Re: SCSI mid layer and high IOPS capable devices

[Christoph Hellwig <hch@infradead.org>]

On Sat, Dec 15, 2012 at 10:40:24AM +0100, Bart Van Assche wrote:
> On 12/14/12 22:06, scameron@beardog.cce.hp.com wrote:
> >[ ... ] how to get the scsi mid layer to provide a wide enough
> >highway for requests destined for very low latency devices.
> 
> While the SCSI mid-layer is processing an I/O request not only the
> queue lock has to be locked and unlocked several times but also the
> SCSI host lock. The reason that it's unavoidable to lock and unlock
> the host lock is because the SCSI core has been designed for SCSI
> equipment that has a queue depth limit per host (shost->can_queue).
> For single LUN devices that model could be changed in a queue depth
> limit per LUN. Also, it's probably not that hard to modify software
> SCSI target implementations such that these have a queue depth limit
> per LUN instead of per host.

We'd also better avoid needing a lock to check these limits, especially
if we normally don't hit them.  The easiest way to get started would
be to simply allow a magic can_queue value that keeps these as unlimited
and only let the driver return one of the busy values from
->queuecommand.  We could then use unlocked list empty checks to see
if anything is in a waiting list and enter a slow path mode.