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From: Keith Busch <kbusch@kernel.org>
To: Hannes Reinecke <hare@suse.de>
Cc: Damien Le Moal <Damien.LeMoal@wdc.com>,
	"Martin K. Petersen" <martin.petersen@oracle.com>,
	linux-scsi <linux-scsi@vger.kernel.org>,
	Tim Walker <tim.t.walker@seagate.com>,
	"linux-nvme@lists.infradead.org" <linux-nvme@lists.infradead.org>,
	Ming Lei <ming.lei@redhat.com>,
	"linux-block@vger.kernel.org" <linux-block@vger.kernel.org>
Subject: Re: [LSF/MM/BPF TOPIC] NVMe HDD
Date: Fri, 14 Feb 2020 23:40:07 +0900
Message-ID: <20200214144007.GD9819@redsun51.ssa.fujisawa.hgst.com> (raw)
In-Reply-To: <2d66bb0b-29ca-6888-79ce-9e3518ee4b61@suse.de>

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.

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Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-10 19:20 Tim Walker
2020-02-10 20:43 ` Keith Busch
2020-02-10 22:25   ` Finn Thain
2020-02-11 12:28 ` Ming Lei
2020-02-11 19:01   ` Tim Walker
2020-02-12  1:47     ` Damien Le Moal
2020-02-12 22:03       ` Ming Lei
2020-02-13  2:40         ` Damien Le Moal
2020-02-13  7:53           ` Ming Lei
2020-02-13  8:24             ` Damien Le Moal
2020-02-13  8:34               ` Ming Lei
2020-02-13 16:30                 ` Keith Busch
2020-02-14  0:40                   ` Ming Lei
2020-02-13  3:02       ` Martin K. Petersen
2020-02-13  3:12         ` Tim Walker
2020-02-13  4:17           ` Martin K. Petersen
2020-02-14  7:32             ` Hannes Reinecke
2020-02-14 14:40               ` Keith Busch [this message]
2020-02-14 16:04                 ` Hannes Reinecke
2020-02-14 17:05                   ` Keith Busch
2020-02-18 15:54                     ` Tim Walker
2020-02-18 17:41                       ` Keith Busch
2020-02-18 17:52                         ` James Smart
2020-02-19  1:31                         ` Ming Lei
2020-02-19  1:53                           ` Damien Le Moal
2020-02-19  2:15                             ` Ming Lei
2020-02-19  2:32                               ` Damien Le Moal
2020-02-19  2:56                                 ` Tim Walker
2020-02-19 16:28                                   ` Tim Walker
2020-02-19 20:50                                     ` Keith Busch
2020-02-14  0:35         ` Ming Lei
2020-02-12 21:52     ` Ming Lei

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