Re: [Qemu-devel] Possibly incorrect data sparsification by qemu-img

[Kevin Wolf]

Am 23.04.2019 um 13:30 hat Martin Kletzander geschrieben:
> Hi,
> 
> I am using qemu-img with nbdkit to transfer a disk image and the update it 
> with
> extra data from newer snapshots.  The end image cannot be transferred because
> the snapshots will be created later than the first transfer and we want to 
> save
> some time up front.  You might think of it as a continuous synchronisation.  
> It
> looks something like this:
> 
> I first transfer the whole image:
> 
>  qemu-img convert -p $nbd disk.raw
> 
> Where `$nbd` is something along the lines of `nbd+unix:///?socket=nbdkit.sock`
> 
> Then, after the next snapshot is created, I can update it thanks to the `-n`
> parameter (the $nbd now points to the newer snapshot with unchanged data 
> looking
> like holes in the file):
> 
>  qemu-img convert -p -n $nbd disk.raw
> 
> This is fast and efficient as it uses block status nbd extension, so it only
> transfers new data.

This is an implementation detail. Don't rely on it. What you're doing is
abusing 'qemu-img convert', so problems like what you describe are to be
expected.

> This can be done over and over again to keep the local
> `disk.raw` image up to date with the latest remote snapshot.
> 
> However, when the guest OS zeroes some of the data and it gets written into 
> the
> snapshot, qemu-img scans for those zeros and does not write them to the
> destination image.  Checking the output of `qemu-img map --output=json $nbd`
> shows that the zeroed data is properly marked as `data: true`.
> 
> Using `-S 0` would write zeros even where the holes are, effectively 
> overwriting
> the data from the last snapshot even though they should not be changed.
> 
> Having gone through some workarounds I would like there to be another way.  I
> know this is far from the typical usage of qemu-img, but is this really the
> expected behaviour or is this just something nobody really needed before?  If 
> it
> is the former, would it be possible to have a parameter that would control 
> this
> behaviour?  If the latter is the case, can that behaviour be changed so that 
> it
> properly replicates the data when `-n` parameter is used?
> 
> Basically the only thing we need is to either:
> 
> 1) write zeros where they actually are or
> 
> 2) turn off explicit sparsification without requesting dense image (basically
>    sparsify only the par that is reported as hole on the source) or
> 
> 3) ideally, just FALLOC_FL_PUNCH_HOLE in places where source did report data,
>    but qemu-img found they are all zeros (or source reported HOLE+ZERO which, 
> I
>    believe, is effectively the same)
> 
> If you want to try this out, I found the easiest reproducible way is using
> nbdkit's data plugin, which can simulate whatever source image you like.

I think what you _really_ want is a commit block job. The problem is
just that you don't have a proper backing file chain, but just a bunch
of NBD connections.

Can't you get an NBD connection that already provides the condensed form
of the whole snapshot chain directly at the source? If the NBD server
was QEMU, this would actually be easier than providing each snapshot
individually.

If this isn't possible, I think you need to replicate the backing chain
on the destination instead of converting into the same image again and
again so that qemu-img knows that it must take existing data of the
backing file into consideration:

    qemu-img convert -O qcow2 nbd://... base.qcow2
    qemu-img convert -O qcow2 -F qcow2 -B base.qcow2 nbd://... overlay1.qcow2
    qemu-img convert -O qcow2 -F qcow2 -B overlay1.qcow2 nbd://... 
overlay2.qcow2
    ...

And at the end you can merge the snapshot chain (using a commit or
stream bĺock job, or qemu-img commit/rebase).

Kevin

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