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

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Message-ID: <1405536631.3165.23.camel@dabdike.int.hansenpartnership.com>
Date:	Wed, 16 Jul 2014 18:50:31 +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 14:45 -0400, Martin K. Petersen wrote:
> >>>>> "James" == James Bottomley <jbottomley@...allels.com> writes:
> 
> >> 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.
> 
> James> Hang on ... unless we apply Christoph or my fix, we'll get the
> James> same issue with every raid driver (that's about 10 I think) that
> James> set no_write_same when they hit a >2TB RAID volume,
> 
> no_write_same is set for all the RAID controller drivers (54b2b50c20a6).
> 
> If a WRITE SAME(10/16) command fails and the UNMAP bit is not set we'll
> set no_write_same=1 and disable REQ_WRITE_SAME support.
> 
> If a WRITE SAME(10/16) command fails and the UNMAP bit is set we'll
> disable REQ_DISCARD support.
> 
> Not sure where the 10 vs. 16 byte 2TB limitation comes into play here?

It's the code we identified in sd.c:read_capacity_16():

	if (buffer[14] & 0x80) { /* LBPME */
		sdkp->lbpme = 1;

		if (buffer[14] & 0x40) /* LBPRZ */
			sdkp->lbprz = 1;

		sd_config_discard(sdkp, SD_LBP_WS16);
	}

If the inquiry shows LBPME set, we'll do write same regardless of the
no_write_same bit.  That's why for SPC-2 devices it only shows up on
>2TB devices because they force RC16.

James

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