[1904] in linux-scsi channel archive
Re: SCSI throughput question
daemon@ATHENA.MIT.EDU (Leonard N. Zubkoff)
Sat May 24 13:24:11 1997
Date: Sat, 24 May 1997 10:21:34 -0700
From: "Leonard N. Zubkoff" <lnz@dandelion.com>
To: myles@puck.nether.net
CC: linux-scsi@vger.rutgers.edu
In-reply-to: <19970523131757.06820@austin.uyema.net> (message from Myles Uyema
on Fri, 23 May 1997 13:17:57 -1000)
Date: Fri, 23 May 1997 13:17:57 -1000
From: Myles Uyema <myles@puck.nether.net>
I want to get a Buslogic Ultra-Wide SCSI controller, but I currently
know very little about SCSI. Let's say I have a 4x Plextor SCSI cdrom
drive. I also have (two) 2.1 gig Ultra-WIDE scsi hard drives on the same
Buslogic Controller. Is the "bandwidth" lowered during cdrom access?
The bandwidth isn't lowered per se, but any time the CD-ROM spends transferring
data across the SCSI bus is time that's not available for other devices. Let's
suppose your CD-ROM only negotiates SCSI-2 speed (5.0 mega-transfers/second),
rather than the 20.0 mega-transfers/second the disks are capable of (40MB/sec
for Wide Ultra). A 4X CD-ROM can transfer at most 614400 bytes/second which
will consume 12.3% of the available time not counting command overhead. That
still leaves more time than can be consumed by two 2.1GB Wide UltraSCSI drives
running full speed.
What you really want to avoid is putting any devices that don't implement
disconnect/reconnect onto a fast bus. Without disconnect/reconnect, the device
will tie up the bus during seeking as well as during data transfer.
Leonard
