Re: [PATCH] virtio-blk: make the queue depth the max supportable by the hypervisor

[Rusty Russell <rusty@rustcorp.com.au>]

Theodore Ts'o <tytso@mit.edu> 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.