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Re: Reducing size of WAL record headers

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>
Cc: Simon Riggs <simon(at)2ndQuadrant(dot)com>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Reducing size of WAL record headers
Date: 2013-01-09 20:59:00
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Lists: pgsql-hackers
On Wed, Jan  9, 2013 at 10:54:33PM +0200, Heikki Linnakangas wrote:
> On 09.01.2013 22:36, Simon Riggs wrote:
> >Overall, the WAL record is MAXALIGN'd, so with 8 byte alignment we
> >waste 4 bytes per record. Or put another way, if we could reduce
> >record header by 4 bytes, we would actually reduce it by 8 bytes per
> >record. So looking for ways to do that seems like a good idea.
> Agreed.
> >The WAL record header starts with xl_tot_len, a 4 byte field. There is
> >also another field, xl_len. The difference is that xl_tot_len includes
> >the header, xl_len and any backup blocks. Since the header is fixed,
> >the only time xl_tot_len != SizeOfXLogRecord + xl_len is when we have
> >backup blocks.
> >
> >We can re-arrange the record layout so that we remove xl_tot_len and
> >add another (maxaligned) 4 byte field (-->  8 bytes) after the record
> >header, xl_bkpblock_len that only exists if we have backup blocks.
> >This will then save 8 bytes from every record that doesn't have backup
> >blocks, and be the same as now with backup blocks.
> Here's a better idea:
> Let's keep xl_tot_len as it is, but move xl_len at the very end of
> the WAL record, after all the backup blocks. If there are no backup
> blocks, xl_len is omitted. Seems more robust to keep xl_tot_len, so
> that you require less math to figure out where one record ends and
> where the next one begins.

OK, crazy idea, but can we just record xl_len as a difference against
xl_tot_len, and shorten the xl_len field?

  Bruce Momjian  <bruce(at)momjian(dot)us>

  + It's impossible for everything to be true. +

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