Re: qcow2 preallocation and backing files

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

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.

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.

Then, to improve this mode handling in qcow2, to not allocate all L2 tables, we
may add "zero" bit to L1 table entry.

-- 
Best regards,
Vladimir