023 and es1024
 run-puppet-agent -e "switchover to es1024"
# Enable es5 on MW (REVERT PATCH https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/606663/)

Clean up tasks:
# change events for query killer:
   events_coredb_master.sql on the new master es1024
   events_coredb_slave.sql on the new slave es1023 
# Update DNS: https://gerrit.wikimedia.org/r/609899
# Clean up lag in orchestrator. Typically you need to go to the new master's heartbeat database and clean up the old master server_id from that table (with replication enabled):
select * from heartbeat; -- Get the server_id from the old master
delete from heartbeat where server_id=171970778; -- example id for db1183
# Update/resolve phabricator ticket about failover

Misc section failover checklist (example with m2)

OLD MASTER: db1065

NEW MASTER: db1132

Check configuration differences between new and old master

$ pt-config-diff h=db1068.eqiad.wmnet,F=/root/.my.cnf h=db1081.eqiad.wmnet,F=/root/.my.cnf

Silence alerts on all hosts
Topology changes: move everything under db1132

db-switchover --timeout=1--only-slave-move db1065.eqiad.wmnet db1132.eqiad.wmnet

Disable puppet @db1065, puppet @db1132

 puppet agent --disable "switchover to db1132"

Merge gerrit: https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/519975/
Run puppet on dbproxy1002 and dbproxy1007 and check the config

puppet agent -tv && cat /etc/haproxy/conf.d/db-master.cfg

Start the failover

!log Failover m2 from db1065 to db1132 - T226952
 root@cumin1001:~/wmfmariadbpy/wmfmariadbpy# db-switchover --skip-slave-move db1065 db1132

Reload haproxies

dbproxy1002:   systemctl reload haproxy && echo "show stat" | socat /run/haproxy/haproxy.sock stdio
dbproxy1007:   systemctl reload haproxy && echo "show stat" | socat /run/haproxy/haproxy.sock stdio

kill connections on the old master (db1065)

 pt-kill --print --kill --victims all --match-all F=/dev/null,S=/run/mysqld/mysql.sock

# 
# START Equivalent manual steps
# Disable GTID on db1132
# Disable heartbeat @db1065
  killall perl
# Set old m2 master in read only
  ./mysql.py -h db1065 -e "SET GLOBAL read_only=1"
# Confirm new master has catched up DONE
  host=db1065 echo "== $host =="; ./mysql.py -h $host -e "show master status\G show slave status\G" | grep ' File:\| Position:\|Relay_Master_Log_File:\|Exec\_Master\_Log\_Pos:'; host=db1132; echo "== $host =="; ./mysql.py -h $host -e "select @@hostname; show master status\G show slave status\G" | grep ' File:\| Position:\|Relay_Master_Log_File:\|Exec\_Master\_Log\_Pos:'
 # Reload haproxies
 dbproxy1002:   systemctl reload haproxy && echo "show stat" | socat /run/haproxy/haproxy.sock stdio
 dbproxy1007:   systemctl reload haproxy && echo "show stat" | socat /run/haproxy/haproxy.sock stdio
 # Restart puppet on old and new masters (for heartbeat): db1065 and db1132
  puppet agent --enable && puppet agent -tv
# Set new master as read-write and stop slave
  mysql -h db1132.eqiad.wmnet -e "STOP SLAVE; SET GLOBAL read_only=0;"
# RESET SLAVE ALL on new master
  ./mysql.py -h db1132.eqiad.wmnet -e "reset slave all;"
# Change old master to replicate from new master DONE
  ./mysql.py -hdb1065: change master to master_host='db1132.eqiad.wmnet', master_user='repl', master_password=, master_port=3306, master_log_file='xx', master_log_pos=xx, master_ssl=1;
if all looks good: start slave; 
# Update tendril and zarcillo master server id for m2 
  UPDATE shards set master_id = 1628 where name='m2' LIMIT 1;
 mysql.py -h db1115 zarcillo -e "UPDATE masters SET instance = 'db1132' WHERE dc = 'eqiad' and section = 'm2' LIMIT 1" 
# Enable GTID on the old master db1065
# STOP equivalent manual steps

