Re: Postgres Replaying WAL slowly

On Jun 30, 2014, at 4:57 PM, Jeff Frost <jeff(at)pgexperts(dot)com> wrote:

>
> On Jun 30, 2014, at 4:04 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Ah ... that's more like a number I can believe something would have
>> trouble coping with. Did you see a noticeable slowdown with this?
>> Now that we've seen that number, of course it's possible there was an
>> even higher peak occurring when you saw the trouble.
>>
>> Perhaps there's an O(N^2) behavior in StandbyReleaseLocks, or maybe
>> it just takes awhile to handle that many locks.
>>
>> Did you check whether the locks were all on temp tables of the
>> ON COMMIT DROP persuasion?
>
>
> Unfortunately not, because I went for a poor man's: SELECT count(*) FROM pg_locks WHERE mode = 'AccessExclusiveLock'
> run in cron every minute.
>
> That said, I'd bet it was mostly ON COMMIT DROP temp tables.
>
> The unfortunate thing is I wouldn't know how to correlate that spike with the corresponding slowdown because the replica is about 5.5hrs lagged at the moment.
>
> Hopefully it will get caught up tonight and we can see if there's a correlation tomorrow.

And indeed it did catch up overnight and the lag increased shortly after a correlating spike in AccessExclusiveLocks that were generated by temp table creation with on commit drop.