Re: question about zeroes in the wal file names

At Sun, 18 Aug 2019 16:17:03 +0200, Luca Ferrari <fluca1978@gmail.com> wrote in
<CAKoxK+67oS+PSygdmRHbn-GSjE_TY+t6Gzk6CRzcw-HVGswzOA@mail.gmail.com>
> I'm just curious to better understand the naming convention behind wal
> files, because I've seen on a system of mine that the wals created
> were:
> 
> 000000050000020E000000FF
>  000000050000020F00000000
> 
> while I was expecting 20E0x100. So I digged into the code and I've
> seen, from the XLogFileName macro, that the last part is built as the
> reminder of the number of segments per wal file:
> 
> #define XLogFileName(fname, tli, logSegNo, wal_segsz_bytes)    \
>     snprintf(fname, MAXFNAMELEN, "%08X%08X%08X", tli,        \
>              (uint32) ((logSegNo) / XLogSegmentsPerXLogId(wal_segsz_bytes)), \
>              (uint32) ((logSegNo) % XLogSegmentsPerXLogId(wal_segsz_bytes)))
> 
> 
> and with the default wal size of 16 MB that gives a remainder of 256
> (FF + 1). Assuming I haven't missed anything, this means that there
> are 6 zeroes that will never change in the last 8 chars of the wal
> filename.  Is therefore this only done to handle PostgreSQL WAL sizes
> of 4 GB each?

I think that's right. I don't see it used in other than WAL file
names.  As the macro name suggests, the 4GB block is internally
called as "xlog id" (XLogId). The block size is determined so
that the location within it can be handled as a 32-bit
integer. Older versions of postgresql defined XLogRecPtr as a
pair of two 32-bit integers.

xlogdefs.h.~REL9_1_0~:17
/*
 * Pointer to a location in the XLOG.  These pointers are 64 bits wide,
 * because we don't want them ever to overflow.
 *
 * NOTE: xrecoff == 0 is used to indicate an invalid pointer.  This is OK
 * because we use page headers in the XLOG, so no XLOG record can start
 * right at the beginning of a file.
 *
 * NOTE: the "log file number" is somewhat misnamed, since the actual files
 * making up the XLOG are much smaller than 4Gb.  Each actual file is an
 * XLogSegSize-byte "segment" of a logical log file having the indicated
 * xlogid.    The log file number and segment number together identify a
 * physical XLOG file.    Segment number and offset within the physical file
 * are computed from xrecoff div and mod XLogSegSize.
 */
typedef struct XLogRecPtr
{
    uint32        xlogid;            /* log file #, 0 based */
    uint32        xrecoff;        /* byte offset of location in log file */
} XLogRecPtr;


regards.

-- 
Kyotaro Horiguchi
NTT Open Source Software Center