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Re: Extra XLOG in Checkpoint for StandbySnapshot

From: Andres Freund <andres(at)2ndquadrant(dot)com>
To: Amit Kapila <amit(dot)kapila(at)huawei(dot)com>
Cc: 'Simon Riggs' <simon(at)2ndQuadrant(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Extra XLOG in Checkpoint for StandbySnapshot
Date: 2013-01-08 15:27:01
Message-ID: 20130108152701.GA12111@awork2.anarazel.de (view raw or flat)
Thread:
Lists: pgsql-hackers
On 2013-01-08 20:33:28 +0530, Amit Kapila wrote:
> On Tuesday, January 08, 2013 8:01 PM Andres Freund wrote:
> > On 2013-01-08 19:51:39 +0530, Amit Kapila wrote:
> > > On Monday, January 07, 2013 7:15 PM Andres Freund wrote:
> > > > On 2013-01-07 19:03:35 +0530, Amit Kapila wrote:
> > > > > On Monday, January 07, 2013 6:30 PM Simon Riggs wrote:
> > > > > > On 7 January 2013 12:39, Amit Kapila <amit(dot)kapila(at)huawei(dot)com>
> > > > wrote:
> > > > > >
> > > > > > > So We can modify to change this in function
> > LogStandbySnapshot as
> > > > > > below:
> > > > > > >                 running = GetRunningTransactionData();
> > > > > > >                 if (running->xcnt > 0)
> > > > > > >                         LogCurrentRunningXacts(running);
> > > > > > >
> > > > > > > So this check will make sure that if there is no operation
> > > > happening
> > > > > > i.e. no
> > > > > > > new running transaction, then no need to log running
> > transaction
> > > > > > snapshot
> > > > > > > and hence further checkpoint operations will be skipped.
> > > > > > >
> > > > > > > Let me know if I am missing something?
> > > > > >
> > > > > > It's not the same test. The fact that nothing is running at
> > that
> > > > > > moment is not the same thing as saying nothing at all has run
> > since
> > > > > > last checkpoint.
> > > > >
> > > > > But isn't the functionality of LogStandbySnapshot() is to log
> > "all
> > > > running
> > > > > xids" and "all current
> > > > > AccessExclusiveLocks". For RunningTransactionLocks, WAL is
> > avoided in
> > > > > similar way.
> > > >
> > > > The information that no transactions are currently running allows
> > you
> > > > to
> > > > build a recovery snapshot, without that information the standby
> > won't
> > > > start answering queries. Now that doesn't matter if all standbys
> > > > already
> > > > have built a snapshot, but the primary cannot know that.
> > >
> > > Can't we make sure that checkpoint operation doesn't happen for below
> > conds.
> > > a. nothing has happened during or after last checkpoint
> > > OR
> > > b. nothing except snapshotstanby WAL has happened
> > >
> > > Currently it is done for point a.
> > >
> > > > Having to issue a checkpoint while ensuring transactions are
> > running
> > > > just to get a standby up doesn't seem like a good idea to me :)
> > >
> > > Simon:
> > > > If you make the correct test, I'd be more inclined to accept the
> > premise.
> > >
> > > Not sure, what exact you are expecting from test?
> > > The test is do any one operation on system and then keep the system
> > idle.
> > > Now at each checkpoint interval, it logs WAL for SnapshotStandby.
> >
> > I can't really follow what you want to do here. The snapshot is only
> > logged if a checkpoint is performed anyway?  As recovery starts at (the
> > logical) checkpoint's location we need to log a snapshot exactly
> > there. If you want to avoid activity when the system is idle you need
> > to
> > prevent checkpoints from occurring itself.
>
> Even if the checkpoint is scheduled, it doesn't perform actual operation if
> there's nothing logged between
> current and previous checkpoint due to below check in CreateCheckPoint()
> function.
> if (curInsert == ControlFile->checkPoint +
>                         MAXALIGN(SizeOfXLogRecord + sizeof(CheckPoint)) &&
>                         ControlFile->checkPoint ==
> ControlFile->checkPointCopy.redo)
>
> But if we set the wal_level as hot_standby, it will log snapshot, now next
> time again when function CreateCheckPoint()
> will get called due to scheduled checkpoint, the above check will fail and
> it will again log snapshot, so this will continue, even if the system is
> totally idle.
> I understand that it doesn't cause any problem, but I think it is better if
> the repeated log of snapshot in this scenario can be avoided.

ISTM in that case you "just" need a way to cope with the additionally
logged record in the above piece of code. Not logging seems to be the
entirely wrong way to go at this.
I admit its not totally simple, but making HS less predictable seems
like a cure *far* worse than the disease.

Greetings,

Andres Freund

--
 Andres Freund	                   http://www.2ndQuadrant.com/
 PostgreSQL Development, 24x7 Support, Training & Services


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