[8857] in linux-scsi channel archive
Re: PATCH: Enhance queueing/scsi-midlayer to handle kiobufs. [Re: Request splits]
daemon@ATHENA.MIT.EDU (Stephen C. Tweedie)
Fri May 19 12:02:15 2000
Date: Fri, 19 May 2000 16:55:02 +0100
From: "Stephen C. Tweedie" <sct@redhat.com>
To: "Stephen C. Tweedie" <sct@redhat.com>,
Chaitanya Tumuluri <chait@getafix.engr.sgi.com>,
Eric Youngdale <eric@andante.org>,
Alan Cox <alan@lxorguk.ukuu.org.uk>,
Douglas Gilbert <dgilbert@interlog.com>, linux-scsi@vger.rutgers.edu,
chait@sgi.com, linux-mm@kvack.org
Message-ID: <20000519165502.G9961@redhat.com>
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In-Reply-To: <20000519084842.A3373@skull.piratehaven.org>; from bapper@piratehaven.org on Fri, May 19, 2000 at 08:48:42AM -0700
Hi,
On Fri, May 19, 2000 at 08:48:42AM -0700, Brian Pomerantz wrote:
> > The real solution is probably not to increase the atomic I/O size, but
> > rather to pipeline I/Os. That is planned for the future, and now there
>
> That really depends on the device characteristics. This Ciprico
> hardware I've been working with really only performs well if the
> atomic I/O size is >= 1MB. Once you introduce additional transactions
> across the bus, your performance drops significantly. I guess it is a
> tradeoff between latency and bandwidth. Unless you mean the low level
> device would be handed a vector of kiobufs and it would build a single
> SCSI request with that vector,
ll_rw_block can already do that, but...
> then I suppose it would work well but
> the requests would have to make up a contiguous chunk of drive space.
... a single request _must_, by definition, be contiguous. There is
simply no way for the kernel to deal with non-contiguous atomic I/Os.
I'm not sure what you're talking about here --- how can an atomic I/O
be anything else? We can do scatter-gather, but only from scattered
memory, not to scattered disk blocks.
--Stephen
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