On Fri, Apr 26, 2019 at 10:14:16AM +0200, Paolo Bonzini wrote: > On 23/04/19 14:04, Stefan Hajnoczi wrote: > >> In addition, does Virtio-scsi support Batch I/O Submission feature > >> which may be able to increase the IOPS via reducing the number of > >> system calls? > > > > I don't see obvious batching support in drivers/scsi/virtio_scsi.c. > > The Linux block layer supports batching but I'm not sure if the SCSI > > layer does. > > I think he's referring to QEMU, in which case yes, virtio-scsi does > batch I/O submission. See virtio_scsi_handle_cmd_req_prepare and > virtio_scsi_handle_cmd_req_submit in hw/scsi/virtio-scsi.c, they do > blk_io_plug and blk_io_unplug in order to batch I/O requests from QEMU > to the host kernel.
This isn't fully effective since the guest driver kicks once per request. Therefore QEMU-level batching you mentioned only works if QEMU is slower at handling virtqueue kicks than the guest is at submitting requests. I wonder if this is something that can be improved. Stefan
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