Theodore Ts'o [off-list ref] writes:

The current virtio block sets a queue depth of 64, which is
insufficient for very fast devices.  It has been demonstrated that
with a high IOPS device, using a queue depth of 256 can double the
IOPS which can be sustained.

As suggested by Venkatash Srinivas, set the queue depth by default to
be one half the the device's virtqueue, which is the maximum queue
depth that can be supported by the channel to the host OS (each I/O
request requires at least two VQ entries).

Also allow the queue depth to be something which can be set at module
load time or via a kernel boot-time parameter, for
testing/benchmarking purposes.

Note that with indirect descriptors (which is supported by Almost
Everyone), we can actually use the full index, so this value is a bit
pessimistic.  But it's OK as a starting point.

Cheers,
Rusty.