[Engineering] Important news about Analytics databases

Wed Nov 8 11:02:37 UTC 2017

Hi everybody,

the Analytics team needs to make some changes to the current configuration
and deployment of the Analytics databases. Before starting a little refresh
to be on the same page:

- db1046 - eventlogging master database
- db1047 - also known as analytics-slave.eqiad.wmnet - replicates via mysql
s1/s2 and the log database (on db1046) using a custom replication script.
- dbstore1002 - also known as analytics-store.eqiad.wmnet and
x1-analytics-slave.eqiad.wmnet - replicates most of the S shards and X1 via
mysql, and the log database using a custom replication script.
- db1108 (brand new host) - replicates the log database using a custom
replication script.

We have been suffering during the past months some space and performance
issues on dbstore1002 (https://phabricator.wikimedia.org/T168303), so we
came up with the following plan:

- db1108, a brand new host with SSD disks, replaces db1047 and becomes the
CNAME of analytics-slave.eqiad.wmnet. This new host will be a replica of
the log database only, no other database will be replicated.
- dbstore1002 will loose the support of the log database, that will be
dropped from the host.
- db1047 will eventually be decommissioned (after backing up data and alert
people beforehand - T156844).

This will allow us to:
1) Reduce the load on dbstore1002 and free a lot of space on the host.
2) Offer a more performant way to query eventlogging analytics data.
3) Reduce the current performance issues that we have been experiencing
while trying to sanitize/purge old event-logging data (https://phabricator.
wikimedia.org/T156933)

The plan is the following:

- November 13th: the analytics-slave CNAME moves from db1047 to db1108
- November 20th: the log database will be dropped from
dbstore1002/analytics-store together with the event-logging replication
script
- December 4th: shutdown of db1047 (prior backup of non-log database tables)

More info in https://phabricator.wikimedia.org/T156844

To summarize what will change from the users perspective:

- dbstore1002 (analytics-store) will offer all the S/X shards replication
(wikis) and all the databases like staging that everybody is used to work
with. It will only loose the support of the log database.
- db1108 will offer the log database replication and a staging database.
- the db1047's (analytics-slave) staging database will be moved or copied
with a different name (like staging_db1047) to dbstore1002.

Please let us know in the task your opinion in T156844, we'd love to hear
some feedback before proceeding, especially about extra requirements that
we haven't thought of.

Thanks!

Luca (on behalf of the Analytics team)
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