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Re: WAL Log numbering

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Justin Clift <justin(at)postgresql(dot)org>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: WAL Log numbering
Date: 2001-09-18 06:40:59
Message-ID: 22812.1000795259@sss.pgh.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Justin Clift <justin(at)postgresql(dot)org> writes:
> I would have though that after 00000000000000FE would be
> 0000000000000100, not 0000000100000000.

This is the intended behavior, I believe.  The low-order half is a
32-bit byte offset DIV XLogSegSize --- we could compress out the zero
bits, but only at the cost of wiring an assumption about XLogSegSize
into the filename format.  The reason that 0/FF is missing from the
sequence is stated in xlog.h:

/*
 * We break each logical log file (xlogid value) into 16Mb segments.
 * 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) (16*1024*1024))
#define XLogSegsPerFile (((uint32) 0xffffffff) / XLogSegSize)
#define XLogFileSize	(XLogSegsPerFile * XLogSegSize)

> Just checked through the Interactive docs (not sure which version of 7.1
> they are) and says the numbers should be sequential.

This would seem to be an oversimplification in the docs.

			regards, tom lane

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Subject: a small doubt
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Subject: WAL Log numbering

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