Re: WAL Log numbering

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