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    SubjectRe: File System Performance
    Linus Torvalds wrote:
    > In article <>,
    > Andrew Morton <> wrote:
    > >
    > >It's tar. It cheats. It somehow detects that the
    > >output is /dev/null, and so it doesn't read the input files.
    > Probably the kernel.
    > If you do a mmap()+write(), the write() to /dev/null won't even read the
    > mmap contents, which in turn will cause the pages to never be brought
    > in.
    > Anything which uses mmap+write will show this.

    Actually, tar _is_ doing funnies with /dev/null. Changelog says:

    1995-12-21 François Pinard <>

    * buffer.c: Rename a few err variables to status.
    * extract.c: Rename a few check variables to status.

    Corrections to speed-up the sizeing pass in Amanda:
    * tar.h: Declare dev_null_output.
    * buffer.c (open_archive): Detect when archive is /dev/null.
    (flush_write): Avoid writing to /dev/null.
    * create.c (dump_file): Do not open file if archive is being
    written to /dev/null, nor read file nor restore times.
    Reported by Greg Maples and Tor Lillqvist.

    One wonders why.

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