Re: Random performance hit, unknown cause.

Claudio Freire <klaussfreire@gmail.com> wrote:
> On Thu, Apr 12, 2012 at 3:41 PM, Brian Fehrle
> <brianf@consistentstate.com> wrote:
>> This morning, during our nightly backup process (where we grab a
>> copy of the data directory), we started having this same issue.
>> The main thing that I see in all of these is a high disk wait on
>> the system. When we are performing 'well', the %wa from top is
>> usually around 30%, and our load is around 12 - 15. This morning
>> we saw a load  21 - 23, and an %wa jumping between 60% and 75%.
>>
>> The top process pretty much at all times is the WAL Sender
>> Process, is this normal?
>
> Sounds like vacuum to me.

More particularly, it seems consistent with autovacuum finding a
large number of tuples which had reached their freeze threshold.
Rewriting the tuple in place with a frozen xmin is a WAL-logged
operation.

-Kevin