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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 >>>> vDPA device simulator core >>>> vDPA simulator for networking device >>>> vDPA simulator for block device >>>> VDUSE (vDPA Device in Userspace) support >>>> Intel IFC VF vDPA driver >>>> Virtio PCI bridge vDPA driver >>>> 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. > >>> 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 > Thanks for including VDUSE results! ublk looks great here and worth > considering even in cases where NBD or VDUSE is already being used. > > Stefan + Andrey Zhadchenko