Messages in this thread |  | | | Date | Mon, 12 Nov 2001 14:16:23 -0800 | | From | Andrew Morton <> | | Subject | Re: File System Performance |
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Linus Torvalds wrote: > > In article <3BF04289.8FC8B7B7@zip.com.au>, > Andrew Morton <akpm@zip.com.au> 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 <pinard@iro.umontreal.ca>
* 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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