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30 Chapter 3 Arrays, Disks, and RAID Levels
Distributed Sparing (supported only by RAIDCore VST)
RAIDCore’s proprietary distributed sparing feature reserves space on each disk in an array. This space is used
when the data from a failed disk is being regenerated during a Restore task.
Whereas other methods of sparing (such as dedicated spares) provide the same level of protection, distributed
sparing provides better performance. All disks are active in an array and are not sitting idle, as is the case with
dedicated spares.
Another advantage of distributed spares is that because all disks are active, a disk cannot fail and go unnoticed,
as is the case with dedicated or global spares.
With distributed sparing, each array has its own dedicated fail-over spaces. This averts the potential problem of
having insufficient space to start a fail-over on the single disk that has been assigned as a spare.
A distributed spare is assigned at the time an array is created or transformed. Distributed spares are valid only
for:
• RAID 5 (four or more disks)
• RAID 50 (four or more disks per RAID 5 set)
• RAID 10 and RAID 10n (six or more disks).
This spare type is the most protective, because space is allocated when the array is created. Like a dedicated
spare, this spare type is assigned to a specific array.
If an array was created without a distributed spare, the array must be transformed to add a distributed spare. A
distributed spare can be assigned only when an array is being created or transformed.
• If there is insufficient unused space on the disks in an array, a distributed spare cannot be added without
adding an additional disk and transforming it to an array (with distributed sparing enabled).
• The distributed spare option uses the equivalent storage of one of the disks in the array.
For example: the total capacity of six disks is being used in a RAID 5 array, and distributed sparing has been
enabled.
• The capacity of the array is the same as a four-disk RAID 0.
• The capacity of one disk is lost to the RAID 5 parity data.
• The capacity of another disk is lost to the distributed sparing option.
For additional information, see Add or Remove Distributed Spares (supported only by RAIDCore VST) on
page 92.
Dedicated Sparing
A dedicated spare is a disk that is assigned as an alternate disk for a specific array. Should a disk fail in that
array, the alternate disk is used to replace the failed disk and the array is rebuilt.
A dedicated spare can be assigned to any redundant array type, and up to four spares can be assigned to an
array.
NOTE:
Assigning a dedicated spare does not reserve space on the disk. Therefore, an automatic restore is not
guaranteed if a disk fails. If a disk fails, make room on the disk for the fail-over to complete, or assign a
different disk with enough room. If a dedicated spare is assigned and a disk fails, the restore process starts
automatically, if there is enough space available on the dedicated spare.
NOTE:
A dedicated spare cannot be assigned while a task is running on the array.
NOTE:
When a dedicated spare is assigned while an array is being created or transformed, the first disk
selected to create the array is assigned by default as the dedicated spare.