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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 8/12/19 3:11 PM, Max Reitz wrote: > On 12.08.19 20:39, John Snow wrote: >> >> >> On 7/25/19 11:55 AM, Max Reitz wrote: >>> Hi, >>> >>> 69f47505ee66afaa513305de0c1895a224e52c45 changed block_status so that= it >>> 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 ran= ges >>> appear as zeroes. >>> >>> 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.) >>> >>> For vdi, vmdk, and vpc, the fix is rather simple, because they really >>> have different subformats depending on whether an image is preallocat= ed >>> or not. This makes the check very simple. >>> >>> 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 everyth= ing is allocated >>> because everything was already written to. 69f47505ee added a heuris= tic >>> to qcow2 to get around this, but I think that=E2=80=99s too much for = vhdx. I >>> just left it unfixed, because I don=E2=80=99t care that much, honestl= y (and I >>> don=E2=80=99t think anyone else does). >>> >> >> What's the practical outcome of that, and is the limitation documented >> somewhere? >=20 > The outcome is that it if you preallocate a vhdx image > (subformat=3Dfixed), you=E2=80=99ll see that all sectors contain data, = even if > they may be zero sectors on the filesystem level. >=20 > I don=E2=80=99t think it=E2=80=99s user-visible whatsoever. >=20 But it might mean that doing things with sync=3Dtop might over-allocate data depending on the destination, wouldn't it? That's not crucial, but it's possibly visible, no? >> (I'm fine with not fixing it, I just want it documented somehow.) >=20 > I am really not inclined to start any documentation on the > particularities with which qemu handles vhdx images. >=20 > (Especially so considering we don=E2=80=99t even have any documentation= on the > qcow2 case. The stress in my paragraph was =E2=80=9Cheuristic=E2=80=9D= . If you > preallocate a qcow2 image, but then discard enough sectors that the > heuristic thinks you didn=E2=80=99t, you=E2=80=99ll have the same effec= t. Or if you > grow a preallocated image without preallocating the new area.) >=20 > Max >=20 "But our qcow2 docs are also bad" is the kind of argument I can't *really* disagree with, but... (I wish we did have a documentation manual per-format that mentioned some gotchas and general info about each format, but I can't really ask you to do that now: I just worry when I see patches like this that the knowledge or memory that there ever was a quirk will vanish immediately.) --js