Re: qcow2 preallocation and backing files

[Vladimir Sementsov-Ogievskiy]

20.11.2019 19:46, Kevin Wolf wrote:
> Am 20.11.2019 um 16:58 hat Vladimir Sementsov-Ogievskiy geschrieben:
>> 20.11.2019 18:18, Alberto Garcia wrote:
>>> On Wed 20 Nov 2019 01:27:53 PM CET, Vladimir Semeeausntsov-Ogievskiy wrote:
>>>
>>>> 3. Also, the latter way is inconsistent with discard. Discarded
>>>> regions returns zeroes, not clusters from backing. I think discard and
>>>> truncate should behave in the same safe zero way.
>>>
>>> But then PREALLOC_MODE_OFF implies that the L2 metadata should be
>>> preallocated (all clusters should be QCOW2_CLUSTER_ZERO_PLAIN), at least
>>> when there is a backing file.
>>>
>>> Or maybe we just forbid PREALLOC_MODE_OFF during resize if there is a
>>> backing file ?
>>>
>>
>> Kevin proposed a fix that alters PREALLOC_MODE_OFF behavior if there is
>> a backing file, to allocate L2 metadata with ZERO clusters..
>>
>> I don't think that it's the best thing to do, but it's already done,
>> it works and seems appropriate for rc3..
>>
>> I see now, that change PREALLOC_MODE_OFF behavior may break things,
>> first of all qemu-img create, which creating UNALLOCATED qcow2 by
>> default for years.
> 
> And it still does, because the backing file is added only after giving
> the qcow2 image the right size.
> 
> But you're right, this is more accidental than by design. I wonder if
> there are other problematic cases (and whether merging something like
> this in -rc3 isn't rather risky).
> 
>> Still, I think that it would be safer to always ZERO expanded part of
>> qcow2, regardless of backing file..
>>
>> We may add PREALLOC_MODE_ZERO, and use it in mirror, commit, and some
>> other calls to bdrv_truncate, except for qcow2 image creation of
>> course.
> 
> What do we do with image formats that don't support zero clusters and
> therefore can't provide PREALLOC_MODE_ZERO? Will commit just fail for
> them?


Hmm. consider committing to raw

                        x     y
qcow2 [----------------------]  - full of unallocated clusters
raw   [2987957285235298]        - full of data, but file is short


Before commit, data from [x,y] reads as zero. Therefore, we should zero
expanded part of base..

And this is for base of any format: [x,y] must be zero after commit. So,
if format can't do fast-zero, it should fallback to writing real zeros.

===

Hmm, actually after your patch all formats partly support PREALLOC_MDOE_ZERO,
which in the worst case is done by writing real zeros.

> 
>> Then, to improve this mode handling in qcow2, to not allocate all L2
>> tables, we may add "zero" bit to L1 table entry.
> 
> This would be an incompatible image format change that needs to be
> explicitly enabled by the user. This might limit its usefulness a bit.
> 

Yes, I understand this. Still it may make sense.


-- 
Best regards,
Vladimir