Restart puppet on old and new masters (for heartbeat):db1065 and db1132

 puppet agent --enable && puppet agent -tv

Check services affected (otrs,debmonitor) DEBMONITOR and OTRS looking good
Clean up lag in orchestrator. Typically you need to go to the new master's heartbeat database and clean up the old master server_id from that table (with replication enabled):

select * from heartbeat; -- Get the server_id from the old master
delete from heartbeat where server_id=171970778; -- example id for db1183

change events for query killer:

   events_coredb_master.sql on the new master db1132
   events_coredb_slave.sql on the new slave db1065

Update/resolve phabricator ticket about failover https://phabricator.wikimedia.org/T226952
Create decommissioning ticket for db1065

Special section: x1 master switchover

x1 is a "special" section which cannot be put on read-only on mediawiki, so it needs to rely on the db-switchover script which puts MySQL on read-only. When failing over, please tag the following Teams and people on the phabricator task so they can have a heads up as they are x1 stakeholders and need to know that x1 will have writes blocked for around one minute.

Subscribers: Tgr, JoeWalsh, Dbrant, Ladsgroup, Addshore, Legoktm
Tags: Cognate, Growth-Team, Language-Team, User-notice, UrlShortener, StructuredDiscussions, MediaWiki-extensions-BounceHandler, ContentTranslation, Reading List Service, WikimediaEditorTasks.

Example task: https://phabricator.wikimedia.org/T226358

Decommissioning a mariadb host server checklist

See MariaDB/Decommissioning_a_DB_Host

Depool a broken or lagged replica

From one of the cluster management hosts (cumin1001.eqiad.wmnet, cumin2002.codfw.wmnet):

dbctl instance dbXXXX depool
dbctl config commit

Create a task with the DBA tag so DBAs can follow up and checkout what happened, a proper fix etc

THE SECTION BELOW IS DEPRECATED: Checkout the mediawiki-config repo if you haven't already:

git clone ssh://yourusername@gerrit.wikimedia.org:29418/operations/mediawiki-config
cd mediawiki-config
vim wmf-config/db-eqiad.php

Assuming eqiad is the active DC

A replica can run just one MySQL instance
If a replica runs more than one MySQL instance, it will be specified in its definition with the following format

hostname:port

Example

db1090:3317

This means db1090 has an instance running on 3317 (and probably others in different ports). The port is formed in the following way

3306 -> MySQL standard port
331X -> X meaning the section where the server belongs to. 
1 -> s1
2 -> s2
..
8 -> s8
3320 -> x1
3321 -> m1
3322 -> m2
3323 -> m3
3324 -> m4
3325 -> m5

So db1090:3317 is a replica that runs on s7

A replica can handle more than one type of traffic.

These are the types of traffic we have

main
recentchanges
api
vslow

They are normally specified on the definition files, let's use db-eqiad.php as it is the active datacenter Let's examine s7 where db1090 belongs to: This is the main traffic section where the servers and weights are specified. Servers with weight 1 normally means they are still checked for lag by the MediaWiki LB, but the have almost no main traffic, because they are special replicas and they have more weight for special traffic (check below)

's7' => [
'db1062' => 0,      # D4 2.8TB 128GB, master
'db1079' => 300,    # A2 3.6TB 512GB, api # master for sanitarium db1125
'db1086' => 400,    # B3 3.6TB 512GB, api # candidate master
'db1090:3317' => 1, # C3 3.6TB 512GB, vslow, dump: s2 and s7, old master
'db1094' => 500,    # D2 3.6TB 512GB
'db1098:3317' => 1, # B5 3.6TB 512GB # rc, log: s6 and s7
'db1101:3317' => 1, # C2 3.6TB 512GB # rc, log: s7 and s8

And there is also a special slaves section below, look for it: db1090:3317 is a single vslow slave db1079 and db1086 share API traffic, db1079 has more weight for API because db1086 has more weight for main traffic (see above) db1098:3317 db1101:3317 are recentchanges replicas and they share the same amount of load for all those special services.

's7' => [
 'vslow' => [
  'db1090:3317' => 1,
 ],
 'dump' => [
  'db1090:3317' => 1,
],
 'api' => [
  'db1079' => 3,
  'db1086' => 1,
],
 'watchlist' => [
  'db1098:3317' => 1,
  'db1101:3317' => 1,
],
 'recentchanges' => [
  'db1098:3317' => 1,
  'db1101:3317' => 1,
],
 'recentchangeslinked' => [
  'db1098:3317' => 1,
  'db1101:3317' => 1,
],
 'contributions' => [
  'db1098:3317' => 1,
  'db1101:3317' => 1,
],
 'logpager' => [
  'db1098:3317' => 1,
  'db1101:3317' => 1,
],
],

Locate the broken slave - keep in mind it can be in more than one section. The easiest way is probably to look for it on the file. For this example let's assume db1090:3317 is broken with replication broken, but its other instance (3312) is working fine, so no need to depool it.

