linux-kernel - Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME

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Date:	Wed, 16 Jul 2014 13:47:35 -0400
From:	"Martin K. Petersen" <martin.petersen@...cle.com>
To:	"hch\@infradead.org" <hch@...radead.org>
Cc:	"Martin K. Petersen" <martin.petersen@...cle.com>,
	KY Srinivasan <kys@...rosoft.com>,
	James Bottomley <jbottomley@...allels.com>,
	"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
	"devel\@linuxdriverproject.org" <devel@...uxdriverproject.org>,
	"apw\@canonical.com" <apw@...onical.com>,
	"stable\@vger.kernel.org" <stable@...r.kernel.org>,
	"linux-scsi\@vger.kernel.org" <linux-scsi@...r.kernel.org>,
	"ohering\@suse.com" <ohering@...e.com>,
	"jasowang\@redhat.com" <jasowang@...hat.com>
Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16

>>>>> "Christoph" == hch@...radead org <hch@...radead.org> writes:

Christoph> Oh, we actually have devices that support WRITE SAME with
Christoph> unmap, but not without?  That's defintively a little strange.

Yep :(

There were several SSDs that did not want to support wearing out flash
by writing gobs of zeroes and only support the UNMAP case.

Christoph> Yes, and it did this intentionally.  I really wouldn't expect
Christoph> devices to support WRITE SAME with UNMAP but blow up on a
Christoph> WRITE SAME without it (and not just simple fail it in an
Christoph> orderly way).

*sigh*

Christoph> It definitively seems odd to default to trying WRITE SAME for
Christoph> unmap for a device that explicitly tells us that it doesn't
Christoph> support WRITE SAME.

Maybe it's just a naming thing. I was really trying to convey
no_req_write_same support, not no_write_same_10_or_16.

Christoph> Note that I'm not against your patch - I suspect forcing us
Christoph> to read EVPD pages even for devices that claim to be SPC-2
Christoph> will come in useful in various scenarios.

I don't have a problem with a BLIST_PREFER_UNMAP flag or something like
that. But BLIST_TRY_VPD_PAGES seems more generally useful and it does
fix the problem at hand. That's why I went that route.

-- 
Martin K. Petersen	Oracle Linux Engineering
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