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Re: xlog location arithmetic

From: Bruce Momjian <bruce(at)momjian(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Robert Haas <robertmhaas(at)gmail(dot)com>, Peter Eisentraut <peter_e(at)gmx(dot)net>,Magnus Hagander <magnus(at)hagander(dot)net>,Fujii Masao <masao(dot)fujii(at)gmail(dot)com>,Euler Taveira de Oliveira <euler(at)timbira(dot)com>,Pgsql Hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: xlog location arithmetic
Date: 2012-03-13 00:47:24
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
On Fri, Mar 09, 2012 at 03:04:23PM -0500, Tom Lane wrote:
> The main actual simplification would be in getting rid of the "hole"
> at the end of each 4GB worth of WAL, cf this bit in xlog_internal.h:
> /*
>  * We break each logical log file (xlogid value) into segment files of the
>  * size indicated by XLOG_SEG_SIZE.  One possible segment at the end of each
>  * log file is wasted, to ensure that we don't have problems representing
>  * last-byte-position-plus-1.
>  */
> #define XLogSegSize		((uint32) XLOG_SEG_SIZE)
> #define XLogSegsPerFile (((uint32) 0xffffffff) / XLogSegSize)
> #define XLogFileSize	(XLogSegsPerFile * XLogSegSize)
> If we can't get rid of that and have a continuous 64-bit WAL address
> space then it's unlikely we can actually simplify any logic.
> Now, doing that doesn't break the naming convention exactly; what it
> changes is that there will be WAL files numbered xxxFFFF (for some
> number of trailing-1-bits I'm too lazy to work out at the moment) where
> before there were not.  So the question really is how much external code
> there is that is aware of that specific noncontiguous numbering behavior
> and would be broken if things stopped being that way.

Our current WAL naming is hopelessly arcane, and we would certainly be
benfitting users to simplify it.  Is this a TODO?

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

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

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