Most sites have some kind of inventory of common parts. We discuss spares
in general in Section 4.1.4, but storage deserves a bit of extra attention.
There used to be large differences between the types of drives used in storage
systems and the ones in desktop systems. This meant that it was much
easier for SAs to dedicate a particular pool of spare drives to infrastructure
use. Now that many storage arrays and workgroup servers are built from
off-the-shelf parts, those drives on the shelf might be spares that could be
used in either a desktop workstation or a workgroup storage array. A common
spares pool is usually considered a good thing. However, it may seem
arbitrary to a customer who is denied a new disk but sees one sitting on a
shelf unused, reserved for the next server failure. How can SAs make sure to
reserve enough drives as spares for vital shared storage while not hoarding
drives that are also needed for new desktop systems or individual customer
needs? It is something of a balancing act, and an important component is
a policy that addresses how spares will be distributed. Few SAs are able to
stock as many spares as they would like to have around, so having a system
for allocating them is crucial.
It’s best to separate general storage spares from infrastructure storage
spares. You can make projections for either type, based on failures observed
in the past on similar equipment. If you are tracking shared storage usage—
and you should be, to avoid surprises—you can make some estimates on how
often drives fail, so that you have adequate spares.
For storage growth, include not only the number of drives required to
extend your existing storage but also whatever server upgrades, such as CPU
and memory, might also be needed. If you have planned to expand by acquiring
whole new systems, such as stand-alone network storage arrays, be sure
to include spares for those systems through the end of the fiscal year when
they will be acquired.