linux-kernel - Re: [PATCH v5 2/2] Add support for SCT Write Same

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Message-ID: <yq1popgrsw0.fsf@sermon.lab.mkp.net>
Date:	Wed, 10 Aug 2016 21:33:51 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Shaun Tancheff <shaun.tancheff@...gate.com>
Cc:	Tom Yan <tom.ty89@...il.com>, Shaun Tancheff <shaun@...cheff.com>,
	linux-ide@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>, Tejun Heo <tj@...nel.org>,
	Josh Bingaman <josh.bingaman@...gate.com>
Subject: Re: [PATCH v5 2/2] Add support for SCT Write Same

>>>>> "Shaun" == Shaun Tancheff <shaun.tancheff@...gate.com> writes:

Shaun,

Shaun> You are correct in that we can advertise the larger limit in
Shaun> ata_scsi_dev_config() when only SCT write same is supported
Shaun> rather than fall back to WS10.

I deliberately capped WRITE SAME to 64K blocks unless otherwise reported
by the device because:

        a) Several older drives supported the WRITE SAME(16) command but
        ignored the upper bytes of the transfer length effectively
        turning it into a WRITE SAME(10).

        b) 64K blocks was the sweet spot for older drives as well, a
        size commonly used by RAID array firmwares that were the only
        commonplace users of the WRITE SAME family.

Shaun> I really am not sure what would be considered the correct
Shaun> solution though. I believe that the WRITE SAME defaults are
Shaun> currently being chosen around physical limits.

Lacking a solid reporting facility in the spec, I suggest you just leave
it unset and let the SCSI defaults apply.

Shaun> I think we can also bump the command timeout for WRITE SAME?

The default WRITE SAME timeout is 120s.

-- 
Martin K. Petersen	Oracle Linux Engineering