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From: Steven Schlansker <stevenschlansker@gmail.com>
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To: Olivier Smedts <olivier@gid0.org>
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Cc: freebsd-fs@freebsd.org, Petri Helenius <petri@helenius.fi>
Subject: Re: zero deleted blocks
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On Aug 13, 2012, at 2:27 AM, Olivier Smedts <olivier@gid0.org> wrote:

> 2012/8/13 Petri Helenius <petri@helenius.fi>:
>>=20
>> Hi,
>>=20
>> Is it possible to have ZFS or UFS to zero out the deallocated blocks =
to facilitate VM disk compaction?
>>=20
>> Pete
>=20
> Did you try a classic "dd if=3D/dev/zero of=3Da-file-on-the-pool" ? =
This
> won't give good results if you do this on a filesystem with
> compression enabled.

ZFS uses copy on write =
(http://en.wikipedia.org/wiki/ZFS#Copy-on-write_transactional_model) so =
overwriting with zeroes will actually allocate new space to hold all the =
zeroed data and leave the old data mostly untouched.