On 2018-06-06 13:48, Daniel P. Berrangé wrote:
> On Wed, Jun 06, 2018 at 12:42:28PM +0100, Richard W.M. Jones wrote:
>> On Wed, Jun 06, 2018 at 12:14:07PM +0100, Dr. David Alan Gilbert wrote:
>>> The problem with having a separate file is that you either have to copy
>>> it around with the image or have an archive.  If you have an archive
>>> you have to have an unpacking step which then copies, potentially a lot
>>> of data taking some reasonable amount of time.  Storing a simple bit
>>> of data with the image avoids that.
>>
>> This isn't really true.  For OVA (ie. tar) we don't unpack them.
>> Adding file.offset and file.size in qemu's raw driver was crucial to
>> that optimization.
> 
> Though that assumes you're only using the qcow2 file in read-only mode.
> As soon as you need write access you need to unpack from the OVA so that
> the qcow2 file can grow its length when new sectors are allocated.

Except if the qcow2 file is at the end of the archive.  Then all you
need to do is adjust the length field of the tar file header.

Max