Re: Extra XLOG in Checkpoint for StandbySnapshot

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.

With Regards,
Amit Kapila.