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...
Cheers,
Hannes
--
Dr. Hannes Reinecke Teamlead Storage & Networking
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