Re: DIOC{G,S}STRATEGY not implemented for all disk devices

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Wed, Jan 16, 2008 at 09:22:17PM +1030, Brett Lymn wrote:
> On Wed, Jan 16, 2008 at 10:19:28AM +0200, Alan Barrett wrote:
> >
> > Why don't the DIOC{G,S}STRATEGY ioctls work for all disk devices?
> >
> 
> Perhaps because they are not needed any more - back in the old days
> when disk devices were dumber doing clever ordering on the host
> allowed you to optimise the disk access but in these days of large on
> disk caches and tagged queuing the order in which the commands are
> submitted to the disk may not be the order in which they are executed
> by the disk device so there is little point in doing any strategising
> on the host side (I think)

There is still some point, to e.g. balance between read and writes.
Otherwise you can come to the point where a process doing large writes
will delay reads for a long time.
Of course we may need something even more clever than just 2 priorities.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--