[ovirt-users] [ovirt-devel] [Feture discussion] Full vacuum tool

Wed Dec 7 20:54:22 UTC 2016

In terms of measuring I used pgclu couple of times and it powerfull,easy to
use, and provide very nice HTML reports
http://pgcluu.darold.net/

And also provide autovacum analysis
http://pgcluu.darold.net/example/dolibarr-table-vacuums-analyzes.html

On Wed, Dec 7, 2016 at 9:55 PM, Roy Golan <rgolan at redhat.com> wrote:

>
>
> On 7 December 2016 at 21:44, Roy Golan <rgolan at redhat.com> wrote:
>
>>
>>
>> On 7 December 2016 at 21:00, Michal Skrivanek <mskrivan at redhat.com>
>> wrote:
>>
>>>
>>>
>>> On 07 Dec 2016, at 11:28, Yaniv Kaul <ykaul at redhat.com> wrote:
>>>
>>>
>>>
>>> On Wed, Dec 7, 2016 at 10:57 AM, Roy Golan <rgolan at redhat.com> wrote:
>>>
>>>> Hi all,
>>>>
>>>> This is a discussion on the RFE[1] to provide a tool to perform full
>>>> vacuum on our DBs.
>>>>
>>>> First if you are not familiar with vacuum please read this [2]
>>>>
>>>> # Backgroud
>>>> ovirt 'engine' DB have several busy table with 2 differnt usage patten.
>>>> One is audit_log and the others are the 'v*_statistics' tables and the
>>>> difference between them is mostly inserts vs mostly hot updates.
>>>> Tables with tons of updates creates garbage or 'dead' records that
>>>> should be removed, and for this postgres have the aforementioned autovacuum
>>>> cleaner. It will make the db reuse its already allocated space to perform
>>>> future updates/inserts and so on.
>>>> Autovacuum is essential for a db to function optimally and tweaking it
>>>> is out of the scope of the feature.
>>>>
>>>> Full vacuum is designed to reclaim the disk space and reset the table
>>>> statistics. It is a heavy maintenance task, it takes an exclusive lock on
>>>> the table and may take seconds to minutes. In some situations it is
>>>> effectively a downtime due to the long table lock and should not be running
>>>> when the engine is running.
>>>>
>>>
>>> So, effectively this should be interesting mostly/only for the audit
>>> log. All other busy table are mostly in-place updates
>>>
>>
>> Given that autovacuum is performing well the yes but if it starts to fall
>> behind this may help a bit.
>> audit_log is insert mostly and also delete, we remove a day, each day.
>>
>>>
>>>
>>>> # Critiria
>>>> Provide a way to reclaim disk space claimed by the garbage created over
>>>> time by the engine db and dwh.
>>>>
>>>> # Usage
>>>> Either use it as part of the upgrade procedure (after all dbscipts
>>>> execution)
>>>>
>>>
>>> That does sound as a good start not requiring much user involvement
>>>
>>> or just provide the tool and admin will run in on demand
>>>> - engine db credentials read from /etc/ovirt-engine/engine.conf.d/
>>>> - invocation:
>>>>  ```
>>>>  tool: [dbname(default engine)] [table: (default all)]
>>>>  ```
>>>> - if we invoke it on upgrade than an installation plugin should be
>>>> added to invoke with default, no interaction
>>>>
>>>
>>> +1
>>>
>>> - since VACUUM ANALYZE is consider a recommended maintenance task we can
>>>> to it by default and ask the user for FULL.
>>>>
>>>
>>> When would you run it? ANALYZE nightly?
>>>
>>> No I'd still avoid doing this repeatedly, autovaccum should handle that
>> as well, but this would cover situations where it isn't functioning
>> optimally.
>>
>> I think  its worth adding a report of the db status and the rate of the
>> autovacuum (a slight midifed version of the query mskrivanek ran on one of
>> the production systems [3])  that will go to the logcollector. Perhaps the
>> output of the ANALYZE will help as well.
>>
>> [3] https://gist.github.com/rgolangh/049cff30b89c5b29284ceee80a3
>> 5dbb4#file-table_status_by_dead_rows-sql
>>
>
>
> Very interesting collection of pg scrips to measure bloat and vacuum -
> needs access to postgres objects though
>
> - https://github.com/pgexperts/pgx_scripts
> - https://github.com/pgexperts/pgx_scripts/blob/master/bloat/
> table_bloat_check.sql
> - https://github.com/pgexperts/pgx_scripts/blob/master/
> vacuum/last_autovacuum.sql
>
>
>>
>>
>>>>
>>> Will the user know to answer intelligently if vacuum is needed or not?
>>> Except for 'yes, you need it', we cannot even provide a time estimate (I
>>> assume a disk space estimate is available!)
>>>
>>> perhaps we can estimate the bloat, there should be a github script to
>> calculate that [4] not sure how good it is.
>>
>>> I would suggest to run ANALYZE for sure and provide an option at the end
>>> of installation, to run the required command line - so make it as
>>> accessible as possible, but not part of the flow.
>>>
>>>
>>> If there are no significant gains why bother any other time but on
>>> upgrade when it can be run unconditionally?
>>>
>>>
>>> I'm wondering if the community can run ANALYZE on their database, and we
>>> can estimate how many are in dire need for full vacuum already.
>>> Y.
>>>
>>> I'll send a different mail for that.
>>
>>
>>>
>>> - remote db is supported as well, doesn't have to be local
>>>>
>>>
>>> Well, not sure if we need to bother. It was introduced for large
>>> deployments where the host can't fit both engine and db load. Do we still
>>> have this issue? I wouldn't say so for 4.1. It may be very niche case
>>>
>>> Running full vacuum is anyway a psql command, so there is no hidden cost
>> here (to the development side I mean)
>>
>>
>>> Thanks,
>>> michal
>>>
>>>
>>>> # Questions
>>>>  - Will remote dwh have the credentials under
>>>> /etc/ovirt-engine/engine.conf.d?
>>>>  - Should  AAA schema be taken into account as well?
>>>>
>>>> Please review, thanks
>>>> Roy
>>>>
>>>> [1] https://bugzilla.redhat.com/show_bug.cgi?id=1388430
>>>> [2] https://www.postgresql.org/docs/9.2/static/runtime-config-au
>>>> tovacuum.html
>>>> [3] https://www.postgresql.org/docs/devel/static/sql-vacuum.html
>>>>
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>>>>
>>>>
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>>>
>>
>
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-- 
-Eldad
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