Re: Block alignment of qcow2 compress driver

["Richard W.M. Jones" <rjones@redhat.com>]

On Fri, Jan 28, 2022 at 12:57:47PM +0100, Hanna Reitz wrote:
> On 28.01.22 12:48, Richard W.M. Jones wrote:
> >On Fri, Jan 28, 2022 at 12:39:11PM +0100, Hanna Reitz wrote:
> >>So I actually don’t know why it works for you.  OTOH, I don’t
> >>understand why the block size affects you over NBD, because I would
> >>have expected qemu to internally auto-align requests when they are
> >>not aligned (in bdrv_co_pwritev_part()).
> >I checked it again and my hack definitely fixes nbdcopy.  But maybe
> >that's expected if qemu-nbd is auto-aligning requests?  (I'm only
> >accessing the block layer through qemu-nbd, not with qemu-io)
> 
> It’s not just qemu-io, with your diff[3] I get the same EINVAL over
> NBD, too:
> 
> $ ./qemu-img create -f qcow2 test.qcow2 64M
> Formatting 'test.qcow2', fmt=qcow2 cluster_size=65536
> extended_l2=off compression_type=zlib size=67108864
> lazy_refcounts=off refcount_bits=16
> 
> $ ./qemu-nbd --fork --image-opts \
> driver=compress,file.driver=qcow2,file.file.driver=file,file.file.filename=test.qcow2
> 
> $ ./qemu-io -c 'write 0 32k' -f raw nbd://localhost
> write failed: Invalid argument

Strange - is that error being generated by qemu's nbd client code?

Here's my test not involving qemu's client code:

$ qemu-nbd --version
qemu-nbd 6.2.0 (qemu-6.2.0-2.fc36)

$ qemu-img create -f qcow2 output.qcow2 1M
Formatting 'output.qcow2', fmt=qcow2 cluster_size=65536 extended_l2=off compression_type=zlib size=1048576 lazy_refcounts=off refcount_bits=16

$ qemu-nbd --fork --image-opts driver=compress,file.driver=qcow2,file.file.driver=file,file.file.filename=output.qcow2

$ nbdsh -u nbd://localhost
nbd> h.get_strict_mode()
31
nbd> h.set_strict_mode(31 & ~nbd.STRICT_ALIGN)
nbd> h.get_strict_mode()
15
nbd> h.pwrite(b'1'*1024, 0)
nbd> exit

So an unaligned 1K write works (after disabling libnbd's client-side
alignment checks).

> I just changed that line of code [2], as shown in [4].  I suppose
> the better thing to do would be to have an option for the NBD server
> to force-change the announced request alignment, because it can
> expect the qemu block layer code to auto-align requests through
> RMW.  Doing it in the client is wrong, because the NBD server might
> want to detect that the client sends unaligned requests and reject
> them (though ours doesn’t, it just traces such events[5] – note that
> it’s explicitly noted there that qemu will auto-align requests).

I know I said I didn't care about performance (in this case), but is
there in fact a penalty to sending unaligned requests to the qcow2
layer?  Or perhaps it cannot compress them?

Rich.

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