Subject: Re: UVM/other problems for desktop users in current?
To: Jukka Marin , Peter Seebach <email@example.com>
From: Daniel Carosone <firstname.lastname@example.org>
Date: 12/18/2002 18:33:27
On Wed, Dec 18, 2002 at 09:23:32AM +0200, Jukka Marin wrote:
> Yep. And I'd appreciate it if someone explained _why_ decent IDE with DMA
> is so bad when compared to SCSI. Especially if you only have one device
> per port and the raw sustained performance is 30 MB/s or more with only a
> little CPU loading.
The main reason is that there's only one outstanding operation to
the disk at once. SCSI disks, with tagged queueing, allow multiple
outstanding requests, that the drive can respond to in the order
that best suits its internal state (disk head position, mostly).
So bandwidth is one thing, scattered transaction rate is another.
Now, recent UDMA modes (I can't remember if it's in 5/100 or only
6/133) apparently also support this for IDE - but NetBSD isn't
using the capability. I don't know that anyone else is either.