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List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Kevin Wolf , Vladimir Sementsov-Ogievskiy , qemu-devel@nongnu.org Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 7/25/19 11:55 AM, Max Reitz wrote: > Hi, >=20 > 69f47505ee66afaa513305de0c1895a224e52c45 changed block_status so that i= t > would only go down to the protocol layer if the format layer returned > BDRV_BLOCK_RECURSE, thus indicating that it has no sufficient > information whether a given range in the image is zero or not. > Generally, this is because the image is preallocated and thus all range= s > appear as zeroes. >=20 > However, it only implemented this preallocation detection for qcow2. > There are more formats that support preallocation, though: vdi, vhdx, > vmdk, vpc. (Funny how they all start with =E2=80=9Cv=E2=80=9D.) >=20 > For vdi, vmdk, and vpc, the fix is rather simple, because they really > have different subformats depending on whether an image is preallocated > or not. This makes the check very simple. >=20 > vhdx is more like qcow2, where after the image has been created, it > isn=E2=80=99t clear whether it=E2=80=99s been preallocated or everythin= g is allocated > because everything was already written to. 69f47505ee added a heuristi= c > to qcow2 to get around this, but I think that=E2=80=99s too much for vh= dx. I > just left it unfixed, because I don=E2=80=99t care that much, honestly = (and I > don=E2=80=99t think anyone else does). >=20 What's the practical outcome of that, and is the limitation documented somewhere? (I'm fine with not fixing it, I just want it documented somehow.) >=20 > Max Reitz (3): > vdi: Make block_status recurse for fixed images > vmdk: Make block_status recurse for flat extents > vpc: Do not return RAW from block_status >=20 > block/vdi.c | 3 ++- > block/vmdk.c | 3 +++ > block/vpc.c | 2 +- > 3 files changed, 6 insertions(+), 2 deletions(-) >=20