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Re: Reducing size of WAL record headers

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Reducing size of WAL record headers
Date: 2013-01-09 22:06:49
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Lists: pgsql-hackers
Simon Riggs <simon(at)2ndQuadrant(dot)com> writes:
> 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.

I think this is extremely premature, in view of the ongoing discussions
about shoehorning logical replication and other kinds of data into the
WAL stream.  It seems quite likely that we'll end up eating some of
that padding space to support those features.  So whacking a lot of code
around in service of squeezing the existing padding out could very
easily end up being wasted work, in fact counterproductive if it
degrades either code readability or robustness.

Let's wait till we see where the logical rep stuff ends up before we
worry about saving 4 bytes per WAL record.

			regards, tom lane

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