Re: [Qemu-devel] [PATCH] qcow2: Add full image preallocation option

[Kevin Wolf <kwolf@redhat.com>]

Am 27.01.2011 16:58, schrieb Daniel P. Berrange:
> On Thu, Jan 27, 2011 at 04:52:14PM +0100, Kevin Wolf wrote:
>> This adds a preallocation=full mode to qcow2 image creation, which does not
>> only allocate metadata for the whole image, but also writes zeros to it,
>> creating a non-sparse image file.
>>
>> Signed-off-by: Kevin Wolf <kwolf@redhat.com>

> Is there a way you can calculate the total size of the qcow2
> file upfront, and just use a single posix_fallocate() call to
> do the zero-filled allocation of all the data blocks. It is
> many orders of magnitude faster than truely writing blocks of
> zero'd data on modern filesystems.  I guess if you're using
> compression or encryption, we'd really have to go the slow
> path, but for regular usage it'd be better to take a fast
> path.

Encryption doesn't really change anything with respect to cluster
allocations, but combining compression with preallocation doesn't make
any sense. We should probably forbid that.

To get the size of the image, it should be enough to get the offset of
the last cluster as the allocation is done sequentially. However, we
don't have a bdrv_fallocate (yet). I'm not sure how to emulate this for
drivers that don't support it directly, but maybe we could just ignore
it for them.

So yes, optimizing it should be possible from the qcow2 side of things,
but it requires at least some additional code in other places.

Kevin