Re: ublk-qcow2: ublk-qcow2 is available

Ming Lei
2022-10-04T09:46:32Z

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?

> # 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.

thanks,
Ming