Re: WAL Log numbering

Hi Bruce,

Bruce Momjian wrote:
>
> Attached is a patch that changes "sequential" to "ever-increasing".

That's a good idea. :) I was trying to think of the right wording, but
I could only think of sentences that were too complex. That one's nice
and simple.

Regards and best wishes,

Justin Clift

>
> > 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
> >
>
> --
> Bruce Momjian | http://candle.pha.pa.us
> pgman(at)candle(dot)pha(dot)pa(dot)us | (610) 853-3000
> + If your life is a hard drive, | 830 Blythe Avenue
> + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
>
> ------------------------------------------------------------------------
> Index: doc/src/sgml/wal.sgml
> ===================================================================
> RCS file: /cvsroot/pgsql/doc/src/sgml/wal.sgml,v
> retrieving revision 1.9
> diff -c -r1.9 wal.sgml
> *** doc/src/sgml/wal.sgml 2001/09/09 23:52:12 1.9
> --- doc/src/sgml/wal.sgml 2001/09/22 03:58:39
> ***************
> *** 137,143 ****
> divided into 8 kB pages. The log record headers are described in
> <filename>access/xlog.h</filename>; record content is dependent on
> the type of event that is being logged. Segment files are given
> ! sequential numbers as names, starting at
> <filename>0000000000000000</filename>. The numbers do not wrap, at
> present, but it should take a very long time to exhaust the
> available stock of numbers.
> --- 137,143 ----
> divided into 8 kB pages. The log record headers are described in
> <filename>access/xlog.h</filename>; record content is dependent on
> the type of event that is being logged. Segment files are given
> ! ever-increasing numbers as names, starting at
> <filename>0000000000000000</filename>. The numbers do not wrap, at
> present, but it should take a very long time to exhaust the
> available stock of numbers.

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