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Re: WAL Log numbering

From: Bruce Momjian <pgman(at)candle(dot)pha(dot)pa(dot)us>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Justin Clift <justin(at)postgresql(dot)org>, pgsql-bugs(at)postgresql(dot)org
Subject: Re: WAL Log numbering
Date: 2001-09-22 03:58:57
Message-ID: 200109220358.f8M3wv624974@candle.pha.pa.us (view raw or flat)
Thread:
Lists: pgsql-bugs
Attached is a patch that changes "sequential" to "ever-increasing".

> 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
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster
> 

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  Bruce Momjian                        |  http://candle.pha.pa.us
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