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List-Id: List-Unsubscribe: , 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 8/13/19 10:48 AM, Max Reitz wrote: > On 12.08.19 23:45, John Snow wrote: >> >> >> 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 th= at it >>>>> would only go down to the protocol layer if the format layer return= ed >>>>> 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 r= anges >>>>> appear as zeroes. >>>>> >>>>> However, it only implemented this preallocation detection for qcow2= . >>>>> There are more formats that support preallocation, though: vdi, vhd= x, >>>>> 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 real= ly >>>>> have different subformats depending on whether an image is prealloc= ated >>>>> 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 every= thing is allocated >>>>> because everything was already written to. 69f47505ee added a heur= istic >>>>> to qcow2 to get around this, but I think that=E2=80=99s too much fo= r vhdx. I >>>>> just left it unfixed, because I don=E2=80=99t care that much, hones= tly (and I >>>>> don=E2=80=99t think anyone else does). >>>>> >>>> >>>> What's the practical outcome of that, and is the limitation document= ed >>>> somewhere? >>> >>> 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. >>> >>> I don=E2=80=99t think it=E2=80=99s user-visible whatsoever. >>> >> >> But it might mean that doing things with sync=3Dtop might over-allocat= e >> data depending on the destination, wouldn't it? >> >> That's not crucial, but it's possibly visible, no? >=20 > I don=E2=80=99t think it has anything to do with sync=3Dtop because whe= ther a > block is zero on the protocol level has nothing to do with whether it i= s > allocated on the format level. >=20 > It may make a difference for convert which uses block_status to inquire > the zero status. However, it also does zero-detection, so... >=20 Oh, okay then. Probably... fine, but I have a nagging doubt relating to some of the fallbacks in e.g. qcow2 that tend to inflate zeroes in some cases (or used to. Maybe it's been fixed since.) ...but I can't point to anything, so it's fine, and I'm just drawing things out for no reason. Reviewed-by: John Snow >>>> (I'm fine with not fixing it, I just want it documented somehow.) >>> >>> I am really not inclined to start any documentation on the >>> particularities with which qemu handles vhdx images. >>> >>> (Especially so considering we don=E2=80=99t even have any documentati= on 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 eff= ect. Or if you >>> grow a preallocated image without preallocating the new area.) >>> >>> Max >>> >> >> "But our qcow2 docs are also bad" is the kind of argument I can't >> *really* disagree with, but... >=20 > My main argument is that nobody would read the vhdx docs anyway. >=20 > Max >=20 That's the sort of thing I'd like to change, but I guess I haven't really made good on that desire in any way, so what good is that? --js