Re: [PATCH 0/6][RFC] virtio-blk: Change I/O path from request to BIO

[Paolo Bonzini <pbonzini@redhat.com>]

On 01/01/2012 05:45 PM, Stefan Hajnoczi wrote:
> By the way, drivers for solid-state devices can set QUEUE_FLAG_NONROT
> to hint that seek time optimizations may be sub-optimal.  NBD and
> other virtual/pseudo device drivers set this flag.  Should virtio-blk
> set it and how does it affect performance?

By itself is not a good idea in general.

When QEMU uses O_DIRECT, the guest should not use QUEUE_FLAG_NONROT 
unless it is active for the host disk as well.  (In doubt, as is the 
case for remote hosts accessed over NFS, I would also avoid NONROT and 
allow more coalescing).

When QEMU doesn't use O_DIRECT, instead, using QUEUE_FLAG_NONROT and 
leaving optimizations to the host may make some sense.

In Xen, the back-end driver is bio-based, so the scenario is like QEMU 
with O_DIRECT.  I remember seeing worse performance when switching the 
front-end to either QUEUE_FLAG_NONROT or the noop scheduler.  This was 
with RHEL5 (2.6.18), but it might still be true in more recent kernels, 
modulo benchmarking of course.  Still, the current in-tree xen-blkfront 
driver does use QUEUE_FLAG_NONROT unconditionally, more precisely its 
synonym QUEUE_FLAG_VIRT.

Still, if benchmarking confirms this theory, QEMU could expose a hint 
via a feature bit.  The default could be simply "use QUEUE_FLAG_NONROT 
iff not using O_DIRECT", or it could be more complicated with help from 
sysfs.

Paolo