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

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Date:	Wed, 10 Aug 2016 21:47:40 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Tom Yan <tom.ty89@...il.com>
Cc:	Shaun Tancheff <shaun.tancheff@...gate.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

>>>>> "Tom" == Tom Yan <tom.ty89@...il.com> writes:

Tom> Well that is actually the minimum. Modern SSDs often support more
Tom> than one-block payload (e.g. 8, 16...).  It's just our SCSI disk
Tom> driver statically limit it to the minimum. Though it allows only
Tom> 0xffffffff / 512 = 8388607 (SD_MAX_WS16_BLOCKS) blocks per WRITE
Tom> SAME (16) command anyway,

Yes, because that's the biggest command we can express in the block
layer.

Tom> so we can at most allow only a 2-block (well, or 3-block) payload.

We tried turning on multi block payloads and it was a massive disaster.
Many drives reported that they supported 8 block payloads but actually
didn't. Instead of playing the blacklist game we capped it at a single
sector.

Many drives from different vendors were affected by this. So we'd have
to make multi block payloads an explicit opt-in like we did for
discard_zeroes_data. However, given that "big" discards are mainly done
synchronously when creating filesystems, I am not sure there is any real
benefit to this.

-- 
Martin K. Petersen	Oracle Linux Engineering