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. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-top is 'top' for virtual machines. Tiny program with many powerful monitoring features, net stats, disk stats, logging, etc. http://people.redhat.com/~rjones/virt-top
