Re: [PATCH] 64 bit scsi read/write

Jonathan Lundell
2001-07-15T17:45:11Z

At 5:10 AM +1200 2001-07-16, Chris Wedgwood wrote:
>On Sat, Jul 14, 2001 at 09:45:44AM +0100, Alan Cox wrote:
>
> As far as I can tell none of them at least in the IDE world
>
>Can you test with the code I posted a hour or so ago please?

AC's comment was about whether the drive's cache would be written out
on power failure, which is another issue, a little harder to test
(and not easily testable by writing a single sector). I raise the
related question of what happens to the write cache on a bus reset on
SCSI drives.

>I ask this because I tested writes to:
>
> -- buffered devices
>
> -- ide with caching on
>
> -- ide with caching off
>
> -- scsi (caching on?)
>
>To a buffered device, I get something silly like 63000
>writes/second. No big surprises there (other than Linux is bloody lean
>these days).
>
>To a SCSI device (10K RPM SCSI-3 160 drive), I get something like 167
>writes/second, which seems moderately sane if caching is disabled.

My impression, based a a little but not much research, is that most
SCSI drives disable write caching by default. IBM SCSI drives may be
an exception to this.

>To a cheap IDE drive (5400 RPM?) with caching off, I get about 87
>writes/second.
>
>To the same drive, with caching on, I get almost 4000 writes/second.
>
>This seems to imply, at least for my test IDE drive, you can turn
>caching off --- and its about half as fast as my SCSI drives which
>rotate at about twice the speed (sanity check).
>
>IDE drive: IBM-DTTA-351010, ATA DISK drive
>SCSI drive: SEAGATE ST318404LC

--
/Jonathan Lundell.