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

From: John Snow
Subject: Re: [Qemu-block] [PATCH 0/3] block: Make various formats' block_status recurse again
Date: Tue, 13 Aug 2019 18:35:46 -0400
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

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 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...

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 <address@hidden>

>>>> (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.
> Max

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?


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