Re: PATCH: Enhance queueing/scsi-midlayer to handle kiobufs. [Re: Request splits]

["Stephen C. Tweedie" <sct@redhat.com>]

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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