When you create a redundant virtual disk using a RAID controller, you have the opportunity to maintain system operations even when a disk fails. To do so, you would assign a hot spare to the virtual disk. When a disk fails, the redundant data is rebuilt onto the hot spare without interrupting system operations.
Understanding Hot Spares
A hot spare is an unused backup physical disk that can be used to rebuild data from a redundant virtual disk. Hot spares remain in standby mode. When a physical disk that is used in a redundant virtual disk fails, the assigned hot spare is activated to replace the failed physical disk without interrupting the system or requiring your intervention. If a virtual disk using the failed physical disk is not redundant, then the data is permanently lost without any method (unless you have a backup) to restore the data.
Hot spare implementation is different for different controllers. See the following sections for more information.
Considerations for Hot Spares on PERC 5/E, PERC 5/i, PERC 6/E, PERC 6/I, and CERC 6/I Controllers
On the PERC 5/E, PERC 5/i, PERC 6/E, PERC 6/I, and CERC 6/I controllers, assigning a hot spare is equivalent to assigning a physical disk to replace another physical disk if it fails. If more than one redundant virtual disk resides on the physical disk, then all redundant portions of the physical disk are rebuilt.
NOTE: When rebuilding a physical disk, you need to delete any nonredundant
virtual disks (such as RAID 0) that reside on the physical disk before rebuilding the
physical disk.
When creating a virtual disk, the physical disks included in the virtual disk can be different sizes. When assigning a hot spare to a RAID 1 or 5 virtual disk, the hot spare only needs to be the same size (or larger) as the smallest physical disk included in the virtual disk.
This is because when using a PERC 5/E, PERC 5/i, PERC 6/E, PERC 6/I, and CERC 6/I controller, you can assign physical disks of different sizes to a virtual disk. When you have fully consumed a smaller physical disk with a virtual disk, however, any portion of larger physical disks that are not consumed by the virtual disk become unusable. Therefore, there is no data on the unused portion of a larger disk that needs to be rebuilt. A redundant virtual disk will also be either striped or mirrored in equal portions across its member physical disks. The amount of data requiring a rebuild will therefore not be larger than the smallest physical disk.
A RAID 10 or 50 virtual disk may include spans that have physical disks of different sizes. In this case, you should identify the span that has the largest "small" physical disk. The hot spare should be large enough to rebuild this physical disk. For example, if one span has three physical disks that are 60 MB, 60 MB and 40 MB and another span has physical disks that are 60 MB, 60 MB, and 50 MB, then the hot spare must be 50 MB or larger.
A dedicated hot spare can only be assigned to the set of virtual disks that share the same physical disks. A global hot spare is assigned to all redundant virtual disks on the controller. A global hot spare must be the same size (or larger) as the smallest physical disk included in any virtual disk on the controller.
After you have assigned a global hot spare, any new virtual disks created on the controller will not be protected by the hot spare if the controller is a SAS controller and the disk size is larger than the global hot spare.
In this case, you can unassign the global hot spare after creating a new virtual disk and then assign a new and larger hot spare to cover all redundant virtual disks on the controller. See "RAID Controller Technology: SAS" to determine whether the controller is using SAS technology.
Dedicated Hot Spare Considerations
The following considerations apply to dedicated hot spares:
Considerations for RAID 10 and RAID 50. If you have created a RAID 10 or RAID 50 virtual disk that does not fully consume its member physical disks, then you will not be able to assign a dedicated hot spare to the RAID 10 or RAID 50 virtual disk. Storage Management does not allow you to create RAID 10 and RAID 50 virtual disks from partial physical disks. You will therefore not encounter this situation if you are using Storage Management to create your virtual disks. If, however, the RAID 10 or 50 virtual disk was created using another application and if it does contain partial physical disks, then you will not be able to assign a dedicated hot spare to the virtual disk.
Considerations for Multiple Dedicated Hot Spares. Storage Management does not allow you to assign more than one dedicated hot spare to a virtual disk. On some controllers, you can use the BIOS to assign more than one dedicated hot spare. In this case, Storage Management recognizes the hot spares assigned in the BIOS but does not allow you to assign additional dedicated hot spares.
Global Hot Spare Considerations on a SAS 6/iR
The SAS 6/iR controller enables you to assign two global hot spares. The controller firmware remembers the hot spare assignment even after the physical disks that you assigned as hot spares have been removed. In other words, in the case of a disk removal, the firmware may assume that a hot spare is present when it is not. In this case, the firmware may prevent you from assigning a new global hot spare as the firmware assumes that a global hot spare is already assigned.
When a physical disk fails in a redundant virtual disk, the failed disk is rebuilt onto the hot spare. In this case, the controller firmware reassigns the slot containing the failed disk as the hot spare. In this circumstance, a disk not previously assigned as a global hot spare becomes a hot spare through failure or removal.
To ensure that the controller firmware always has a healthy physical disk as a global hot spare, do the following:
When removing a physical disk that is assigned as a global hot spare, unassign the hot spare before removal and reassign another physical disk as the global hot spare.
Immediately replace any physical disk that has failed or been removed. This ensures that a healthy disk resides in a slot that the controller firmware assumes is a hot spare.
