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

[Kevin Wolf]

Am 23.04.2019 um 16:26 hat Martin Kletzander geschrieben:
> On Tue, Apr 23, 2019 at 02:12:18PM +0200, Kevin Wolf wrote:
> > 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
> >    ...
> > 
> 
> I thought of this, but (to be honest) I did not know that `-B` would
> work for nbd.

It still depends on the NBD server providing the right block allocation
status, but that's no worse than what you needed for -n. But whether -B
can be used at all depends on the target format, not the source.

> Does it assume that data are to be taken from the base image if and
> only if the source (be it nbd server or just a plain file) says there
> is a hole?  If yes, then it could nicely solve the issue.

I haven't tested it now, but yes, that's what I remember it to do.

Looking at the code, the requirement seems to be that the NBD server
flags the sparse blocks as a HOLE, but not as ZERO.

Kevin

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