[PATCH] dd: add a flag to discard cached data

[Pádraig Brady]

I noticed fadvise(DONTNEED) getting some love in the kernel recently
   http://lkml.org/lkml/2011/2/17/169
Which prompted me to implement this. With the attached one can now:

  # Advise to drop cache for whole file
  dd if=ifile iflag=nocache count=0

  # Ensure drop cache for whole file
  dd of=ofile oflag=nocache conv=notrunc,fdatasync count=0

  # Drop cache for part of file
  dd if=ifile iflag=nocache skip=10 count=10 of=/dev/null

  # Stream data just using readahead cache
  dd if=ifile of=ofile iflag=nocache oflag=nocache

When count=0, i.e. when only manipulating the cache,
we propagate the posix_fadvise() return to the exit status.

Note this will invalidate the cache even if another
process has the file open. That could be avoided with mincor:
   http://insights.oetiker.ch/linux/fadvise.html
However, I don't think that's needed for a tool like dd.

cheers,
Pádraig.

dd-nocache.diff
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