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Re: Reducing size of WAL record headers

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Simon Riggs <simon(at)2ndQuadrant(dot)com>
Cc: Heikki Linnakangas <hlinnakangas(at)vmware(dot)com>,PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: Reducing size of WAL record headers
Date: 2013-01-09 21:43:27
Message-ID: 20130109214327.GB8545@momjian.us (view raw or flat)
Thread:
Lists: pgsql-hackers
On Wed, Jan  9, 2013 at 09:15:16PM +0000, Simon Riggs wrote:
> On 9 January 2013 21:02, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com> wrote:
> 
> >> OK, crazy idea, but can we just record xl_len as a difference against
> >> xl_tot_len, and shorten the xl_len field?
> >
> >
> > Hmm, so it would essentially be the length of all the backup blocks. perhaps
> > rename it to xl_bkpblk_len.
> >
> > However, that would cap the total size of backup blocks to 64k. Which would
> > not be enough with 32k BLCKSZ.
> 
> Since that requires a recompile anyway, why not make XLogRecord
> smaller only for 16k BLCKSZ or less?
> 
> Problem if we do that is that xl_len is used extensively in _redo
> routines, so its a much more invasive patch.

I would just make it int16 on <=16k block size, and int32 on >16k
blocks.

-- 
  Bruce Momjian  <bruce(at)momjian(dot)us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

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


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