Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)anarazel(dot)de>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Simon Riggs <simon(at)2ndquadrant(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Changing WAL Header to reduce contention during ReserveXLogInsertLocation()
Date: 2018-01-12 22:43:00
Message-ID: 23021.1515796980@sss.pgh.pa.us
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Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-01-12 17:24:54 -0500, Tom Lane wrote:
>> Andres Freund <andres(at)anarazel(dot)de> writes:
>>> Right. I wonder if it be reasonable to move that to a page's header
>>> instead of individual records? To avoid torn page issues we'd have to
>>> reduce the page size to a sector size, but I'm not sure that's that bad?

>> Giving up a dozen or two bytes out of every 512 sounds like quite an
>> overhead.

> It's not nothing, that's true. But if it avoids 8 bytes in every record,
> that'd probably at least as much in most usecases.

Fair point. I don't have a very good handle on what "typical" WAL record
sizes are, but we might be fine with that --- some quick counting on the
fingers says we'd break even with an average record size of ~160 bytes,
and be ahead below that.

We'd need to investigate the page-crossing overhead carefully though.

regards, tom lane

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