RE: Disk sizes and performance

Robin,

The number of disk arms is only one element in the story. (It was the way I
looked for disk performance in the past).
We use for the large disk capacities EMC disk subsystem, with 3 Gigabyte
cache in the EMC box.  This means that you have very good disk performance.
(Disadvantage of EMC may be the higher price)
I think IBM comes out right now with a new disk subsystem with an
intelligent cache to improve the performance, and to avoid the problems with
the disk arms. 
 
Paul 

-----Original Message-----
From: Robin Sapiro [mailto:robin.sapiro@home.com]
Sent: Monday, August 02, 1999 2:34 AM
To: MIDRANGE-L@midrange.com
Subject: Disk sizes and performance


I am looking for some ball park estimates on when to use a greater number of
small disk drives versus a smaller number of larger disk drives.

Conventional wisdom is that the more disk arms you have the better the
performance. Obviously this is directly related to the amount of I/O being
done on the system. I feel however that there is some point (total disk
required) at which adding additional disk arms does not have any significant
improvement on performance and at that point it is more beneficial to use
larger disks which results in financial savings in the form or lesss $/GB,
less controllers, less racks etc.

Any opinions as to what the magic number might be. ie if less than n GB on a
system, use 4 GB drives. for n-m GB on a system use 8.5 GB drives and for
greater than m GB on a system use 17 GB drives.

Also what is the general opinion as to the DASD % utilisation at which I/O
performance begins to degrade.

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