Understanding Hot Spares Go to Table of Contents for Storage Management Online Help
Dell OpenManage™ Storage Management User's Guide
Considerations for Hot Spares on PERC 2, 2/Si, 3/Si, 3/Di, and CERC SATA1.5/6ch Controllers
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.
A hot spare is an unused backup array disk that can be used to rebuild data from a redundant virtual disk. Hot spares remain in standby mode. When an array disk that is used in a redundant virtual disk fails, the assigned hot spare is activated to replace the failed array disk without interrupting the system or requiring your intervention. If a virtual disk using the failed array 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.
For these controllers, assigning a hot spare is equivalent to assigning an array disk to replace another array disk. If more than one redundant virtual disk resides on the array disk, then all redundant portions of the array disk are rebuilt. Note however, that you need to delete any non-redundant virtual disks (such as RAID 0) that reside on the array disk before rebuilding the array disk. See Considerations for Hot Spares on PERC 3/SC, 3/DCL, 3/DC, 3/QC, 4/SC, 4/DC, 4e/DC, 4/Di, 4e/Si, 4e/Di, and CERC ATA100/4ch Controllers for more information.
For these controllers, a hot spare is assigned to a virtual disk. When an array disk fails, only the portion of the array disk containing the virtual disk is rebuilt onto the hot spare. Data or space on the array disk not included in the virtual disk will not be rebuilt. See Considerations for Hot Spares on PERC 2, 2/Si, 3/Si, 3/Di, and CERC SATA1.5/6ch Controllers for more information.
The following sections describe procedures for assigning a hot spare:
When creating a virtual disk, the array disks included in the virtual disk can be different sizes. When assigning a hot spare on a PERC 3/SC, 3/DCL, 3/DC, 3/QC, 4/SC, 4/DC, 4e/DC, 4/Di, 4e/Si, 4e/Di, or CERC ATA100/4ch controller, the hot spare only needs to be the same size (or larger) as the smallest array disk included in the virtual disk.
This is because when using a PERC 3/SC, 3/DCL, 3/DC, 3/QC, 4/SC, 4/DC, 4e/DC, 4/Di, 4e/Si, 4e/Di, or CERC ATA100/4ch controller, you can assign array disks of different sizes to a virtual disk. Once you have fully consumed a smaller array disk with a virtual disk, however, any portion of larger array 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 array disks. The amount of data requiring a rebuild will therefore not be larger than the smallest array disk.
A dedicated hot spare can only be assigned to a single virtual disk. 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 array disk included in the largest virtual disk on the controller.
Once 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 partition size of the disk 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.
On the PERC 3/SC, 3/DCL, 3/DC, 3/QC, 4/SC, 4/DC, 4e/DC, 4/Di, 4e/Si, 4e/Di, and CERC ATA100/4ch controllers, the virtual disk state is not updated until the controller performs an I/O operation. This means that when a redundant virtual disk is degraded on one of these controllers, the hot spare will not be activated until the controller performs an I/O operation. See I/O Requirements for Detecting Drive Status Changes for more information.
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Note: The Assign and Unassign Dedicated Hot Spare command is not available on the PERC 2/SC and PERC 2/DC controllers. |
If you have created a RAID 10 or RAID 50 virtual disk that does not fully consume its member array 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 array disks. You will therefore not encounter this situation if you are using Storage Management to create your virtual disks. If you use the controller BIOS, however, you may be able to create a RAID 10 or RAID 50 virtual disk that uses partial array disks. In this situation, you will not be able to assign a dedicated hot spare to these RAID 10 or RAID 50 virtual disks.
If you have a hot spare assigned to a virtual disk and an array disk in the virtual disk fails, the failed array disk may change from Online state to Ready state without displaying a Failed state. This occurs when the hot spare is activated before the array disk is able to report the Failed state. Because the Failed state is not reported, the "Device failed: Array disk" event 2048 is not generated.
Once the hot spare is activated it changes to the Rebuilding state. If you review the event log and identify a "rebuilding" event such as 2064 or 2065, you can assume that an array disk has failed.
A dedicated hot spare on PERC 2, 2/Si, 3/Si, 3/Di, and CERC SATA1.5/6ch controllers rebuilds the portion of a redundant virtual disk that resides on the failed array disk. On these controllers, individual array disks may be included in more than one virtual disk. (Assigning a portion of an array disk to a virtual disk does not preclude the remaining portion of the array disk from being used by other virtual disks.)
You can assign the same dedicated hot spare to more than one virtual disk. In this case, the hot spare attempts to rebuild all portions of redundant virtual disks that reside on a failed array disk. To increase the likelihood that the hot spare is able to rebuild all virtual disks, you should do the following:
Once the hot spare is activated to rebuild a particular virtual disk, it is no longer available for rebuilding other virtual disks should an additional array disk fail. For this reason, when a hot spare is activated it is automatically unassigned from the remaining virtual disks. To maintain data protection, you will need to add a new hot spare and assign it to the other virtual disks.
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Note The Assign and Unassign Dedicated Hot Spare command is not available on the CERC SATA1.5/2s controller. |
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