Re: ublk-qcow2: ublk-qcow2 is available
From: Jason Wang
Date: Fri Oct 21 2022 - 02:30:53 EST
在 2022/10/21 13:33, Yongji Xie 写道:
On Tue, Oct 18, 2022 at 10:54 PM Stefan Hajnoczi <stefanha@xxxxxxxxx> wrote:
On Tue, 18 Oct 2022 at 09:17, Yongji Xie <xieyongji@xxxxxxxxxxxxx> wrote:
On Tue, Oct 18, 2022 at 2:59 PM Ming Lei <tom.leiming@xxxxxxxxx> wrote:
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@xxxxxxxxx> 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@xxxxxxxxx> wrote:
On Sat, 8 Oct 2022 at 04:43, Ziyang Zhang <ZiyangZhang@xxxxxxxxxxxxxxxxx> 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@xxxxxxxxx> 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.
Maybe, but I think we can try to optimize it.
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.
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:
I might have misunderstood what you mean by virtio ring before. My
previous understanding of the virtio ring does not include the
virtio-blk driver.
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.
Yes, this is the bottleneck of the virtio-blk driver, even in the VM
case. We are also trying to optimize this lock.
One way to mitigate it is making submission and completion happen in
the same core.
QEMU sizes virtio-blk device num-queues to match the vCPU count. The
virtio-blk driver is a blk-mq driver, so submissions and completions
for a given virtqueue should already be processed by the same vCPU.
Unless the device is misconfigured or the guest software chooses a
custom vq:vCPU mapping, there should be no vq lock contention between
vCPUs.
I can think of a reason why submission and completion require
coordination: descriptors are occupied until completion. The
submission logic chooses free descriptors from the table. The
completion logic returns free descriptors so they can be used in
future submissions.
Yes, we need to maintain a head pointer of the free descriptors in
both submission and completion path.
Not necessarily after IN_ORDER?
Thanks
Other ring designs expose the submission ring head AND tail index so
that it's clear which submissions have been processed by the other
side. Once processed, the descriptors are no longer occupied and can
be reused for future submissions immediately. This means that
submission and completion do not share state.
This is for the split virtqueue layout. For the packed layout I think
there is a similar dependency because descriptors are used for both
submission and completion.
I have CCed Michael Tsirkin in case he has any thoughts on the
independence of submission and completion in the vring design.
BTW I have written about difference in the VIRTIO, NVMe, and io_uring
descriptor ring designs here:
https://blog.vmsplice.net/2022/06/comparing-virtio-nvme-and-iouring-queue.html
Good to know that!
Thanks,
Yongji