Spare disks
Spares are disks that are not active members of any particular array, but have been configured
to be used when a disk in one of the arrays fails. If a spare is present, it will immediately be used
to begin rebuilding the information that was on the failed disk, using parity information from the
other member disks. During the rebuilding process, the array is operating in a reduced state and,
unless it is a RAID6 or RAID1+0 array, it cannot tolerate another disk failure in the same array.
If another disk fails at this time, the array becomes inaccessible and information stored there
must be restored from backup.
After the rebuild of the data onto the spare is completed, when a replacement drive is inserted
to replace the failed drive, the system will automatically transfer the data from the spare onto the
replacement drive and return the spare to an available-spare state. It is important to note that
the process of rebuilding the spare or the replacement drive must not be interrupted or the process
will be aborted.
Some administrators have multiple spare disks, so that multiple arrays can experience failure
and successfully recover, before administrative intervention would be required to replace the
spare or failed disk. When assigning a spare to an array, the administrator chooses which arrays
and how many arrays are protected by that spare.
Array sizing
As a general rule, the greater the number of drives that are included in an array, the greater the
performance level that can be achieved. However, performance considerations are offset by fault
tolerance considerations. The greater the number of drives in an array, the higher the probability
of one or more disk failures in that array. The administrator must strike a balance between
performance and fault tolerance.
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Installation
