On 2016-02-02 10:12:25 +0530, Amit Kapila wrote:
> @@ -8239,7 +8262,7 @@ CreateCheckPoint(int flags)
>   if ((flags & (CHECKPOINT_IS_SHUTDOWN |
>   {
> - if
> (prevPtr == ControlFile->checkPointCopy.redo &&
> + if (GetProgressRecPtr() == prevPtr &&
> prevPtr / XLOG_SEG_SIZE == curInsert / XLOG_SEG_SIZE)
>   {
> I think such a check won't consider all the WAL-activity happened
> during checkpoint (after checkpoint start, till it finishes) which was
> not the intention of this code without your patch.


> I think both this and previous patch (hs-checkpoints-v1 ) approach
> won't fix the issue in all kind of scenario's.


> Let me try to explain what I think should fix this issue based on
> discussion above, suggestions by Andres and some of my own
> thoughts:

> Have a new variable in WALInsertLock that would indicate the last
> insertion position (last_insert_pos) we write to after holding that lock.
> Ensure that we don't update last_insert_pos while logging standby
> snapshot (simple way is to pass a flag in XLogInsert or distinguish
> it based on type of record (XLOG_RUNNING_XACTS) or if you can
> think of a more smarter way).  Now both during checkpoint and
> in bgwriter, to find out whether there is any activity since last
> time, we need to check all the WALInsertLocks for latest insert position
> (by referring last_insert_pos) and compare it with the latest position
> we have noted during last such check and decide whether to proceed
> or not based on comparison result.  If you think it is not adequate to
> store last_insert_pos in WALInsertLock, then we can think of having
> it in PGPROC.

Yes, that's pretty much what I was thinking of.

> Yet another idea that occurs to me this morning is that why not
> have a variable in shared memory in XLogCtlInsert or XLogCtl
> similar to CurrBytePos/PrevBytePos which will be updated on
> each XLOGInsert apart from the XLOGInsert for standby snapshots
> and use that in a patch somewhat close to what you have in
> hs-checkpoints-v1.

That'll require holding locks longer...


Andres Freund

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