  Choosing a disk storage upgrade? 
 The Question is:
 
I'm looking to add storage capacity to my CI cluster
by purchasing an sw800 cabinet with dual redundant controllers
and dual power supplies per shelf, and configure some RAID 3/5 arrays.
 
When purchasing disks,
would you get better performance by purchasing 4.3GB drives
or half as many 9.1 GB drives to get the same RAID volume capacity?
4.3 GB drives are 7200 RPM where 9.1GB drives are 10,000 RPM
You can get 1-9.1 gb drive for less $$ than 2-4.3GB drives.
 
Would you get better performance with the increased number of
spindles with the slower drives? Using lower capacity drives reduces
your expansion capabilities by using twice as many shelf bays.
 
What about the same scenareo with 9.1 vs 18 GB drives?
 
How to choose... Please, Oh Wizard, give me some clues....
 
 The Answer is:
 
  There is no answer to this question without substantially more site
  characterization information.
 
  The OpenVMS performance management manual may be of interest.
 
  In isolation, more spindles will provide better I/O throughput, and
  fewer spindles will provide more storage expansion.
 
  That said, disk performance is seldom viewed in isolation.  You must take
  into account application requirements, CPU I/O completion capacity, the
  available host memory for caching, host controller bandwidth (the CI,
  in this case), SCSI bandwidth, storage controller bandwidth, available
  storage controller interconnect (CI) bandwidth, and various other factors.
  Disk-specific factors include the transfer rate and particularly the
  spiral read read, as well as the access time -- characteristics of the
  application I/O activity dictate if access time (many small I/Os) or
  spiral read rate (fewer larger I/Os) will be of central interest.
 
  Additional considerations include expansion capabilities, as you stated,
  and the ability of the controllers to perform RAID functions such as
  striping and shadowing (mirroring).  Also of interest will be your level
  of interest in managing the low-level behaviour of the applications and
  of the disk I/O subsystem.  (RAID controllers can often be configured to
  transparently divide I/O operations across multiple disk spindles, thus
  increasing I/O throughput.)
 
  In this particular configuration, you will particularly want to consider
  the bandwidth of the CI adapters and the activity on the CI buses.
 
  Unless your present storage configuration is severely I/O bound and you
  have similar storage technologies with those of your planned upgrade,
  you will likely see improvements from any storage upgrade.  (Barring an
  existing I/O performance limit -- CI bandwidth, for instance -- or an
  existing non-I/O performance limit -- eg: CPU performance -- that is not
  specifically related to the disk or RAID controller performance.)  Thus
  the use of the larger disks may be more cost-effective.
 
 
