Re: ublk-qcow2: ublk-qcow2 is available

Denis V. Lunev
2022-10-05T12:38:13Z

On 10/5/22 14:21, Stefan Hajnoczi wrote:
> On Wed, 5 Oct 2022 at 00:19, Ming Lei <tom.leiming@gmail.com> wrote:
>> On Tue, Oct 04, 2022 at 09:53:32AM -0400, Stefan Hajnoczi wrote:
>>> On Tue, 4 Oct 2022 at 05:44, Ming Lei <tom.leiming@gmail.com> wrote:
>>>> On Mon, Oct 03, 2022 at 03:53:41PM -0400, 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!
>>>>>
>>>>>> 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
>>>>>>
>>>>>> - 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?
>>>> Yeah.
>>>>
>>>>>> 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
>>>> Not found virtio_vdpa module even though I enabled all the following
>>>> options:
>>>>
>>>> --- vDPA drivers
>>>> <M> vDPA device simulator core
>>>> <M> vDPA simulator for networking device
>>>> <M> vDPA simulator for block device
>>>> <M> VDUSE (vDPA Device in Userspace) support
>>>> <M> Intel IFC VF vDPA driver
>>>> <M> Virtio PCI bridge vDPA driver
>>>> <M> vDPA driver for Alibaba ENI
>>>>
>>>> BTW, my test environment is VM and the shared data is done in VM too, and
>>>> can virtio_vdpa be used inside VM?
>>> I hope Xie Yongji can help explain how to benchmark VDUSE.
>>>
>>> virtio_vdpa is available inside guests too. Please check that
>>> VIRTIO_VDPA ("vDPA driver for virtio devices") is enabled in "Virtio
>>> drivers" menu.
>>>
>>>>> # 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
>>>>>
>>>>>> - 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.
>>>> Maybe not true.
>>>>
>>>> ublk-qcow2 uses io_uring to handle all backend IO(include meta IO) completely,
>>>> and so far single io_uring/pthread is for handling all qcow2 IOs and IO
>>>> command.
>>> qemu-nbd doesn't use io_uring to handle the backend IO, so we don't
>> I tried to use it via --aio=io_uring for setting up qemu-nbd, but not succeed.
>>
>>> know whether the benchmark demonstrates that ublk is faster than NBD,
>>> that the ublk-qcow2 implementation is faster than qemu-nbd's qcow2,
>>> whether there are miscellaneous implementation differences between
>>> ublk-qcow2 and qemu-nbd (like using the same io_uring context for both
>>> ublk and backend IO), or something else.
>> The theory shouldn't be too complicated:
>>
>> 1) io uring passthough(pt) communication is fast than socket, and io command
>> is carried over io_uring pt commands, and should be fast than virio
>> communication too.
>>
>> 2) io uring io handling is fast than libaio which is taken in the
>> test on qemu-nbd, and all qcow2 backend io(include meta io) is handled
>> by io_uring.
>>
>> https://github.com/ming1/ubdsrv/blob/master/tests/common/qcow2_common
>>
>> 3) ublk uses one single io_uring to handle all io commands and qcow2
>> backend IOs, so batching handling is common, and it is easy to see
>> dozens of IOs/io commands handled in single syscall, or even more.
> I agree with the theory but theory has to be tested through
> experiments in order to validate it. We can all learn from systematic
> performance analysis - there might even be bottlenecks in ublk that
> can be solved to improve performance further.
>
>>> I'm suggesting measuring changes to just 1 variable at a time.
>>> Otherwise it's hard to reach a conclusion about the root cause of the
>>> performance difference. Let's learn why ublk-qcow2 performs well.
>> Turns out the latest Fedora 37-beta doesn't support vdpa yet, so I built
>> qemu from the latest github tree, and finally it starts to work. And test kernel
>> is v6.0 release.
>>
>> Follows the test result, and all three devices are setup as single
>> queue, and all tests are run in single job, still done in one VM, and
>> the test images are stored on XFS/virito-scsi backed SSD.
>>
>> The 1st group tests all three block device which is backed by empty
>> qcow2 image.
>>
>> The 2nd group tests all the three block devices backed by pre-allocated
>> qcow2 image.
>>
>> Except for big sequential IO(512K), there is still not small gap between
>> vdpa-virtio-blk and ublk.
>>
>> 1. run fio on block device over empty qcow2 image
>> 1) qemu-nbd
>> running qcow2/001
>> run perf test on empty qcow2 image via nbd
>> fio (nbd(/mnt/data/ublk_null_8G_nYbgF.qcow2), libaio, bs 4k, dio, hw queues:1)...
>> randwrite: jobs 1, iops 8549
>> randread: jobs 1, iops 34829
>> randrw: jobs 1, iops read 11363 write 11333
>> rw(512k): jobs 1, iops read 590 write 597
>>
>>
>> 2) ublk-qcow2
>> running qcow2/021
>> run perf test on empty qcow2 image via ublk
>> fio (ublk/qcow2( -f /mnt/data/ublk_null_8G_s761j.qcow2), libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0).
>> randwrite: jobs 1, iops 16086
>> randread: jobs 1, iops 172720
>> randrw: jobs 1, iops read 35760 write 35702
>> rw(512k): jobs 1, iops read 1140 write 1149
>>
>> 3) vdpa-virtio-blk
>> running debug/test_dev
>> run io test on specified device
>> fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)...
>> randwrite: jobs 1, iops 8626
>> randread: jobs 1, iops 126118
>> randrw: jobs 1, iops read 17698 write 17665
>> rw(512k): jobs 1, iops read 1023 write 1031
>>
>>
>> 2. run fio on block device over pre-allocated qcow2 image
>> 1) qemu-nbd
>> running qcow2/002
>> run perf test on pre-allocated qcow2 image via nbd
>> fio (nbd(/mnt/data/ublk_data_8G_sc0SB.qcow2), libaio, bs 4k, dio, hw queues:1)...
>> randwrite: jobs 1, iops 21439
>> randread: jobs 1, iops 30336
>> randrw: jobs 1, iops read 11476 write 11449
>> rw(512k): jobs 1, iops read 718 write 722
>>
>> 2) ublk-qcow2
>> running qcow2/022
>> run perf test on pre-allocated qcow2 image via ublk
>> fio (ublk/qcow2( -f /mnt/data/ublk_data_8G_yZiaJ.qcow2), libaio, bs 4k, dio, hw queues:1, uring_comp: 0, get_data: 0).
>> randwrite: jobs 1, iops 98757
>> randread: jobs 1, iops 110246
>> randrw: jobs 1, iops read 47229 write 47161
>> rw(512k): jobs 1, iops read 1416 write 1427
>>
>> 3) vdpa-virtio-blk
>> running debug/test_dev
>> run io test on specified device
>> fio (vdpa(/dev/vdc), libaio, bs 4k, dio, hw queues:1)...
>> randwrite: jobs 1, iops 47317
>> randread: jobs 1, iops 74092
>> randrw: jobs 1, iops read 27196 write 27234
>> rw(512k): jobs 1, iops read 1447 write 1458
> Thanks for including VDUSE results! ublk looks great here and worth
> considering even in cases where NBD or VDUSE is already being used.
>
> Stefan
+ Andrey Zhadchenko