Re: ublk-qcow2: ublk-qcow2 is available

Denis V. Lunev
2022-10-03T23:58:09Z

On 10/3/22 21:53, Stefan Hajnoczi wrote:
> On Fri, Sep 30, 2022 at 05:24:11PM +0800, Ming Lei wrote:
>> ublk-qcow2 is available now.
> Cool, thanks for sharing!
yep

>> So far it provides basic read/write function, and compression and snapshot
>> aren't supported yet. The target/backend implementation is completely
>> based on io_uring, and share the same io_uring with ublk IO command
>> handler, just like what ublk-loop does.
>>
>> Follows the main motivations of ublk-qcow2:
>>
>> - building one complicated target from scratch helps libublksrv APIs/functions
>> become mature/stable more quickly, since qcow2 is complicated and needs more
>> requirement from libublksrv compared with other simple ones(loop, null)
>>
>> - there are several attempts of implementing qcow2 driver in kernel, such as
>> ``qloop`` [2], ``dm-qcow2`` [3] and ``in kernel qcow2(ro)`` [4], so ublk-qcow2
>> might useful be for covering requirement in this field
There is one important thing to keep in mind about all partly-userspace
implementations though:
* any single allocation happened in the context of the
userspace daemon through try_to_free_pages() in
kernel has a possibility to trigger the operation,
which will require userspace daemon action, which
is inside the kernel now.
* the probability of this is higher in the overcommitted
environment

This was the main motivation of us in favor for the in-kernel
implementation.

>> - performance comparison with qemu-nbd, and it was my 1st thought to evaluate
>> performance of ublk/io_uring backend by writing one ublk-qcow2 since ublksrv
>> is started
>>
>> - help to abstract common building block or design pattern for writing new ublk
>> target/backend
>>
>> So far it basically passes xfstest(XFS) test by using ublk-qcow2 block
>> device as TEST_DEV, and kernel building workload is verified too. Also
>> soft update approach is applied in meta flushing, and meta data
>> integrity is guaranteed, 'make test T=qcow2/040' covers this kind of
>> test, and only cluster leak is reported during this test.
>>
>> The performance data looks much better compared with qemu-nbd, see
>> details in commit log[1], README[5] and STATUS[6]. And the test covers both
>> empty image and pre-allocated image, for example of pre-allocated qcow2
>> image(8GB):
>>
>> - qemu-nbd (make test T=qcow2/002)
> Single queue?
>
>> randwrite(4k): jobs 1, iops 24605
>> randread(4k): jobs 1, iops 30938
>> randrw(4k): jobs 1, iops read 13981 write 14001
>> rw(512k): jobs 1, iops read 724 write 728
> Please try qemu-storage-daemon's VDUSE export type as well. The
> command-line should be similar to this:
>
> # modprobe virtio_vdpa # attaches vDPA devices to host kernel
> # modprobe vduse
> # qemu-storage-daemon \
> --blockdev file,filename=test.qcow2,cache.direct=of|off,aio=native,node-name=file \
> --blockdev qcow2,file=file,node-name=qcow2 \
> --object iothread,id=iothread0 \
> --export vduse-blk,id=vduse0,name=vduse0,num-queues=$(nproc),node-name=qcow2,writable=on,iothread=iothread0
> # vdpa dev add name vduse0 mgmtdev vduse
>
> A virtio-blk device should appear and xfstests can be run on it
> (typically /dev/vda unless you already have other virtio-blk devices).
>
> Afterwards you can destroy the device using:
>
> # vdpa dev del vduse0
but this would be anyway limited by a single thread doing AIO in
qemu-storage-daemon, I believe.

>> - ublk-qcow2 (make test T=qcow2/022)
> There are a lot of other factors not directly related to NBD vs ublk. In
> order to get an apples-to-apples comparison with qemu-* a ublk export
> type is needed in qemu-storage-daemon. That way only the difference is
> the ublk interface and the rest of the code path is identical, making it
> possible to compare NBD, VDUSE, ublk, etc more precisely.
>
> I think that comparison is interesting before comparing different qcow2
> implementations because qcow2 sits on top of too much other code. It's
> hard to know what should be accounted to configuration differences,
> implementation differences, or fundamental differences that cannot be
> overcome (this is the interesting part!).
>
>> randwrite(4k): jobs 1, iops 104481
>> randread(4k): jobs 1, iops 114937
>> randrw(4k): jobs 1, iops read 53630 write 53577
>> rw(512k): jobs 1, iops read 1412 write 1423
>>
>> Also ublk-qcow2 aligns queue's chunk_sectors limit with qcow2's cluster size,
>> which is 64KB at default, this way simplifies backend io handling, but
>> it could be increased to 512K or more proper size for improving sequential
>> IO perf, just need one coroutine to handle more than one IOs.
>>
>>
>> [1] https://github.com/ming1/ubdsrv/commit/9faabbec3a92ca83ddae92335c66eabbeff654e7
>> [2] https://upcommons.upc.edu/bitstream/handle/2099.1/9619/65757.pdf?sequence=1&isAllowed=y
>> [3] https://lwn.net/Articles/889429/
>> [4] https://lab.ks.uni-freiburg.de/projects/kernel-qcow2/repository
>> [5] https://github.com/ming1/ubdsrv/blob/master/qcow2/README.rst
>> [6] https://github.com/ming1/ubdsrv/blob/master/qcow2/STATUS.rst

interesting...

Den