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Date:   Thu, 6 Oct 2022 19:24:34 +0800
From:   Ming Lei <tom.leiming@gmail.com>
To:     Stefan Hajnoczi <stefanha@gmail.com>
Cc:     Stefan Hajnoczi <stefanha@redhat.com>, io-uring@vger.kernel.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        Kirill Tkhai <kirill.tkhai@openvz.org>,
        Manuel Bentele <development@manuel-bentele.de>,
        qemu-devel@nongnu.org, Kevin Wolf <kwolf@redhat.com>,
        rjones@redhat.com, Xie Yongji <xieyongji@bytedance.com>,
        "Denis V. Lunev" <den@openvz.org>,
        Stefano Garzarella <sgarzare@redhat.com>
Subject: Re: ublk-qcow2: ublk-qcow2 is available
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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?

ublk has code to deal with device idle, and it may apply the similar
cache approach intelligently in future.

But I think here the final solution could be applying zero copy for
avoiding the big chunk copy, or use hardware engine.


Thanks,
Ming