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.
