Re: [LSF/MM/BPF TOPIC] NVMe HDD

[Keith Busch <kbusch@kernel.org>]

On Fri, Feb 14, 2020 at 05:04:25PM +0100, Hannes Reinecke wrote:
> On 2/14/20 3:40 PM, Keith Busch wrote:
> > On Fri, Feb 14, 2020 at 08:32:57AM +0100, Hannes Reinecke wrote:
> > > On 2/13/20 5:17 AM, Martin K. Petersen wrote:
> > > > People often artificially lower the queue depth to avoid timeouts. The
> > > > default timeout is 30 seconds from an I/O is queued. However, many
> > > > enterprise applications set the timeout to 3-5 seconds. Which means that
> > > > with deep queues you'll quickly start seeing timeouts if a drive
> > > > temporarily is having issues keeping up (media errors, excessive spare
> > > > track seeks, etc.).
> > > > 
> > > > Well-behaved devices will return QF/TSF if they have transient resource
> > > > starvation or exceed internal QoS limits. QF will cause the SCSI stack
> > > > to reduce the number of I/Os in flight. This allows the drive to recover
> > > > from its congested state and reduces the potential of application and
> > > > filesystem timeouts.
> > > > 
> > > This may even be a chance to revisit QoS / queue busy handling.
> > > NVMe has this SQ head pointer mechanism which was supposed to handle
> > > this kind of situations, but to my knowledge no-one has been
> > > implementing it.
> > > Might be worthwhile revisiting it; guess NVMe HDDs would profit from that.
> > 
> > We don't need that because we don't allocate enough tags to potentially
> > wrap the tail past the head. If you can allocate a tag, the queue is not
> > full. And convesely, no tag == queue full.
> > 
> It's not a problem on our side.
> It's a problem on the target/controller side.
> The target/controller might have a need to throttle I/O (due to QoS settings
> or competing resources from other hosts), but currently no means of
> signalling that to the host.
> Which, incidentally, is the underlying reason for the DNR handling
> discussion we had; NetApp tried to model QoS by sending "Namespace not
> ready" without the DNR bit set, which of course is a totally different
> use-case as the typical 'Namespace not ready' response we get (with the DNR
> bit set) when a namespace was unmapped.
> 
> And that is where SQ head pointer updates comes in; it would allow the
> controller to signal back to the host that it should hold off sending I/O
> for a bit.
> So this could / might be used for NVMe HDDs, too, which also might have a
> need to signal back to the host that I/Os should be throttled...

Okay, I see. I think this needs a new nvme AER notice as Martin
suggested. The desired host behavior is simiilar to what we do with a
"firmware activation notice" where we temporarily quiesce new requests
and reset IO timeouts for previously dispatched requests. Perhaps tie
this to the CSTS.PP register as well.