On Tue, Feb 06 2001, [EMAIL PROTECTED] wrote:
> >It depends on the device driver. Different controllers will have
> >different maximum transfer size. For IDE, for example, we get wakeups
> >all over the place. For SCSI, it depends on how many scatter-gather
> >entries the driver can push into a single on-the-wire request. Exceed
> >that limit and the driver is forced to open a new scsi mailbox, and
> >you get independent completion signals for each such chunk.
SCSI does not build a request bigger than the low level driver
can handle. If you exceed the scatter count in a single request,
you just stop and fire of that request, later on restarting I/O
on the remainder.
> I see. I remember Jens Axboe mentioning something like this with IDE.
> So, in this case, you want every such chunk to check if its completed
> filling up a buffer and then trigger a wakeup on that ?
Yes. Which is why dealing with buffer heads are so nice in this
regard, you never have problems with ending I/O on a single "piece".
> But, does this also mean that in such a case combining requests beyond this
> limit doesn't really help ? (Reordering requests to get contiguity would
> help of course in terms of seek times, I guess, but not merging beyond this
> limit)
There's a slight benefit in building bigger requests than the driver
can handle, in that you can have more I/O pending on the queue. It's
not worth spending too much time on though.
--
Jens Axboe
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