SCSI & storage

[Trent Shipley]

Our design engineer has decided to use SCSI-3 instead of fiber channels for
mass storage.

The reason is that on the level of one drive, SCSI-3 is about 50% faster
than current optical connections to storage.  It is also a less expensive
commodity level solution, but the main determinant is the speed.

Since I am the junior DB analyst/administrator/developer/designer I have no
say in the final decision, but can get blamed if the final decision is
sub-optimal.

As the junior DB jack-of-all-trades, I am more concerned with the overall
cost-of-bandwidth to storage than with the theoretical performance of a
given device on a specified channel.  PCI slots and density are secondary
concerns.  Power usage is a tertiary concern because it effects our
customers TCO.

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I have repeatedly read that there should be no more than 1 IDE device on a
channel or 3 SCSI devices if you want sustained throughput.  However, all
these sources quote a SUN oriented analysis of system performance tuning
published in the early 1990's.

Questions:

1) Is the 3 SCSI drives per "channel":
   a) still current.
   b) applicable to a given SCSI chain
   c) applicable to a given SCSI controller no matter how many chains it
says it supports.

2) We mirror all our drives, and use RAID 10 on all larger installations.
If we stick with SCSI-3 should we:
   a) Only put 2 drives on a chain
   b) Put four drives on a chain.
   c) Put three drives on a chain but use a partitioning scheme to balance
mirroring.  (I sense the technicians who have to build the thing preparing
to lynch me . . .)

3) How many drives can go on an optical channel before they start to
interfere with each other and bottleneck?

4)  Fiber solutions are slower per drive.  Without regard to cost, can I get
more bandwidth out of an array of RAID 10 if they use a fiber optic protocol
instead of SCSI-3?

5) Assuming RAID 10, which costs less per Mbit-sec, SCSI-3 or fiber channel?
   a)  Assume 7 devices.  RAID 10 uses six drives and the seventh is a
spare.
   b)  On a WinTel box, when will you be forced to use a fiber-optical
solution because you run out of slots for SCSI cards?
   c)  Is there a point when the SCSI and Fiber Channel cost lines cross?




Trent Shipley

Work:
(602) 522-7502
mailto:tshipley@symbio-tech.com
http://www.symbio-tech.com