Re: ublk-qcow2: ublk-qcow2 is available

Jason Wang
2022-10-21T06:28:46Z

在 2022/10/18 14:59, Ming Lei 写道:
> On Mon, Oct 17, 2022 at 07:11:59PM +0800, Yongji Xie wrote:
>> On Fri, Oct 14, 2022 at 8:57 PM Ming Lei <tom.leiming@gmail.com> wrote:
>>> On Thu, Oct 13, 2022 at 02:48:04PM +0800, Yongji Xie wrote:
>>>> On Wed, Oct 12, 2022 at 10:22 PM Stefan Hajnoczi <stefanha@gmail.com> wrote:
>>>>> On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang <ZiyangZhang@linux.alibaba.com> wrote:
>>>>>> On 2022/10/5 12:18, Ming Lei 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'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
>>>>>>>
>>>>>>>
>>>>>> Hi All,
>>>>>>
>>>>>> We are interested in VDUSE vs UBLK, too. And I have tested them with nullblk backend.
>>>>>> Let me share some results here.
>>>>>>
>>>>>> I setup UBLK with:
>>>>>> ublk add -t loop -f /dev/nullb0 -d QUEUE_DEPTH -q NR_QUEUE
>>>>>>
>>>>>> I setup VDUSE with:
>>>>>> qemu-storage-daemon \
>>>>>> --chardev socket,id=charmonitor,path=/tmp/qmp.sock,server=on,wait=off \
>>>>>> --monitor chardev=charmonitor \
>>>>>> --blockdev driver=host_device,cache.direct=on,filename=/dev/nullb0,node-name=disk0 \
>>>>>> --export vduse-blk,id=test,node-name=disk0,name=vduse_test,writable=on,num-queues=NR_QUEUE,queue-size=QUEUE_DEPTH
>>>>>>
>>>>>> Here QUEUE_DEPTH is 1, 32 or 128 and NR_QUEUE is 1 or 4.
>>>>>>
>>>>>> Note:
>>>>>> (1) VDUSE requires QUEUE_DEPTH >= 2. I cannot setup QUEUE_DEPTH to 1.
>>>>>> (2) I use qemu 7.1.0-rc3. It supports vduse-blk.
>>>>>> (3) I do not use ublk null target so that the test is fair.
>>>>>> (4) I setup fio with direct=1, bs=4k.
>>>>>>
>>>>>> ------------------------------
>>>>>> 1 job 1 iodepth, lat（usec)
>>>>>> vduse ublk
>>>>>> seq-read 22.55 11.15
>>>>>> rand-read 22.49 11.17
>>>>>> seq-write 25.67 10.25
>>>>>> rand-write 24.13 10.16
>>>>> Thanks for sharing. Any idea what the bottlenecks are for vduse and ublk?
>>>>>
>>>> I think one reason for the latency gap of sync I/O is that vduse uses
>>>> workqueue in the I/O completion path but ublk doesn't.
>>>>
>>>> And one bottleneck for the async I/O in vduse is that vduse will do
>>>> memcpy inside the critical section of virtqueue's spinlock in the
>>>> virtio-blk driver. That will hurt the performance heavily when
>>>> virtio_queue_rq() and virtblk_done() run concurrently. And it can be
>>>> mitigated by the advance DMA mapping feature [1] or irq binding
>>>> support [2].
>>> Hi Yongji,
>>>
>>> Yeah, that is the cost you paid for virtio. Wrt. userspace block device
>>> or other sort of userspace devices, cmd completion is driven by
>>> userspace, not sure if one such 'irq' is needed.
>> I'm not sure, it can be an optional feature in the future if needed.
>>
>>> Even not sure if virtio
>>> ring is one good choice for such use case, given io_uring has been proved
>>> as very efficient(should be better than virtio ring, IMO).
>>>
>> Since vduse is aimed at creating a generic userspace device framework,
>> virtio should be the right way IMO.
> OK, it is the right way, but may not be the effective one.
>
>> And with the vdpa framework, the
>> userspace device can serve both virtual machines and containers.
> virtio is good for VM, but not sure it is good enough for other
> cases.

Well, virtio is not yet limited to virt and has been widely used in bare
metal, containers, automotive and even edge in production environment
for years. A lot of vendors has shipped their software or hardware
virtio/vDPA products.

>
>> Regarding the performance issue, actually I can't measure how much of
>> the performance loss is due to the difference between virtio ring and
>> iouring. But I think it should be very small. The main costs come from
>> the two bottlenecks I mentioned before which could be mitigated in the
>> future.
> Per my understanding, at least there are two places where virtio ring is
> less efficient than io_uring:
>
> 1) io_uring uses standalone submission queue(SQ) and completion queue(CQ),
> so no contention exists between submission and completion; but virtio queue
> requires per-vq lock in both submission and completion.

Virtio is not limited in its layout of the queue. I've used to proposed
SQ/CQ model in the spec in the past but vendors complains a third format
immediate after the second. Maybe it's time to revisit that, but it
needs to be fully benchmarked and proved at first.

>
> 2) io_uring can use single system call of io_uring_enter() for both
> submitting and completing, so one context switch is enough, together
> with natural batch processing for both submission and completion, and
> it is observed that dozens or more than one hundred of IOs can be
> covered in single syscall; virtio requires one notification for submission and
> another one for completion,

You can queue several buffers before a kick to the virtqueue, with a
polling device and driver, you don't even need any kick/notification. I
don't see much difference here.

> looks at least two context switch are required
> for handling one IO(s).

For virtio, the queue layout or ring design should not be bottleneck for
the block device at least. I can give your some numbers measured by PPS
(since network traffic is more queue layout sensitive than block):

1) vDPA vendor can achieve 30Mpps or even higher
2) software userspace virtio backends like vhost-user can do almost the
same or even higher

This is a strong hint that virtio ring should be sufficient for block.
For NFV/wire-speed like 100G we do need more work on optimization on the
queue/descriptor format.

Thanks

>
>>> ublk uses io_uring pt cmd for handling both io submission and completion,
>>> turns out the extra latency can be pretty small.
>>>
>>> BTW, one un-related topic, I saw the following words in
>>> Documentation/userspace-api/vduse.rst:
>>>
>>> ```
>>> Note that only virtio block device is supported by VDUSE framework now,
>>> which can reduce security risks when the userspace process that implements
>>> the data path is run by an unprivileged user.
>>> ```
>>>
>>> But when I tried to start qemu-storage-daemon for creating vdpa-virtio
>>> block by nor unprivileged user, 'Permission denied' is still returned,
>>> can you explain a bit how to start such process by unprivileged user?
>>> Or maybe I misunderstood the above words, please let me know.
>>>
>> Currently vduse should only allow privileged users by default. But
>> sysadmin can change the permission of the vduse char device or pass
>> the device fd to an unprivileged process IIUC.
> I appreciate if you may provide a bit detailed steps for the above?
>
> BTW, I changed privilege of /dev/vduse/control to normal user, but
> qemu-storage-daemon still returns 'Permission denied'. And if the
> char dev is /dev/vduse/vduse0N, which is created by qemu-storage-daemon,
> so how to change user of qemu-storage-daemon to unprivileged after
> /dev/vduse/vduse0N is created?
>
>
>
> Thanks,
> Ming
>