Date: Thu, 24 Nov 2016 16:51:29 -0300
From: Alvaro Herrera <alvherre@2ndquadrant.com>
To: Phil Frost <phil@postmates.com>
Cc: pgsql-admin@postgresql.org
Subject: Re: Replica lag, high read IO, vacuum index scanning bug?
Message-ID: <20161124195129.uuunjnwb4qz4k4bc@alvherre.pgsql>
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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


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