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From: James Bottomley <James.Bottomley@HansenPartnership.com>
To: Ming Lei <ming.lei@redhat.com>, Yufen Yu <yuyufen@huawei.com>
Cc: "linux-block@vger.kernel.org" <linux-block@vger.kernel.org>,
	john.garry@huawei.com, "axboe@kernel.dk" <axboe@kernel.dk>,
	hare@suse.de, Bart Van Assche <bvanassche@acm.org>
Subject: Re: [Question] abort shared tags for SCSI drivers
Date: Thu, 16 Jan 2020 07:17:44 -0800
Message-ID: <1579187864.3551.10.camel@HansenPartnership.com> (raw)
In-Reply-To: <20200116090347.GA7438@ming.t460p>

On Thu, 2020-01-16 at 17:03 +0800, Ming Lei wrote:
> On Thu, Jan 16, 2020 at 12:06:02PM +0800, Yufen Yu wrote:
> > Hi, all
> > 
> > Shared tags is introduced to maintains a notion of fairness between
> > active users. This may be good for nvme with multiple namespace to
> > avoid starving some users. Right?
> 
> Actually nvme namespace is LUN of scsi world.
> 
> Shared tags isn't for maintaining fairness, it is just natural sw
> implementation of scsi host's tags, since every scsi host shares
> tags among all LUNs. If the SCSI host supports real MQ, the tags
> is hw-queue wide, otherwise it is host wide.

From the standards point of view, this statement is incorrect, it's the
ITLQ that identifies the Nexus, so the same tag *may* be in use for
different commands on different LUNS according to standards.

However, some device drivers choose to use ITQ as the identifier
because it fits with the ITLQ standard and they can use Q as a per
target mailbox identifier.

I don't think this affects the analysis, but I just didn't want people
to think it was always true.  We have some older drivers that will
allocate the same tag for different LUNS and this was incredibly
prevalent in the SPI world where the tag was only  bits.

James


  parent reply index

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-01-16  4:06 Yufen Yu
2020-01-16  9:03 ` Ming Lei
2020-01-16 12:17   ` [Question] about " John Garry
2020-01-16 15:17   ` James Bottomley [this message]
2020-01-17  7:19   ` Yufen Yu
2020-01-17 10:16     ` Ming Lei

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