On Sun, 25 Nov 2012 17:39:24 +0100, Pawel Jakub Dawidek <p...@freebsd.org> wrote:

On Sun, Nov 25, 2012 at 04:32:08PM +0000, Martin Matuska wrote:
Author: mm
Date: Sun Nov 25 16:32:07 2012
New Revision: 243524
URL: http://svnweb.freebsd.org/changeset/base/243524

Log:
  MFV r243013 and r243267:

  Import the zio nop-write improvement from Illumos. To reduce I/O,
  nop-write omits overwriting data if the checksum (cryptographically
  secure) of new data matches the checksum of existing data.
  It also saves space if snapshots are in use.

  It currently works only on datasets with enabled compression, disabled
  deduplication and sha256 checksums.

  IllumOS 13887:196932ec9e6a and 13888:7204b3392a58
  3236 zio nop-write

This is a nice change, although I don't understand why it depends on
compression being turned on. It won't save any space.


This question is also discussed here.
http://comments.gmane.org/gmane.os.illumos.zfs/200
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