[HACKERS] Is XLogFileInit() hoping to force space allocation?

[Tom Lane]

I'm wondering about the intent of this snippet in xlog.c:

    fd = BasicOpenFile(tpath, O_RDWR | O_CREAT | O_EXCL | PG_BINARY, S_IRUSR | 
S_IWUSR);
    if (fd < 0)
        elog(STOP, "InitCreate(logfile %u seg %u) failed: %m",
             logId, logSeg);

    if (lseek(fd, XLogSegSize - 1, SEEK_SET) != (off_t) (XLogSegSize - 1))
        elog(STOP, "lseek(logfile %u seg %u) failed: %m",
             logId, logSeg);

    if (write(fd, "", 1) != 1)
        elog(STOP, "write(logfile %u seg %u) failed: %m",
             logId, logSeg);

    if (fsync(fd) != 0)
        elog(STOP, "fsync(logfile %u seg %u) failed: %m",
             logId, logSeg);

    if (lseek(fd, 0, SEEK_SET) < 0)
        elog(STOP, "lseek(logfile %u seg %u off %u) failed: %m",
             log, seg, 0);

    close(fd);

If the idea here is to force XLogSegSize bytes of disk space to be
allocated, it's a loser.  Most Unix file systems that I know about
will treat the file as containing a "hole", and only allocate the
single block in which data has actually been written.  The fact
that 'ls' shows the file as 16MB is a user-interface artifact of ls;
du will tell you the grim truth:

$ initdb
...
$ ls -l data/pg_xlog
total 328
-rw-------   1 postgres   users      16777216 Nov 27 00:44 0000000000000000
$ du data/pg_xlog/0000000000000000
328     data/pg_xlog/0000000000000000

I don't know whether you consider it important to force the logfile
to be fully allocated before you start using it; but if you do,
the above code will not get the job done.

                        regards, tom lane