[Devel] [Engine-devel] vds_dynamic refactor
Liran Zelkha
liran.zelkha at gmail.com
Mon Apr 7 05:51:00 UTC 2014
On Sun, Apr 6, 2014 at 11:03 PM, Gilad Chaplik <gchaplik at redhat.com> wrote:
> ----- Original Message -----
> > From: "Liran Zelkha" <liran.zelkha at gmail.com>
> > To: "Gilad Chaplik" <gchaplik at redhat.com>
> > Cc: "Kobi Ianko" <kobi at redhat.com>, devel at linode01.ovirt.org,
> "engine-devel" <engine-devel at ovirt.org>
> > Sent: Sunday, April 6, 2014 8:51:02 PM
> > Subject: Re: [Devel] [Engine-devel] vds_dynamic refactor
> >
> > On Sun, Apr 6, 2014 at 6:32 PM, Gilad Chaplik <gchaplik at redhat.com>
> wrote:
> >
> > > ----- Original Message -----
> > > > From: "Liran Zelkha" <liran.zelkha at gmail.com>
> > > > To: "Kobi Ianko" <kobi at redhat.com>
> > > > Cc: "Gilad Chaplik" <gchaplik at redhat.com>, devel at linode01.ovirt.org,
> > > "engine-devel" <engine-devel at ovirt.org>
> > > > Sent: Sunday, April 6, 2014 3:40:13 PM
> > > > Subject: Re: [Devel] [Engine-devel] vds_dynamic refactor
> > > >
> > > > On Sun, Apr 6, 2014 at 3:37 PM, Kobi Ianko <kobi at redhat.com> wrote:
> > > >
> > > > > Joining in...
> > > > > From my point of view, in real life a user should have that many
> VDSs
> > > on
> > > > > one Engine (from a DB point of view).
> > > > > Modern DB system handles tables with millions of records and many
> > > > > relations, Do we really have a performance issue here?
> > > > > We could prefer a more easy to maintain implantation in this case
> over
> > > DB
> > > > > performance
> > > > >
> > > > > Yes we do. We make many queries on the VDS view, which is a VERY
> > > complex
> > > > view.
> > > >
> > >
> > > Actually I quite agree with Kobi, what is the plan for VMs? why do we
> > > start with VDS...
> > > what is the biggest deploy do you know of?
> > >
> > We start with VDS because in an idle system, with 200 hosts and several
> > thousands VMs, this is what you get as the top queries against the
> > database. Look at how many times getvds is called.
> > [image: Inline image 1]
> > BTW - the second query is an example of abusing the dynamic query
> > mechanism. The 4th query (an update command) is a set of useless
> > update_vds_dynamic commands.
> >
> > For reference, the explain plan of get VDS is something like this:
> >
> > QUERY PLAN
> >
> >
> -----------------------------------------------------------------------------------------------------------------------------------------------------------
> > Nested Loop (cost=9.30..46.75 rows=6 width=9060) (actual
> > time=0.063..0.068 rows=1 loops=1)
> > Join Filter: (vds_static.vds_id = vds_statistics.vds_id)
> > -> Seq Scan on vds_statistics (cost=0.00..1.01 rows=1 width=109)
> > (actual time=0.008..0.008 rows=1 loops=1)
> > -> Nested Loop (cost=9.30..45.64 rows=6 width=8983) (actual
> > time=0.048..0.052 rows=1 loops=1)
> > Join Filter: (vds_groups.vds_group_id = vds_static.vds_group_id)
> > -> Nested Loop Left Join (cost=0.00..9.29 rows=1 width=1389)
> > (actual time=0.013..0.013 rows=1 loops=1)
> > -> Seq Scan on vds_groups (cost=0.00..1.01 rows=1
> > width=1271) (actual time=0.003..0.003 rows=1 loops=1)
> > -> Index Scan using pk_storage_pool on storage_pool
> > (cost=0.00..8.27 rows=1 width=134) (actual time=0.008..0.008 rows=1
> > loops=1)
> > Index Cond: (vds_groups.storage_pool_id = id)
> > -> Hash Right Join (cost=9.30..36.28 rows=6 width=7610)
> (actual
> > time=0.033..0.037 rows=1 loops=1)
> > Hash Cond: (vds_spm_id_map.vds_id = vds_static.vds_id)
> > -> Seq Scan on vds_spm_id_map (cost=0.00..22.30
> rows=1230
> > width=20) (actual time=0.003..0.003 rows=1 loops=1)
> > -> Hash (cost=9.29..9.29 rows=1 width=7606) (actual
> > time=0.019..0.019 rows=1 loops=1)
> > Buckets: 1024 Batches: 1 Memory Usage: 2kB
