Disk sizes and performance

["Robin Sapiro" <robin.sapiro@xxxxxxxx>]

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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