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    SubjectRe: File System Performance
    Linus Torvalds wrote:
    > On Mon, 12 Nov 2001, Lionel Bouton wrote:
    > >
    > > Seems not the case with gnu tar : write isn't even called once on the fd
    > > returned by open("/dev/null",...). In fact a "grep write" on the strace
    > > output is empty in the "tar cf /dev/null" case. Every file in the tar-ed
    > > tree is stat-ed but no-one is read-ed.
    > Wow. What a sleazy optimization - it can't be anything but a special case.
    > How do they do it anyway? By matching on the name Or by knowing what the
    > minor/major numbers of /dev/null are supposed to be on that particular
    > operating system?

    /* Detect if outputting to "/dev/null". */
    static char const dev_null[] = "/dev/null";
    struct stat dev_null_stat;

    dev_null_output =
    (strcmp (archive_name_array[0], dev_null) == 0
    || (! _isrmt (archive)
    && stat (dev_null, &dev_null_stat) == 0
    && S_ISCHR (archive_stat.st_mode)
    && archive_stat.st_rdev == dev_null_stat.st_rdev));

    > And what's the _point_ of the optimization? I've never heard of a "tar
    > benchmark"..

    Ask Al. He understands those GNU folks.

    It's actually a bug. I may _want_ an ISREG /dev/null...

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