From: | Andres Freund <andres(at)anarazel(dot)de> |
---|---|
To: | Tomas Vondra <tomas(dot)vondra(at)2ndquadrant(dot)com> |
Cc: | Stephen Frost <sfrost(at)snowman(dot)net>, Robert Haas <robertmhaas(at)gmail(dot)com>, Michael Paquier <michael(at)paquier(dot)xyz>, Bruce Momjian <bruce(at)momjian(dot)us>, PostgreSQL Hackers <pgsql-hackers(at)lists(dot)postgresql(dot)org> |
Subject: | Re: finding changed blocks using WAL scanning |
Date: | 2019-04-23 16:34:54 |
Message-ID: | 20190423163454.7mbljqagrsbqhmlc@alap3.anarazel.de |
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Lists: | pgsql-hackers |
Hi,
On 2019-04-23 18:07:40 +0200, Tomas Vondra wrote:
> Well, the thing is that for prefetching to be possible you actually have
> to be a bit behind. Otherwise you can't really look forward which blocks
> will be needed, right?
>
> IMHO the main use case for prefetching is when there's a spike of activity
> on the primary, making the standby to fall behind, and then hours takes
> hours to catch up. I don't think the cases with just a couple of MBs of
> lag are the issue prefetching is meant to improve (if it does, great).
I'd be surprised if a good implementation didn't. Even just some smarter
IO scheduling in the startup process could help a good bit. E.g. no need
to sequentially read the first and then the second block for an update
record, if you can issue both at the same time - just about every
storage system these days can do a number of IO requests in parallel,
and it nearly halves latency effects. And reading a few records (as in a
few hundred bytes commonly) ahead, allows to do much more than that.
Greetings,
Andres Freund
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