Re: [Qemu-devel] is there a limit on the number of in-flight I/O operations?

[Josh Durgin <jdurgin@redhat.com>]

On 08/27/2015 09:49 AM, Stefan Hajnoczi wrote:
> On Mon, Aug 25, 2014 at 03:50:02PM -0600, Chris Friesen wrote:
>> The only limit I see in the whole call chain from
>> virtio_blk_handle_request() on down is the call to
>> bdrv_io_limits_intercept() in bdrv_co_do_writev().  However, that doesn't
>> provide any limit on the absolute number of inflight operations, only on
>> operations/sec.  If the ceph server cluster can't keep up with the aggregate
>> load, then the number of inflight operations can still grow indefinitely.
>
> We probably shouldn't rely on QEMU I/O throttling to keep memory usage
> reasonable.

Agreed.

> Instead rbd should be adjusted to support iovecs as you suggested.  That
> way no bounce buffers are needed.

Yeah, this is pretty simple to do. Internally librbd has 
iovec-equivalents. I'm not sure this is the main source of extra memory 
usage
though.

I suspect the main culprit here is rbd cache letting itself burst too
large, rather than the bounce buffers.

Andrey, does this still occur with caching off?

Josh