This host is a vslow host, so we need to replace it with another host from the same section. Let's chose db1079 to replace it.

If the host is a special host and servers some special traffic (vslow,api, recentchanges, it needs to be replaced if it is the only one, we cannot leave a section without a slave)

We need to look for it and comment it out from every single line where db1090:3317 appears. However DO NOT COMMENT the line with its definition which is already really explicit:

    'db1090:3317' => '10.64.32.116:3317', # do not remove or comment out

So this should be the diff once we are ready to commit: https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/447984/1/wmf-config/db-eqiad.php

git add wmf-config/db-eqiad.php
git commit
git review

Once we have +2 on gerrit and the change has been merged automatically:

ssh deployment.eqiad.wmnet
cd /srv/mediawiki-staging/
git fetch ; git rebase
git show (this should show our change)
scap sync-file wmf-config/db-eqiad.php "Depool db1090:3317 as it is broken"

Once the change is deployed, we should be able to see our change on: https://noc.wikimedia.org/conf/highlight.php?file=db-eqiad.php

If a host is completely down and it has multiple instances it needs to be depooled from all the instances Let's say db1090 had a hardware crash and it is powered off, that means that both instances 3312 and 3317 are down. We need to also depool the instance db1090:3312 with the same method as described above.

Create a task with the DBA tag so DBAs can follow up and checkout what happened, a proper fix etc

Testing servers

As of today (June 2021) there are two testing hosts on a shard called test-s4:

DB master (writable): db1124.eqiad.wmnet 256GB RAM + RAID10
DB slave: (read-only): db1125.eqiad.wmnet 256GB RAM + RAID10

Whilst these hosts are normally up, it is not warranted they will always be available or up.

These hosts have static data, and is not updated with production data. More tables or wikis can be requested if needed by filing a ticket to DBAs). It is not warranted that they'll be included, but they can be studied and some expectations about ETA and if it is possible to have that data will be given on the ticket.

Access to that hosts is granted individually to those who already have an NDA and cluster access if really needed. Please file a ticket and tag DBA to let us know why you need access so we can start a discussion about it.

People with access

Data Persistence Members

Stretch + 10.1 -> Buster + 10.4 known issues

Fixed: ~~Optimizer flag rowid_filter needs to be disabled until it is fixed upstream (and checked) as it causes a regression: T245489 Upstream: https://jira.mariadb.org/browse/MDEV-21794~~
Possible bug with the optimizer choosing the wrong query plan on certain queries, pending upstream pick up: T246069 Upstream: https://jira.mariadb.org/browse/MDEV-21813
Some metrics have been deprecated in 10.4 and need to be removed from grafana dashboards: T244696
prometheus-mysqld-exporter:
- Regression on configuration disallows proper use of mysql auth_socket authentication. Upstream report: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953040
  - Worked around with: https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/576398/ and
- Some flags have changed their configuration flags from - to -- on Buster: Fixed with https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/576368/
- Seems that when the exporter runs before mysql is started and mysql_upgrade run to update internal tables from 10.1 to 10.4 structure, it gets "stuck" and keeps reporting some scrape errors, even though metrics are sent normally: T247290
  - The workaround is to restart prometheus-exporter once mysql_upgrade has been run.
- Sometimes the base prometheus-mysqld-exporter.service is started on multi-instance hosts. This needs to be manually stopped and disabled.
wmf-pt-kill seems to be working fine, but needs more checking on wiki replicas.
Tendril doesn't like this in-place upgrade, so it requires a disable + drop + add + enable after upgrade, otherwise the Act. (last contact) field doesn't get updated.
Fixed: ~~Events on 10.4 starts disabled: T247728 https://jira.mariadb.org/browse/MDEV-21758 - to be fixed on 10.4.13~~
On mysql_upgrade, backup user gets an additional grant (as it has SUPER, needed to handle replication) "DELETE HISTORY". This makes mydumper fail with "broken table detected" on sys and other system tables (my guess is because it can list it, but cannot read from it - this is on purpose, we only want backups from certain databases). Revoking the grant fixed mydumper run:

REVOKE DELETE HISTORY ON *.* from 'dump'@'X.X.X.X';

How to enable a new external storage (es) section

The process of enabling a new external storage section can be complex and require different steps.

