zero deleted blocks

[Steven Hartland]

----- Original Message ----- 
From: "Olivier Smedts" <olivier at gid0.org>

> 2012/8/13 Steven Schlansker <stevenschlansker at gmail.com>:
>>
>> On Aug 13, 2012, at 2:27 AM, Olivier Smedts <olivier at gid0.org> wrote:
>>> 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.
>
> Only until all the free space is consumed !

Also need to ensure compression isnt turned on and its not an device
that does compression on disk / disk controller such as Sandforce
based SSD's as that could resulting in something other than the
intended behavour. 


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