Date: Mon, 13 Aug 2012 09:54:52 -0700 From: Steven Schlansker <stevenschlansker@gmail.com> To: Olivier Smedts <olivier@gid0.org> Cc: freebsd-fs@freebsd.org, Petri Helenius <petri@helenius.fi> Subject: Re: zero deleted blocks Message-ID: <F1CF0B4C-275D-44DF-9B73-323A55B38F30@gmail.com> In-Reply-To: <CABzXLYN%2BYPeuf_-kC1UHPQRVWy-98OQs7M39EGfiyCaOj7fgQw@mail.gmail.com> References: <7BDA02A3-99ED-4F67-9B2D-5AAF1523642E@helenius.fi> <CABzXLYN%2BYPeuf_-kC1UHPQRVWy-98OQs7M39EGfiyCaOj7fgQw@mail.gmail.com>
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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>: >> >> Hi, >> >> Is it possible to have ZFS or UFS to zero out the deallocated blocks to facilitate VM disk compaction? >> >> Pete > > Did you try a classic "dd if=/dev/zero of=a-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.help
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