Re: Synchronizing slots from primary to standby

On Fri, Jan 19, 2024 at 3:55 PM shveta malik <shveta(dot)malik(at)gmail(dot)com> wrote:
>
> On Fri, Jan 19, 2024 at 10:35 AM Masahiko Sawada <sawada(dot)mshk(at)gmail(dot)com> wrote:
> >
> >
> > Thank you for updating the patch. I have some comments:
> >
> > ---
> > + latestWalEnd = GetWalRcvLatestWalEnd();
> > + if (remote_slot->confirmed_lsn > latestWalEnd)
> > + {
> > + elog(ERROR, "exiting from slot synchronization as the
> > received slot sync"
> > + " LSN %X/%X for slot \"%s\" is ahead of the
> > standby position %X/%X",
> > + LSN_FORMAT_ARGS(remote_slot->confirmed_lsn),
> > + remote_slot->name,
> > + LSN_FORMAT_ARGS(latestWalEnd));
> > + }
> >
> > IIUC GetWalRcvLatestWalEnd () returns walrcv->latestWalEnd, which is
> > typically the primary server's flush position and doesn't mean the LSN
> > where the walreceiver received/flushed up to.
>
> yes. I think it makes more sense to use something which actually tells
> flushed-position. I gave it a try by replacing GetWalRcvLatestWalEnd()
> with GetWalRcvFlushRecPtr() but I see a problem here. Lets say I have
> enabled the slot-sync feature in a running standby, in that case we
> are all good (flushedUpto is the same as actual flush-position
> indicated by LogstreamResult.Flush). But if I restart standby, then I
> observed that the startup process sets flushedUpto to some value 'x'
> (see [1]) while when the wal-receiver starts, it sets
> 'LogstreamResult.Flush' to another value (see [2]) which is always
> greater than 'x'. And we do not update flushedUpto with the
> 'LogstreamResult.Flush' value in walreceiver until we actually do an
> operation on primary. Performing a data change on primary sends WALs
> to standby which then hits XLogWalRcvFlush() and updates flushedUpto
> same as LogstreamResult.Flush. Until then we have a situation where
> slots received on standby are ahead of flushedUpto and thus slotsync
> worker keeps one erroring out. I am yet to find out why flushedUpto is
> set to a lower value than 'LogstreamResult.Flush' at the start of
> standby. Or maybe am I using the wrong function
> GetWalRcvFlushRecPtr() and should be using something else instead?
>

Can we think of using GetStandbyFlushRecPtr()? We probably need to
expose this function, if this works for the required purpose.

--
With Regards,
Amit Kapila.