linux-kernel - Re: [PATCH 3.16 106/217] sd: disable discard_zeroes

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Message-ID: <yq1a8keeh3z.fsf@sermon.lab.mkp.net>
Date:	Wed, 27 Apr 2016 20:01:36 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	Rafael David Tinoco <rafael.tinoco@...onical.com>
Cc:	Ben Hutchings <ben@...adent.org.uk>, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org, akpm@...ux-foundation.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Christoph Hellwig <hch@....de>
Subject: Re: [PATCH 3.16 106/217] sd: disable discard_zeroes_data for UNMAP

>>>>> "Rafael" == Rafael David Tinoco <rafael.tinoco@...onical.com> writes:

Rafael> It seems that changing discard method from UNMAP to WRITE
Rafael> SAME(16) without using NDOB bit (as first described in
Rafael> sbc3r35b.pdf) can cause performance problems on big discards
Rafael> (since data-out buffer will be checked for every WRITE SAME
Rafael> command). I think this is happening after this commit, since
Rafael> NDOB bit wasn't implemented with this change (afaik, iirc).

We don't currently use NDOB since there is no VPD parameter to key off
of. I am also not aware of any devices that actually support it.

I am working on some patches that changes how we submit write same and
unmap down the stack to make it easier for the target subsystem to
provide zeroing guarantees. Those changes will clean some of this up but
they are 4.8 material.

-- 
Martin K. Petersen	Oracle Linux Engineering