Re: Re: BUG #13685: Archiving while idle every archive_timeout with wal_level hot_standby

On 2016-01-28 16:40:13 +0900, Michael Paquier wrote:
> OK, so as a first step and after thinking about the whole for a while,
> I have finished with the patch attached. This patch is aimed at
> avoiding unnecessary checkpoints on idle systems when wal_level >=
> hot_standby by centralizing the check to look at if there has some WAL
> activity since the last checkpoint.

That's not what I suggested.

> /*
> + * XLOGHasActivity -- Check if XLOG had some significant activity or
> + * if it is idle lately. This is primarily used to check if there has
> + * been some WAL activity since the last checkpoint that occurred on
> + * system to control the generaton of XLOG record related to standbys.
> + */
> +bool
> +XLOGHasActivity(void)
> +{
> + XLogCtlInsert *Insert = &XLogCtl->Insert;
> + XLogRecPtr redo_lsn = ControlFile->checkPointCopy.redo;
> + uint64 prev_bytepos;
> +
> + /* Check if any activity has happened since last checkpoint */
> + SpinLockAcquire(&Insert->insertpos_lck);
> + prev_bytepos = Insert->PrevBytePos;
> + SpinLockRelease(&Insert->insertpos_lck);
> +
> + return XLogBytePosToRecPtr(prev_bytepos) == redo_lsn;
> +}
>

How should this actually should work reliably, given we *want* to have
included a standby snapshot after the last checkpoint?

In CreateCheckPoint() we have
/*
* Here we update the shared RedoRecPtr for future XLogInsert calls; this
* must be done while holding all the insertion locks.
*
RedoRecPtr = XLogCtl->Insert.RedoRecPtr = checkPoint.redo;

computing the next redo rec ptr and then
if (!shutdown && XLogStandbyInfoActive())
LogStandbySnapshot();
before the final
XLogRegisterData((char *) (&checkPoint), sizeof(checkPoint));
recptr = XLogInsert(RM_XLOG_ID,
shutdown ? XLOG_CHECKPOINT_SHUTDOWN :
XLOG_CHECKPOINT_ONLINE);

so the above condition doesn't really something we want to rely on. Am I
missing what you're trying to do?

Greetings,

Andres Freund