[Engine-devel] DB Performance Monitoring

[Libor Spevak]

On 21.2.2013 17:11, Eli Mesika wrote:
> ----- Original Message -----
>> From: "Libor Spevak" <lspevak at redhat.com>
>> To: engine-devel at ovirt.org
>> Sent: Tuesday, February 19, 2013 5:44:10 PM
>> Subject: [Engine-devel] DB Performance Monitoring
>>
>>
>> Hi,
>> I just wanted to share a partial result from testing of engine db
>> load during operation (SQL queries frequencies). Maybe, we can
>> decide later, if some SQL queries result CACHING can boost
>> throughput for larger deployments.
>>
>> The caching technology can be from a simple HashMap lookup to
>> deployments of something like http://ehcache.org/ (memory database).
>>
>> I prepared two simple scenarios:
>>
>> - oVirt engine, 2 hosts, 1 VM, running 15 minutes (Power on + Up
>> state)
>> - oVirt engine, 2 hosts, 10 VMs from one pool, running 15 minutes
>> (Power on + Up state)
>>
>> Appending 2 spreadsheets with data about the most used SQL queries
>> (generated by PostgreSQL standard pg_statements_stat module).
>>
>> e.g.
>> - 2nd row shows number of granted connections from the db pool (e.g.
>> can be used to set optimal connection pool size) (check of
>> connection health: select 1)
>> - most of the queries are wrapped by a PLSQL function so we see a
>> wrapper:
>>
>> select * from getvdsgroupbyvdsgroupid($1, $2, $3)
>>
>> and near to it the 'real' query:
>>
>> SELECT vds_groups_view.*
>> FROM vds_groups_view
>> WHERE vds_group_id = v_vds_group_id
>> AND (NOT v_is_filtered OR EXISTS (SELECT 1
>> FROM user_vds_groups_permissions_view
>> WHERE user_id = v_user_id AND entity_id = v_vds_group_id))
>>
>> Just as an example, I selected in yellow color some queries, which
>> probably do not change often, but are very frequent.
> Thanks Libor for taking te time to do that.
> The problematic queries are those that involves all kinds of *permissions* checks with the complicated & expensive permission handling views.
> I believe that using Snapshot Materialized Views as defined in http://www.ovirt.org/OVirt-DB-Issues/MaterializedViews will solve the major part of those problems.
> We are currently verifying this approach and my recommendation is to have all *permissions* views as Snapshot Materialized Views that are updated via a cron job.
>
I think there could be performace improvement with M-views, of course. 
There is some overhead just with their maintainance, e.g. refreshing, 
updating existing dependent objects (table name renames, ...), not sure 
about NATIVE implementation level status in PostgreSQL, according to 
e.g. Oracle db capabilities (MV-logs + M-views).

http://wiki.postgresql.org/wiki/Materialized_Views

Still, if the Engine gathers statistics nearly in real time, my question 
would be, if there is a need of round-trip to the database and back at 
all for frequent queries.

>
>> Other tools like 'pg_top' can provide runtime statistics of db
>> processes (cpu, mem, locks, ... views).
>>
>> By enabling debug level logging of PostgreSQL we can check real
>> values to the queries.
>>
>> Of course, it would be useful to run such tests with many hosts and
>> VMs to predict scaling issues.
>>
>> More info about tools configuration:
>> http://www.ovirt.org/Engine_database_performance_monitoring
>>
>> Regards,
>> Libor
>>
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>>

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