On 08/25/2014 09:12 AM, Chris Friesen wrote:
I set up another test, checking the inflight value every second.
Running just "dd if=/dev/zero of=testfile2 bs=1M count=700
oflag=nocache&" gave a bit over 100 inflight requests.
If I simultaneously run "dd if=testfile of=/dev/null bs=1M count=700
oflag=nocache&" then then number of inflight write requests peaks at 176.
I should point out that the above numbers are with qemu 1.7.0, with a
ceph storage backend. qemu is started with
-drive file=rbd:cinder-volumes/.........
From a stacktrace that I added it looks like the writes are coming in
via virtio_blk_handle_output().
Looking at virtio_blk_device_init() I see it calling
virtio_add_queue(vdev, 128, virtio_blk_handle_output);
I wondered if that 128 had anything to do with the number of inflight
requests, so I tried recompiling with 16 instead. I still saw the number
of inflight requests go up to 178 and the guest took a kernel panic in
virtqueue_add_buf() so that wasn't very successful. :)
Following the code path in virtio_blk_handle_write() it looks like it
will bundle up to 32 writes into a single large iovec-based "multiwrite"
operation. But from there on down I don't see a limit on how many
writes can be outstanding at any one time. Still checking the code
further up the virtio call chain.
Chris
