Re: ublk-qcow2: ublk-qcow2 is available

Yongji Xie
2022-10-07T10:04:49Z

On Thu, Oct 6, 2022 at 7:24 PM Ming Lei <tom.leiming@gmail.com> wrote:
>
> On Wed, Oct 05, 2022 at 08:21:45AM -0400, 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.
>
> Indeed, one thing is that ublk uses get user pages to retrieve user pages
> for copying data, this way may add latency for big chunk IO, since
> latency of get user pages should be increased linearly by nr_pages.
>
> I looked into vduse code a bit too, and vduse still needs the page copy,
> but lots of bounce pages are allocated and cached in the whole device
> lifetime, this way can void the latency for retrieving & allocating
> pages runtime with cost of extra memory consumption. Correct me
> if it is wrong, Xie Yongji or anyone?
>

Yes, you are right. Another way is registering the preallocated
userspace memory as bounce buffer.

Thanks,
Yongji