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. 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
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