Re: [PATCH v9 30/34] qcow2: Add prealloc field to QCowL2Meta

[Max Reitz]

On 02.07.20 16:58, Eric Blake wrote:
> On 7/2/20 9:50 AM, Max Reitz wrote:
>> On 28.06.20 13:02, Alberto Garcia wrote:
>>> This field allows us to indicate that the L2 metadata update does not
>>> come from a write request with actual data but from a preallocation
>>> request.
>>>
>>> For traditional images this does not make any difference, but for
>>> images with extended L2 entries this means that the clusters are
>>> allocated normally in the L2 table but individual subclusters are
>>> marked as unallocated.
>>>
>>> This will allow preallocating images that have a backing file.
>>>
>>> There is one special case: when we resize an existing image we can
>>> also request that the new clusters are preallocated. If the image
>>> already had a backing file then we have to hide any possible stale
>>> data and zero out the new clusters (see commit 955c7d6687 for more
>>> details).
>>>
>>> In this case the subclusters cannot be left as unallocated so the L2
>>> bitmap must be updated.
>>>
>>> Signed-off-by: Alberto Garcia <berto@igalia.com>
>>> Reviewed-by: Eric Blake <eblake@redhat.com>
>>> ---
>>>   block/qcow2.h         | 8 ++++++++
>>>   block/qcow2-cluster.c | 2 +-
>>>   block/qcow2.c         | 6 ++++++
>>>   3 files changed, 15 insertions(+), 1 deletion(-)
>>
>> Sounds good, but I’m just not quite sure about the details on
>> falloc/full allocation: With .prealloc = true, writing to the
>> preallocated subclusters will require a COW operation.  That’s not
>> ideal, and avoiding those COWs may be a reason to do preallocation in
>> the first place.
> 
> I'm not sure I follow the complaint.  If a cluster is preallocated but
> the subcluster is marked unallocated, then doing a partial write to that
> subcluster must provide the correct contents for the rest of the
> subcluster (either filling with zero, or reading from a backing file) -
> but this COW can be limited to just the portion of the subcluster, and
> is no different than the COW you have to perform without subclusters
> when doing a write to a preallocated cluster in general.

It was my impression that falloc/full preallocation would create normal
data clusters, not zero clusters, so no COW was necessary when writing
to them.

>> Now, with backing files, it’s entirely correct.  You need a COW
>> operation, because that’s the point of having a backing file.
>>
>> But without a backing file I wonder if it wouldn’t be better to set
>> .prealloc = false to avoid that COW.
> 
> Without a backing file, there is no read required - writing to an
> unallocated subcluster within a preallocated cluster merely has to
> provide zeros to the rest of the write.  And depending on whether we can
> intelligently guarantee that the underlying protocol already reads as
> zeroes when preallocated, we even have an optimization where even that
> is not necessary.  We can still lump it in the "COW" terminology, in
> that our write is more complex than merely writing in place, but it
> isn't a true copy-on-write operation as there is nothing to be copied.

The term “COW” specifically in the qcow2 driver also refers to having to
write zeroes to an area that isn’t written to by the guest as part of
the process of having to allocate a (sub)cluster.

(Of course there is no COW from a backing file if there is no backing file.)

Max

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