> > -> Nested Loop (cost=0.00..9.29 rows=1 width=7606)
> > (actual time=0.012..0.013 rows=1 loops=1)
> > -> Seq Scan on vds_dynamic (cost=0.00..1.01
> > rows=1 width=1895) (actual time=0.006..0.006 rows=1 loops=1)
> > -> Index Scan using pk_vds_static on
> vds_static
> > (cost=0.00..8.27 rows=1 width=5711) (actual time=0.005..0.006 rows=1
> > loops=1)
> > Index Cond: (vds_id =
> vds_dynamic.vds_id)
> > Total runtime: 0.299 ms
> > (19 rows)
> >
> > It's terrible. Adding any additional join will make this worse. Please
> > don't add any more tables...
>
> Thank you for the detailed explanation, my comments:
>
> * a very long time isn't an argument for not adding another table (should
> be neglectable);
> currently we have an unrelated problem, we need to solve it.
>
Of course it is. A very long time for a query that you execute many times
is THE factor. Who said the join has no performance effect? Have you tested
it? Under load? Under many writes/updates?
>
> * > We start with VDS because in an idle system, with 200 hosts and several
> > thousands VMs, this is what you get as the top queries against the
> > database.
>
> so, if fetching VMs takes 10 minutes? and its get called a single time?
>
Where do you see 10 minutes? If you are looking at the red bar it's the
inherent time - total query time * number of queries.
>
> * you didn't reply on my of my suggestion of constructing the VDS records
> in the DB without using joins.
>
If you mean materialized views - we don't have it in Postgres just yet...
And even if we do, since we do many updates to vds_statistics and
vds_dynamic - I'm not sure it will have positive impact on our performance.
If you mean joins in the database - everything that is based on VDS is done
in the database. Part of the problem, since we can cache some information
and only query the dynamic/statistics part of VDS, but that's another
matter.
>
>
> >
> > >
> > > > >
> > > > > ----- Original Message -----
> > > > > > From: "Gilad Chaplik" <gchaplik at redhat.com>
> > > > > > To: "Liran Zelkha" <liran.zelkha at gmail.com>
> > > > > > Cc: devel at linode01.ovirt.org, "engine-devel" <
> engine-devel at ovirt.org
> > > >
> > > > > > Sent: Sunday, April 6, 2014 3:32:26 PM
> > > > > > Subject: Re: [Devel] [Engine-devel] vds_dynamic refactor
> > > > > >
> > > > > > ----- Original Message -----
> > > > > > > From: "Liran Zelkha" <liran.zelkha at gmail.com>
> > > > > > > To: "Gilad Chaplik" <gchaplik at redhat.com>
> > > > > > > Cc: "Itamar Heim" <iheim at redhat.com>, devel at linode01.ovirt.org
> ,
> > > > > > > "engine-devel" <engine-devel at ovirt.org>
> > > > > > > Sent: Sunday, April 6, 2014 3:26:24 PM
> > > > > > > Subject: Re: [Engine-devel] vds_dynamic refactor
> > > > > > >
> > > > > > > On Sun, Apr 6, 2014 at 3:18 PM, Gilad Chaplik <
> gchaplik at redhat.com
> > > >
> > > > > wrote:
> > > > > > >
> > > > > > > > ----- Original Message -----
> > > > > > > > > From: "Itamar Heim" <iheim at redhat.com>
> > > > > > > > > To: "Liran Zelkha" <liran.zelkha at gmail.com>
> > > > > > > > > Cc: "Gilad Chaplik" <gchaplik at redhat.com>,
> > > > > devel at linode01.ovirt.org,
> > > > > > > > "engine-devel" <engine-devel at ovirt.org>
> > > > > > > > > Sent: Sunday, April 6, 2014 11:33:12 AM
> > > > > > > > > Subject: Re: [Engine-devel] vds_dynamic refactor
> > > > > > > > >
> > > > > > > > > On 04/06/2014 11:32 AM, Liran Zelkha wrote:
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > On Sun, Apr 6, 2014 at 11:22 AM, Itamar Heim <
> > > iheim at redhat.com
> > > > > > > > > > <mailto:iheim at redhat.com>> wrote:
> > > > > > > > > >
> > > > > > > > > > On 04/03/2014 07:51 PM, Liran Zelkha wrote:
> > > > > > > > > >
> > > > > > > > > > The problem is with both updates and selects.
