Re: qcow2: Zero-initialization of external data files

[Eric Blake <eblake@redhat.com>]

On 4/9/20 10:01 AM, Max Reitz wrote:
> On 09.04.20 16:32, Eric Blake wrote:
>> On 4/9/20 9:10 AM, Max Reitz wrote:
>>
>>>>
>>>> What happens when an operation attempts to unmap things?  Do we reject
>>>> all unmap operations when data-file-raw is set (thus leaving a cluster
>>>> marked as allocated at all times, if we can first guarantee that
>>>> preallocation set things up that way)?
>>> No, unmap operations currently work.  qcow2_free_any_clusters() passes
>>> them through to the external data file.
>>>
>>> The problem is that the unmap also zeroes the L2 entry, so if you then
>>> write data to the raw file, it won’t be visible from the qcow2 side of
>>> things.  However, I’m not sure whether we support modifications of a raw
>>> file when it is already “in use” by a qcow2 image, so maybe that’s fine.
>>
>> We don't support concurrent modification. But if the guest is running
>> and unmaps things, then shuts off, then we edit the raw file offline,
>> then we restart the guest, the guest should see the results of those
>> offline edits.
> 
> Should it?  The specification doesn’t say anything about that.
> 
> In fact, I think we have always said we explicitly discourage that
> because this might lead to outdated metadata; even though we usually
> meant “dirty bitmaps” by that.

Hmm.  Kevin, I'd really like your opinion here.  The point of the 
raw-external-data flag is to state that "qemu MUST ensure that whatever 
is done to this image while the guest is running is reflected through to 
the raw file, so that after the guest stops, the raw file alone is still 
viable to see what the guest saw".  But as you say, there's a difference 
between "the raw file will read what the guest saw" and "we can now edit 
the raw file without regards to the qcow2 wrapper but later reuse of the 
qcow2 wrapper won't be corrupted by those edits".

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org