Pre steps

Check all the involved servers have replication working and enabled (with SSL). Check replication lag is 0
Check all the involved servers have weight (if not done, check the section below)
Check pt-heartbeat is running
Check all the involved servers have notifications enabled (and Icinga looks clean)
Check the future active master has read_only set to OFF
Check the future servers have all the wikis and blobs_clusterXX tables created. This is an example ticket T245720
Check wikiadmin wikiuser have grants.

Setting up the servers with dbctl

external storage sections normally have the following weights: master: 0 slave1: 100 slave2: 100

In order to be able to do so, use dbctl to generate those configs.

dbctl instance esXXXX edit

And leave it like this for the master

# Editing object eqiad/esXXXX
host_ip: 10.XX.XX.XX
note: 
port: 3306
sections:
  esX: {percentage: 100, pooled: true, weight: 0}

And like this for a slave

# Editing object eqiad/esXXXX
host_ip: 10.XX.XX.XX
note: 
port: 3306
sections:
 esX: {percentage: 100, pooled: true, weight: 100}

Commit the change

dbtcl config commit -m "Set weights for the new es hosts - TXXXXXX"

Mediawiki patches and deployment

In order to be able to set the new external storage there are two steps needed from mediawiki point of view. They can be just one, but in order to catch errors, it better be split into two.

Enable the new section in MW, this should be a NOOP, but would allow testing before going fully live.
- Add the section as described here: https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/577185/
Make sure to double check the blobs_clusterXX entry match the tables number on the future new tables.

Once the change is merged and deployed to the passive DC, use Wikimedia Debug extension to browse the site via mwdebug2001.codfw.wmnet and monitor errors. If everything looks fine, deploy on the active DC.

Use shell.php from a mwmaint host to fetch stuff from those new tables, to make sure grants and everything is fine. Getting a false is a good thing. Everything else might require further research.

root@mwmaint1002:~# mwscript shell.php --wiki=enwiki
Psy Shell (PHP 7.2.26 — cli) by Justin Hileman
>>> ExternalStore::fetchFromURL( 'DB://cluster26/1' )
Wikimedia/Rdbms/DBConnectionError with message 'Cannot access the database: No working replica DB server: Unknown error'
>>> ExternalStore::fetchFromURL( 'DB://cluster25/1' )
=> b"Ý][û~¸»¿vÇ(i╚

If everything looks fine, the next patch should be enabling that section to be writable. https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/577189/

Before fully deploying that change, test it on mwdebug. Generate some writes by writing on your talk page and checking on the new blobs_clusterXX table. It might take a few writes to be able to show up there, as it is balanced with the other existing external storage masters.

Try a few times, and a few projects to make sure the row shows up there and on all the hosts (to double check replication is working as expected).

If the writes are showing up, and nothing else is errorring, you are good to go and fully deploy to all DCs.

Once deployed, monitor for lag, errors and periodically select from the new tables to make sure it is getting new things.

Update spicerack to get these new sections up to date: https://gerrit.wikimedia.org/r/#/c/operations/software/spicerack/+/576297/

How to set a external storage (es) section to read only

Whenever an existing external storage section needs to go read-only, these are the steps that need to be taken.

Create a MW patch that set it to read-only and remove it from the DefaultExternalStore : https://gerrit.wikimedia.org/r/#/c/operations/mediawiki-config/+/578766/

As always, first push to the passive DC, monitor for logs and if all is ok, push to the active DC:

Once the change is live, monitor the master binlog (skip heartbeat entries) and make sure the INSERTS stopped a few seconds after the change is live everywhere.
Disable alerts for 10-15 minutes on the involved servers, to avoid any alerts.
Once writes has stopped, prepare and push a puppet patch to change those hosts to standalone: https://gerrit.wikimedia.org/r/#/c/operations/puppet/+/578816/
Set read_only=1 on the master
Once puppet has run on the master, check that pt-heartbeat has stopped there. Once done, run show master status; to make sure nothing is being written.
Run the following across all the hosts: show master status; show slave status\G and note those positions somewhere.
Reset replication across all the hosts: stop slave; reset slave all
Update tendril to hide that replication tree:

update shards set display=0 where name='esX'

