On 20.11.19 16:58, Vladimir Sementsov-Ogievskiy wrote: > 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.
Well, the good news is that block_resize currently has no such parameter and thus we could make a non-OFF value the default. The bad news is that the reason it has no such parameter is that it would need to be a job in order to support any form of preallocation. So actually I don’t think we can make block_resize write zeroes to the new region by default, because then it needs to be a job, and it just isn’t. (Of course, we can get to it through a deprecation cycle, but we can’t “fix” block_resize directly.) Max
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