[libvirt] [PATCH 3/3] Add volume zeroing

[Dave Allan]

On 02/16/2010 07:36 AM, Daniel P. Berrange wrote:
> On Tue, Feb 16, 2010 at 01:31:58PM +0100, Paolo Bonzini wrote:
>> On 02/16/2010 12:33 PM, Daniel P. Berrange wrote:
>>> The idea of zeroing upon delete, is that we want the currently allocated
>>> extents to be overwritten with zeros. If we truncate to 0 size, then
>>> extend to original size I imagine that would easily allow the filesystem
>>> to give you a new set of extents filled with zeros, leaving the old
>>> extents with data intact as unused space on the FS.
>>
>> Yeah, as long as you use regular files as images you're safe, but you'd
>> expose the old data if you destroyed the partition on which the file
>> resided and used the partition as storage for a new guest.
>>
>> But then in this scenario (delete file system partition and give it out
>> as raw) you could expose information not only about other/old guests,
>> but also about the host.  So for partitions it can be even more
>> important to provide an option to zero the partition _before_ giving it
>> out.  Currently you cannot do that with libvirt.
>
> There is an unused 'flags' parameter in virStorageVolCreate(), where
> we could/should add a VIR_STORAGE_VOL_CREATE_ZEROED  parameter too
> for that scenario
>
> ANother option would be to add an explicit virStorageVolZero() API to
> allow a volume to be wiped independently of create/delete operations.

I like this approach.  Since the code is already written for pretty much 
every permutation: regular non-sparse file, regular sparse file, block 
device, adding the new API isn't a lot of work.  That gives users the 
greatest flexibility for choosing the time of the zero operation, which 
is important since it's time consuming.

Dave