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gh-ost/doc/perks.md
Shlomi Noach 087d1dd64d suuporting dynamic reconfiguration of max-load
2016-06-09 11:25:01 +02:00

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Perks

Listed below are some operation perks that make the DBA happy.

Dynamic reconfiguration

You started with a chunk-size=5000 but you find out it's too much. You want to reduce it. There is no need to kill and restart the migration with a new configuration. You may change the chunk-size dynamically.

gh-ost listens on a unix socket file, and optionally via TCP as well. You may, for example:

$ echo "chunk-size=250" | nc -U /tmp/gh-ost.test.sample_data_0.sock

Likewise, you can change the max-load configuration:

$ echo "max-load=Threads_running=50,threads_connected=1000" | nc -U /tmp/gh-ost.test.sample_data_0.sock

The max-load format must be: some_status=<numeric-threshold>[,some_status=<numeric-threshold>...]. In case of parsing error the command is ignored.

Read more about interactive commands

What's the status?

You do not have to have access to the screen where the migration is issued. You have two ways to get current status:

  1. Use interactive commands. Via unix socket file or via TCP you can get current status:
$ echo status | nc -U /tmp/gh-ost.test.sample_data_0.sock
# Migrating `test`.`sample_data_0`; Ghost table is `test`.`_sample_data_0_gst`
# Migration started at Tue Jun 07 11:45:16 +0200 2016
# chunk-size: 200; max lag: 1500ms; max-load: map[Threads_connected:20]
# Throttle additional flag file: /tmp/gh-ost.throttle
# Serving on unix socket: /tmp/gh-ost.test.sample_data_0.sock
# Serving on TCP port: 10001
Copy: 0/2915 0.0%; Applied: 0; Backlog: 0/100; Elapsed: 40s(copy), 41s(total); streamer: mysql-bin.000550:49942; ETA: throttled, flag-file
  1. gh-ost creates and uses a changelog table for internal bookkeeping. This table has the _osc suffix (the tool creates and announces this table upon startup) If you like, you can SQL your status:
> select * from _sample_data_0_osc order by id desc limit 1 \G
*************************** 1. row ***************************
         id: 325
last_update: 2016-06-08 15:52:13
       hint: copy iteration 0 at 1465393933
      value: Copy: 0/2915 0.0%; Applied: 0; Backlog: 0/100; Elapsed: 1m35s(copy), 1m35s(total); streamer: mysql-bin.000560:60904; ETA: throttled, flag-file

Postpone the cut-over phase

You begin a migration, and the ETA is for it to complete at 04:00am. Not a good time for you, because you happen to want to have eyes on things as the migration completes (ideally, you shouldn't need to, but life is hard).

Today, DBAs are coordinating the migration start time such that it completes in a convenient hour. gh-ost offers an alternative: postpone the final cut-over phase till you're ready.

Execute gh-ost with --postpone-cut-over-flag-file=/path/to/flag.file. As long as this file exists, gh-ost will not take the final cut-over step. It will complete the row copy, and continue to synchronize the tables by continuously applying changes made on the original table onto the ghost table. It can do so on and on and on. When you're finally ready, remove the file and cut-over will take place.