Open Stack

On 03/04, Jay Pipes wrote:
> On 04/03/2018 11:07 AM, Michael Bayer wrote:
> > The MySQL / MariaDB variants we use nowadays default to
> > innodb_file_per_table=ON and we also set this flag to ON in installer
> > tools like TripleO.     The reason we like file per table is so that
> > we don't grow an enormous ibdata file that can't be shrunk without
> > rebuilding the database.  Instead, we have lots of little .ibd
> > datafiles for each table throughout each openstack database.
> >
> > But now we have the issue that these files also can benefit from
> > periodic optimization which can shrink them and also have a beneficial
> > effect on performance.   The OPTIMIZE TABLE statement achieves this,
> > but as would be expected it itself can lock tables for potentially a
> > long time.   Googling around reveals a lot of controversy, as various
> > users and publications suggest that OPTIMIZE is never needed and would
> > have only a negligible effect on performance.   However here we seek
> > to use OPTIMIZE so that we can reclaim disk space on tables that have
> > lots of DELETE activity, such as keystone "token" and ceilometer
> > "sample".
> >
> > Questions for the group:
> >
> > 1. is OPTIMIZE table worthwhile to be run for tables where the
> > datafile has grown much larger than the number of rows we have in the
> > table?
>
> Possibly, though it's questionable to use MySQL/InnoDB for storing transient
> data that is deleted often like ceilometer samples and keystone tokens. A
> much better solution is to use RDBMS partitioning so you can simply ALTER
> TABLE .. DROP PARTITION those partitions that are no longer relevant (and
> don't even bother DELETEing individual rows) or, in the case of Ceilometer
> samples, don't use a traditional RDBMS for timeseries data at all...
>
> But since that is unfortunately already the case, yes it is probably a good
> idea to OPTIMIZE TABLE on those tables.
>
> > 2. from people's production experience how safe is it to run OPTIMIZE,
> > e.g. how long is it locking tables, etc.
>
> Is it safe? Yes.
>
> Does it lock the entire table for the duration of the operation? No. It uses
> online DDL operations:
>
> https://dev.mysql.com/doc/refman/5.7/en/innodb-file-defragmenting.html
>
> Note that OPTIMIZE TABLE is mapped to ALTER TABLE tbl_name FORCE for InnoDB
> tables.
>
> > 3. is there a heuristic we can use to measure when we might run this
> > -.e.g my plan is we measure the size in bytes of each row in a table
> > and then compare that in some ratio to the size of the corresponding
> > .ibd file, if the .ibd file is N times larger than the logical data
> > size we run OPTIMIZE ?
>
> I don't believe so, no. Most things I see recommended is to simply run
> OPTIMIZE TABLE in a cron job on each table periodically.
>
> > 4. I'd like to propose this job of scanning table datafile sizes in
> > ratio to logical data sizes, then running OPTIMIZE, be a utility
> > script that is delivered via oslo.db, and would run for all innodb
> > tables within a target MySQL/ MariaDB server generically.  That is, I
> > really *dont* want this to be a script that Keystone, Nova, Ceilometer
> > etc. are all maintaining delivering themselves.   this should be done
> > as a generic pass on a whole database (noting, again, we are only
> > running it for very specific InnoDB tables that we observe have a poor
> > logical/physical size ratio).
>
> I don't believe this should be in oslo.db. This is strictly the purview of
> deployment tools and should stay there, IMHO.
>

Hi,

As far as I know most projects do "soft deletes" where we just flag the
rows as deleted and don't remove them from the DB, so it's only when we
use a management tool and run the "purge" command that we actually
remove these rows.

Since running the optimize without purging would be meaningless, I'm
wondering if we should trigger the OPTIMIZE also within the purging
code.  This way we could avoid innefective runs of the optimize command
when no purge has happened and even when we do the optimization we could
skip the ratio calculation altogether for tables where no rows have been
deleted (the ratio hasn't changed).

Ideally the ratio calculation and optimization code would be provided by
oslo.db to reduce code duplication between projects.

Cheers,
Gorka.


> > 5. for Galera this gets more tricky, as we might want to run OPTIMIZE
> > on individual nodes directly.  The script at [1] illustrates how to
> > run this on individual nodes one at a time.
> >
> > More succinctly, the Q is:
> >
> > a. OPTIMIZE, yes or no?
>
> Yes.
>
> > b. oslo.db script to run generically, yes or no?
>
> No. Just have Triple-O install galera_innoptimizer and run it in a cron job.
>
> Best,
> -jay
>
> > thanks for your thoughts!
> >
> >
> >
> > [1] https://github.com/deimosfr/galera_innoptimizer
> >
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