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Re: [Qemu-block] [PATCH 0/3] block: Make various formats' block_status r

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again
Date: Tue, 13 Aug 2019 16:48:36 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

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 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 ranges
>>>> 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 “v”.)
>>>> 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.
>>>> vhdx is more like qcow2, where after the image has been created, it
>>>> isn’t clear whether it’s been preallocated or everything is allocated
>>>> because everything was already written to.  69f47505ee added a heuristic
>>>> to qcow2 to get around this, but I think that’s too much for vhdx.  I
>>>> just left it unfixed, because I don’t care that much, honestly (and I
>>>> don’t think anyone else does).
>>> What's the practical outcome of that, and is the limitation documented
>>> somewhere?
>> The outcome is that it if you preallocate a vhdx image
>> (subformat=fixed), you’ll see that all sectors contain data, even if
>> they may be zero sectors on the filesystem level.
>> I don’t think it’s user-visible whatsoever.
> But it might mean that doing things with sync=top might over-allocate
> data depending on the destination, wouldn't it?
> That's not crucial, but it's possibly visible, no?

I don’t think it has anything to do with sync=top because whether a
block is zero on the protocol level has nothing to do with whether it is
allocated on the format level.

It may make a difference for convert which uses block_status to inquire
the zero status.  However, it also does zero-detection, so...

>>> (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’t even have any documentation on the
>> qcow2 case.  The stress in my paragraph was “heuristic”.  If you
>> preallocate a qcow2 image, but then discard enough sectors that the
>> heuristic thinks you didn’t, you’ll have the same effect.  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...

My main argument is that nobody would read the vhdx docs anyway.


> (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

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