Update zarcillo to reflect those hosts as standalone

 update sections set standalone=1 where name='esX';

Slowly give some weight to the old masters, and now standalone - probably best done in a few commits, to slowly warm their buffer pool

dbctl instance esXXXX edit
dbctl config commit -m "Slowly give weight to old and now standalone es masters TXXXXXX"

dbctl at the moment doesn't treat standalone hosts any different from a normal replicating section. So it will be showed normally on either db-eqiad.json and db-codfw.json even though there is no real master anymore but just standalone hosts.

Update Spicerack to reflect that these new sections are RO https://gerrit.wikimedia.org/r/#/c/operations/software/spicerack/+/576297/

IPv6 and MariaDB

MariaDB (and MySQL) should have good support of IPv6. However, due to account handling being based on IP addresses, not DNS, a lot of issues, specially with authentication and grants can arise:

Hosts using DNS will try to contact mysqld using IPv6 first. That will cause timeouts (of several seconds) until it fails back to IPv4
Privileges may fail completely (access denied):
- When setting up and performing mysql backups
- When accessing host from remote mysql root clients (e.g. cumin)
- When using replication
- When using watchdog user for tendril

db1108, as of July 2020, is the only host configured using IPv6 while holding a mysql service.

Tables drift between code and production

There are known differences between tables definitions in MW and the live version in production. We have historically worked hard to the most important ones, but there are still lots minor ones that need to be solved.

Amir created a dashboard where those can be checked/observed: https://drift-tracker.toolforge.org/report/core/

Warming up production databases

This is mostly done before a DC switchover so the databases are not totally cold (aka with nothing cached).

We do this in three different ways:

MW warmup script

From a mw maintenance host make sure you have a urls-cluster.txt file:

root@mwmaint1002:/home/marostegui# cat urls-cluster.txt
# Purpose: Root redirect
https://%server/
# Purpose: Main Page, Skin cache, Sidebar cache, Localisation cache
https://%server/wiki/Main_Page
# Purpose: MobileFrontend, Main Page
https://%mobileServer/wiki/Main_Page
# Purpose: Login page
https://%server/wiki/Special:UserLogin
# Purpose: API, Recent changes
https://%server/w/api.php?format=json&action=query&list=recentchanges

If you want to warm up eqiad, simply run:

nodejs /var/lib/mediawiki-cache-warmup/warmup.js urls-cluster.txt spread appservers.svc.eqiad.wmnet

Normally you might want to run this several times for hours to make sure you touch as many wikis and hosts as possible. In the output of the script you'll be able to see how the loading times start to decrease over time.

This script is pretty harmless and be run without much babysitting in a loop:

while true; do  nodejs /var/lib/mediawiki-cache-warmup/warmup.js urls-cluster.txt spread appservers.svc.eqiad.wmnet ; sleep 15; done

Comparing tables

Using db-compare over several is a good way to bring some of the most accessed tables into memory. This is a very gentle script and can be run on the background for hours/days without many issues.

The idea here is to check data consistency between DCs and bring data into the buffer pool. We have a file with the most important tables to be checked/warmed up, which is called tables_to_check.txt and lives at: https://raw.githubusercontent.com/wikimedia/operations-software/master/dbtools/tables_to_check.txt

The content:

actor actor_id
archive ar_id
change_tag ct_id
comment comment_id
content content_id
logging log_id
pagelinks pl_from
page page_id
revision rev_id
revision_actor_temp revactor_rev
revision_comment_temp revcomment_rev
slots slot_revision_id
text old_id
user user_id
watchlist wl_id

What we normally do is to iterate over all the databases on a given section and go through all the tables across all the wikis there. The idea is to select all the production hosts on the passive DC and compare them against the vslow,dump host on the active DC.

The list of hosts per section can be retrieved from zarcillo database, orchestrator or with the section tool: The list of databases per section can be retrieved from the sX.dblist files present on wmf-config repo

Once those things are ready, we can simply iterate over all the tables with a loop (example of enwiki (s1):

cat git/mediawiki-config/dblists/s1.dblist | grep -v "#" | while read db; do cat main_tables.txt | while read table index; do echo "$db.$table"; db-compare $db $table $index db2146.codfw.wmnet:3306 db1163 db1099:3311 db1105:3311 db1106 db1118 db1119 db1134 db1135 db1164 db1169 db1184 ; done ; done

If differences are found, they need to be investigated BEFORE giving greenlight for the DC switchover.

