linux-kernel - Re: [PATCH 2/3] target: Add max_write_same

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Date:	Thu, 15 Nov 2012 11:23:12 -0800
From:	"Nicholas A. Bellinger" <nab@...ux-iscsi.org>
To:	Christoph Hellwig <hch@...radead.org>
Cc:	target-devel <target-devel@...r.kernel.org>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Christoph Hellwig <hch@....de>,
	"Martin K. Petersen" <martin.petersen@...cle.com>
Subject: Re: [PATCH 2/3] target: Add max_write_same_len device attribute

On Thu, 2012-11-15 at 05:53 -0500, Christoph Hellwig wrote:
> On Thu, Nov 08, 2012 at 08:07:17PM +0000, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@...ux-iscsi.org>
> > 
> > This patch adds a new max_write_same_len device attribute for use with
> > WRITE_SAME w/ UNMAP=0 backend emulation.
> > 
> > Also, update block limits VPD emulation code in spc_emulate_evpd_b0() to
> > set the default MAXIMUM WRITE SAME LENGTH value of zero.
> 
> why do we need an exposed attribute for this?
> 

This is useful for userspace to reduce the allowed maximum from the
default 0xFFFF set by IBLOCK.  Allowing huge WRITE_SAMEs can very much
effect performance (esp. with spinning media), so being able to reduce
the max we accept via a userspace tunable is helpful.

--nab

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