On 2022-05-18 07:30, Stefan Hajnoczi wrote: > Hi Kirill, > I saw your "[PATCH 0/4] dm: Introduce dm-qcow2 driver to attach QCOW2 > files as block device" patch series: > https://lore.kernel.org/linux-kernel/YkME5ZS2CpXuNmN6@xxxxxxxxxxxxx/T/ > > There has been recent work in vDPA (VIRTIO Data Path Acceleration) to > achieve similar functionality. The qemu-storage-daemon VDUSE export > attaches a virtio-blk device to the host kernel and QEMU's qcow2 > implementation can be used: > https://patchew.org/QEMU/20220504074051.90-1-xieyongji@xxxxxxxxxxxxx/ > > A container can then access this virtio-blk device (/dev/vda). Note that > the virtio-blk device is implemented in software using vDPA/VDUSE, there > is no virtio-pci device. > > As a quick comparison with a dm-qcow2 target, this approach keeps the > qcow2 code in QEMU userspace and can take advantage of QEMU block layer > features (storage migration/mirroring/backup, snapshots, etc). On the > other hand, it's likely to be more heavyweight because bounce buffers > are required in VDUSE for security reasons, there is a separate > userspace process involved, and there's the virtio_blk.ko driver and an > emulated virtio-blk device involved. > > Another similar feature that was recently added to QEMU is the > qemu-storage-daemon FUSE export: > > $ qemu-storage-daemon \ > --blockdev file,filename=test.img,node-name=drive0 \ > --export fuse,node-name=drive0,id=fuse0,mountpoint=/tmp/foo > $ ls -alF /tmp/foo > -r--------. 1 me me 10737418240 May 18 07:22 /tmp/foo > > This exports a disk image as a file via FUSE. Programs can access it > like a regular file and qemu-storage-daemon will do the qcow2 I/O on the > underlying file. > > I wanted to mention these options for exposing qcow2 disk images to > processes/containers on the host. Depending on your use cases they might > be interesting. Performance comparisons against VDUSE and FUSE exports > would be interesting since these new approaches seem to be replacing > qemu-nbd. In addition, there was also qemu-tcmu, (more PoC compared to other options): https://lists.gnu.org/archive/html/qemu-devel/2016-10/msg04408.html Fam > > Can you share more about your use cases for the dm-qcow2 target? It > could be useful for everyone I've CCed to be aware of various efforts in > this area. > > Thanks, > Stefan -- dm-devel mailing list dm-devel@xxxxxxxxxx https://listman.redhat.com/mailman/listinfo/dm-devel