Warming up ES hosts with compare

External store hosts are different as they only have one table per wiki, and depending on the section it is name differently, they are all called blobs_clusterX.

In Sept 2021 this is the mapping: Read only sections: es1 blob_cluster1 until blob_cluster23 es2 blob_cluster24 es3 blob_cluster5

Writable sections: es4 blob_cluster26 es5 blob_cluster27

This comparison isn't trustable for data drifts as the PK is a blob_id but can still be used to warm up things. In this case, the iteration follow the same pattern as a normal one, you simply need to check the same table across all the databases existing on the hosts, that can be done with the following loop (as data drift isn't to be trusted, no need to compare against the active DC, so just comparing them within the passive DC):

for i in `mysql.py -hes1023 -e "show databases" -BN`; do db-compare $i blobs_cluster27 blob_id es1023 es1024 es1025; done

Warming up parsercache hosts

For parsercache hosts, the easiest way is to simply do a table count on all its tables.

for i in `mysql.py -hpc1011 parsercache -BN -e "show tables"`; do echo $i; mysql.py -hpc1012 parsercache -e "select count(*) from $i"; mysql.py -hpc1011 parsercache -e "select count(*) from $i";mysql.py -hpc1013 
parsercache -e "select count(*) from $i";  done

Warming up x1 hosts

Hosts in x1 have all the wikis but only a few tables per wiki and we mostly only warm up the echo tables. The databases present on x1 are at echo.dblists on the mediawiki-config repo: https://github.com/wikimedia/operations-mediawiki-config/blob/master/dblists/echo.dblist

The tables to check file is present at: https://raw.githubusercontent.com/wikimedia/operations-software/master/dbtools/echo_tables_to_check.txt

This is the content on Sept 2021:

echo_event event_id
echo_notification notification_event
echo_target_page etp_page

The check can be performed with a simple loop:

cat git/mediawiki-config/dblists/echo.dblist | grep -v "#" | while read db; do cat echo_tables_to_check.txt | while read table index; do echo "$db.$table"; db-compare $db $table $index db2096.codfw.wmnet db1103 db1120 db1137; done; done

Replaying traffic

Probably the most effective way to warm up tables, but the most time consuming one and definitely dangerous. So proceed with caution.

Another way to warm up more concrete tables and hosts is to reply some of the most common/slow queries that are arriving to the active DC and send them to the passive DC. This requires a bit more manual work but this is a high level way of doing it.

Which queries to replay

Tendril

Using slow queries section on tendril and filtering for the last 24h and getting all the SELECTs into a file is a good way to start. Then simply iterate that file over all the hosts in a given section and all the wikis on that section.

Live traffic

If you are not 100% sure of what you are doing, do not proceed with this warm up method. YOU COULD CORRUPT DATA

On the passive DC, you can enable slow query log for a few seconds using the following commands. It highly depends on the day the kind of queries you can grab, but in general you cannot really reuse them as some of them have specific timestamps (especially recentchanges).

First, make sure you are using FILE as a way to log queries and which file is it:

> SHOW VARIABLES LIKE "general_log%";
+------------------+------------+
| Variable_name    | Value      |
+------------------+------------+
| general_log      | OFF        |
| general_log_file | db1169.log |
+------------------+------------+

Now you are ready to enable, capture half a minute or a minute of traffic and then disable it:

SET global general_log = 1; select sleep (30); SET global general_log = 0;

Keep in mind that if forgotten, this file can grow to huge values, so always try to use the enable and disable on the same command so you don't forget to disable it. Unfortunately this captures all queries and not only SELECTs (as log_disabled_statements isn't dynamic)

Once you've got the file, you need to exclude INSERT DELETE UPDATE and ONLY leave SELECTs statements otherwise YOU COULD CORRUPT DATA

If you are not 100% sure of what you are doing, do not proceed with this warm up method.

Once the file only has SELECTs, you can replay them to all the hosts you want to warm up

This page is a part of the SRE Data Persistence technical documentation
(go here for a list of all our pages)