linux-kernel - Re: Integration of SCST in the mainstream Linux kernel

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Message-Id: <1201710871.3292.22.camel@localhost.localdomain>
Date:	Wed, 30 Jan 2008 10:34:31 -0600
From:	James Bottomley <James.Bottomley@...senPartnership.com>
To:	Bart Van Assche <bart.vanassche@...il.com>
Cc:	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>, rdreier@...co.com,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	vst@...b.net, linux-scsi@...r.kernel.org,
	scst-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org
Subject: Re: Integration of SCST in the mainstream Linux kernel


On Wed, 2008-01-30 at 09:38 +0100, Bart Van Assche wrote:
> On Jan 30, 2008 12:32 AM, FUJITA Tomonori <fujita.tomonori@....ntt.co.jp> wrote:
> >
> > iSER has parameters to limit the maximum size of RDMA (it needs to
> > repeat RDMA with a poor configuration)?
> 
> Please specify which parameters you are referring to. As you know I
> had already repeated my tests with ridiculously high values for the
> following iSER parameters: FirstBurstLength, MaxBurstLength and
> MaxRecvDataSegmentLength (16 MB, which is more than the 1 MB block
> size specified to dd).

the 1Mb block size is a bit of a red herring.  Unless you've
specifically increased the max_sector_size and are using an sg_chain
converted driver, on x86 the maximum possible transfer accumulation is
0.5MB.

I certainly don't rule out that increasing the transfer size up from
0.5MB might be the way to improve STGT efficiency, since at an 1GB/s
theoretical peak, that's roughly 2000 context switches per I/O; however,
It doesn't look like you've done anything that will overcome the block
layer limitations.

James


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