Re: Replica lag, high read IO, vacuum index scanning bug?

Phil Frost wrote:
> We are seeing an issue on RDS 9.4.5 where replica lag gets really high.
> This lag seems to be correlated to a saturation of disk read throughput,
> and vacuums of one of our larger and more frequently updated tables.
>
> I've heard that there's a bug where the replica will scan indexes
> unnecessarily when doing the WAL replay when the vacuums happen. Can
> anyone confirm this and maybe point me towards the commit where it was
> fixed? Has there been a backport to the 9.4 series?

Hi Phil,

The 9.4 backport of the patch in question is this commit by yours truly:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=30e3cb307d494990cb38cebff6bfc8b4ce27d7ab

This will appear in the next minor release of the 9.4 series (as well as
9.5), due out sometime during February 2017.

Thanks,

--
Álvaro Herrera                https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services