SCSI & storage

[Paul Dickson]

Take my comments with a grain of salt.  My experience is with fixing
such machines rather than setting them up and benchmarking.


On Thu, 22 Mar 2001 10:46:10 -0700, Trent Shipley wrote:

 
> 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.

Hardware has changed a lot in the decade.  There are hardware RAID
controllers (with CPUs and battery backed-up RAM) and hard drives with
very large buffers and extremely fast rotation speeds.

> 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.

I'm not sure, but I think channel=chain.  I don't think this a
applicable any more.  Be sure you're using a hardware RAID controller.
 
> 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 . . .)

You can put them all on the same channel.  If you're using HW RAID,
you DB software will be the slow point.

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

No experience with this.

> 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?

I believe FC is about 800Mb/s on copper wire, faster if you use
optical but I don't know the number.  For SCSI, the number floating to
the top of my head is 160 MB/s or 1280Mb/s.  Optical FC is therefore
comparable.

> 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

FC lets you have 256 drives rather than 16.  It allows redundant paths
to RAID arrays.  Thus, allows systems to share the arrays for
fail-over clustering.


	-Paul