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

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Date:	Tue, 5 Feb 2008 20:02:06 +0100
From:	"Bart Van Assche" <bart.vanassche@...il.com>
To:	"Erez Zilber" <erezz@...taire.com>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Vladislav Bolkhovitin" <vst@...b.net>,
	James.Bottomley@...senpartnership.com,
	"FUJITA Tomonori" <fujita.tomonori@....ntt.co.jp>,
	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 Feb 5, 2008 6:10 PM, Erez Zilber <erezz@...taire.com> wrote:
> One may claim that STGT should have lower performance than SCST because
> its data path is from userspace. However, your results show that for
> non-IB transports, they both show the same numbers. Furthermore, with IB
> there shouldn't be any additional difference between the 2 targets
> because data transfer from userspace is as efficient as data transfer
> from kernel space.
>
> The only explanation that I see is that fine tuning for iSCSI & iSER is
> required. As was already mentioned in this thread, with SDR you can get
> ~900 MB/sec with iSER (on STGT).

My most recent measurements also show that one can get 900 MB/s with
STGT + iSER on an SDR IB network, but only for very large block sizes
(>= 100 MB). A quote from Linus Torvalds is relevant here (February 5,
2008):

    Block transfer sizes over about 64kB are totally irrelevant for
99% of all people.

Please read my e-mail (posted earlier today) with a comparison for 4
KB - 64 KB block transfer sizes between SCST and STGT.

Bart Van Assche.
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