> > > > > > > > > > For selects - to get all the information for the
> VDS
> > > we
> > > > > have
> > > > > > > > > > multiple
> > > > > > > > > > joins. Adding another one will hurt performance
> even
> > > > > more.
> > > > > > > > > > For updates - we have vds_static thats hardly
> > > changed.
> > > > > > > > > > vds_statistics
> > > > > > > > > > that changes all the time. vds_dynamic is not
> changed
> > > > > allot -
> > > > > > > > but
> > > > > > > > > > is
> > > > > > > > > > updated all the time because of the status. I
> think
> > > it's
> > > > > best
> > > > > > > > to
> > > > > > > > > > split
> > > > > > > > > > it to the two existing tables (BTW - relevant
> for VM
> > > as
> > > > > well)
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > but we don't update it unless the status has changed,
> > > which
> > > > > is a
> > > > > > > > > > rare occurance?
> > > > > > > > > >
> > > > > > > > > > Actually - no. We can definitely see times we are
> updating
> > > > > > > > > > vds_dynamic
> > > > > > > > > > with no reason at all. I tried to create patches for
> that -
> > > but
> > > > > it
> > > > > > > > > > happens from many different places in the code.
> > > > > > > > >
> > > > > > > > > what would be updated vds_dyanmic for status not
> originating in
> > > > > update
> > > > > > > > > run time info?
> > > > > > > >
> > > > > > > > We have separate DB flows for that (updateStatus and
> > > > > > > > updatePartialVdsDynamicCalc and more in
> > > VdsDynamicDAODbFacadeImpl).
> > > > > > > > A question: do you know if we update status in
> updateVdsDynamic?
> > > :-)
> > > > > not
> > > > > > > > sure but I found a possible race for pending resources (cpu,
> > > mem),
> > > > > LOL
> > > > > > > > :-)
> > > > > > > >
> > > > > > > > I think we do but not sure. Will check.
> > > > > >
> > > > > > Of course it is, that was a rhetorical question :-) (a lot of
> > > emoticons
> > > > > and
> > > > > > LOLs ;-))
> > > > > >
> > > > > > >
> > > > > > >
> > > > > > > > Still holds my original thought for having vds_on_boot.
> > > > > > > >
> > > > > > > >
> > > > > > > >
> > > > > > > Let's talk f2f on Tuesday?
> > > > > >
> > > > > > I'd prefer to reach conclusions here, I'd like everyone to be
> > > involved
> > > > > in a
> > > > > > root issue like this one.
> > > > >
> > > >
> > > > What is the update frequency of this field?
> > > >
> > >
> > > which field?
> > > status? pending resources? on boot fields?
> > > iinm, status is updated mostly by user actions, at least in positive
> > > scenarios, and not that often.
> > >
> > >
> > > > > >
> > > > > > >
> > > > > > _______________________________________________
> > > > > > Devel mailing list
> > > > > > Devel at ovirt.org
> > > > > > http://lists.ovirt.org/mailman/listinfo/devel
> > > > > >
> > > > >
> > > >
> > >
> >
>
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