Re: Confusing comment in xlog.c or am I missing something?

On 02.05.2013 08:16, Amit Langote wrote:
> I was just going through the xlog.c and I came across following which
> confused me:
>
> Given,
>
> src/include/access/xlogdefs.h
>
> #define XLogSegsPerFile (((uint32) 0xffffffff) / XLogSegSize)
> #define XLogFileSize    (XLogSegsPerFile * XLogSegSize)
>
> Also,
> typedef struct XLogRecPtr
>   {
>           uint32          xlogid;                 /* log file #, 0 based */
>           uint32          xrecoff;                /* byte offset of location
> in log file */
>   } XLogRecPtr;
>
>
> Check for crossing of xlog "segment" boundary?
>
> src/include/access/trasam/xlog.c
>
> /* Check for crossing of xlog segment boundary */
> if (RecPtr->xrecoff>= XLogFileSize)
> {
>          (RecPtr->xlogid)++;
>          RecPtr->xrecoff = 0;
> }
>
> Is that xlog "file" boundary or am I missing something?

The WAL space is divided into 4GB "log files", which are not physical 
files but purely logical constructs. The XLogRecPtr.xlogid field 
indicates the logical log file, and xrecoff is the offset within the 
file. Each logical log file is divided into "log segments", which are 
the physical files you see in pg_xlog. See also the comment above the 
definition of XLogRecPtr.

This split was necessary to deal with more than 4 GB of WAL, using only 
32-bit integers. In 9.3, that's changed, and XLogRecPtr is a 64-bit 
integer. The whole concept of "logical log files" is gone, so it's a lot 
less confusing now.

- Heikki