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Re: Hot Standby query cancellation and Streaming Replication integration

From: Greg Stark <gsstark(at)mit(dot)edu>
To: Josh Berkus <josh(at)agliodbs(dot)com>
Cc: Greg Smith <greg(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Hot Standby query cancellation and Streaming Replication integration
Date: 2010-03-01 18:46:29
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Lists: pgsql-hackers
On Mon, Mar 1, 2010 at 5:50 PM, Josh Berkus <josh(at)agliodbs(dot)com> wrote:
> I don't think that defer_cleanup_age is a long-term solution.  But we
> need *a* solution which does not involve delaying 9.0.

So I think the primary solution currently is to raise max_standby_age.

However there is a concern with max_standby_age. If you set it to,
say, 300s. Then run a 300s query on the slave which causes the slave
to fall 299s behind. Now you start a new query on the slave -- it gets
a snapshot based on the point in time that the slave is currently at.
If it hits a conflict it will only have 1s to finish before the
conflict causes the query to be cancelled.

In short in the current setup I think there is no safe value of
max_standby_age which will prevent query cancellations short of -1. If
the slave has a constant stream of queries and always has at least one
concurrent query running then it's possible that the slave will run
continuously max_standby_age-epsilon behind the master and cancel
queries left and right, regardless of how large max_standby_age is.

To resolve this I think you would have to introduce some chance for
the slave to catch up. Something like refusing to use a snapshot older
than max_standby_age/2  and instead wait until the existing queries
finish and the slave gets a chance to catch up and see a more recent
snapshot. The problem is that this would result in very unpredictable
and variable response times from the slave. A single long-lived query
could cause replay to pause for a big chunk of max_standby_age and
prevent any new query from starting.

Does anyone see any way to guarantee that the slave gets a chance to
replay and new snapshots will become visible without freezing out new
queries for extended periods of time?


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