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

Brian Pomerantz
2000-05-19T15:48:42Z

On Fri, May 19, 2000 at 04:09:58PM +0100, Stephen C. Tweedie wrote:
> Hi,
>
> On Thu, May 18, 2000 at 12:55:04PM -0700, Chaitanya Tumuluri wrote:
>
> > I've had the same question in my mind. I've also wondered why raw I/O was
> > restricted to only KIO_MAX_SECTORS at a time.
>
> Mainly for resource limiting --- you don't want to have too much user
> memory pinned permanently at once.
>
> 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, then I suppose it would work well but
the requests would have to make up a contiguous chunk of drive space.

BAPper