[Qemu-devel] 4k seq read splitting for virtio-blk - possible workarounds?

[Andrey Korolyov]

Hi,

during the test against generic storage backend with NBD frontend we
found that the virtio block device is always splitting a single read
range request to 4k ones, bringing the overall performance of the
sequential reads far below virtio-scsi. Random reads are going
relatively well on small blocks due to small overhead comparing to
sequential ones and writes are ok in all cases. Multiread slightly
improves the situation, but it would be nice to see complete
pass-through of range read requests down to backend without an
intermediate splitting.

Samples measured on an NBD backend during 128k sequential reads for
both virtio-blk and virtio-scsi are attached. Please let me know if it
looks like that I missed something or this behavior is plainly wrong.

Thanks!

virtio-blk.txt
Description: Text document

virtio-scsi.txt
Description: Text document