Re: [HACKERS] xlogfilename

On Thu, 20 Jul 2017 11:02:25 +0200
Michael Paquier <michael.paquier@gmail.com> wrote:

> On Thu, Jul 20, 2017 at 10:58 AM, DEV_OPS <devops@ww-it.cn> wrote:
> > I think you may reference to function: pg_xlogfile_name   in
> > src/backend/access/transam/xlogfuncs.c,  it use XLogFileName  defined in
> > src/include/access/xlog_internal.h
> >
> > #define XLogFileName(fname, tli, logSegNo)  \
> >     snprintf(fname, MAXFNAMELEN, "%08X%08X%08X", tli,       \
> >              (uint32) ((logSegNo) / XLogSegmentsPerXLogId), \
> >              (uint32) ((logSegNo) % XLogSegmentsPerXLogId))
> >
> >
> > hope it's helpful for you
>
> The first 8 characters are the timeline number in hexadecimal format.
> The next 8 characters indicate a segment number, which gets
> incremented every 256 segments in hexa format. The last 8 characters
> indicate the current segment number in hexa format.

As far as I understand, XLOG is a logical big file of 256 * 16 MB,
and this is split to multiple physical files of 16MB which are called
WAL segments. The second 8 characters indicate the id of the logical
xlog file, and the last 8 characters indicate the sequencial number of
the segment in this xlog.