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

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Date:	Wed, 16 Jul 2014 18:02:14 +0000
From:	James Bottomley <jbottomley@...allels.com>
To:	"martin.petersen@...cle.com" <martin.petersen@...cle.com>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hch@...radead.org" <hch@...radead.org>,
	"devel@...uxdriverproject.org" <devel@...uxdriverproject.org>,
	"apw@...onical.com" <apw@...onical.com>,
	"kys@...rosoft.com" <kys@...rosoft.com>,
	"stable@...r.kernel.org" <stable@...r.kernel.org>,
	"linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>,
	"ohering@...e.com" <ohering@...e.com>,
	"jasowang@...hat.com" <jasowang@...hat.com>
Subject: Re: [PATCH 4/8] Drivers: scsi: storvsc: Filter WRITE_SAME_16

On Wed, 2014-07-16 at 13:47 -0400, Martin K. Petersen wrote:
> >>>>> "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.

Hang on ... unless we apply Christoph or my fix, we'll get the same
issue with every raid driver (that's about 10 I think) that set
no_write_same when they hit a >2TB RAID volume, so I think we need both
fixes.

James

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