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Re: Keepalive for max_standby_delay

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Simon Riggs <simon(at)2ndquadrant(dot)com>, Josh Berkus <josh(at)agliodbs(dot)com>, Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Keepalive for max_standby_delay
Date: 2010-07-02 20:52:08
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Lists: pgsql-hackers
Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Fri, Jul 2, 2010 at 4:36 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>> In the archive case, you're presumably trying to catch up, and so it
>> makes sense to kill queries faster so you can catch up.

> On the flip side, the timeout for the WAL segment is for 16MB of WAL,
> whereas the timeout for SR is normally going to be for a much smaller
> chunk (right?).  So even with the same value for both, it seems like
> queries will be killed more aggressively during archive recovery.

True, but I suspect useful settings will be significantly larger than
those times anyway, so it kind of comes out in the wash.  For
walreceiver the expected time to process each new chunk is less than
wal_sender_delay (if it's not, you better get a faster slave machine).
For the archive case, it probably takes more than 200ms to process a
16MB segment, but still only a few seconds.  So if you have both the
max-delay parameters set to 30 seconds, the minimum "normal" grace
periods are 29.8 sec vs maybe 25 sec, not all that different.

			regards, tom lane

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