Dell PERC 4/SI - Manual (Page 14)

See RAID Configuration and Management for detailed procedures for configuring arrays and logical drives, and spanning the drives.
Table 2-2. Spanning for RAID 10 and RAID 50
Hot Spares
A hot spare is an extra, unused disk drive that is part of the disk subsystem. It is usually in standby mode, ready for service if a drive fails. Hot spares permit
you to replace failed drives without system shutdown or user intervention. PERC 4/Di/Si and 4e/Di/Si implement automatic and transparent rebuilds of failed
drives using hot spare drives, providing a high degree of fault tolerance and zero downtime.
The PERC 4/Di/Si and 4e/Di/Si RAID management software allows you to specify physical drives as hot spares. When a hot spare is needed, the RAID controller
assigns the hot spare that has a capacity closest to and at least as great as that of the failed drive to take the place of the failed drive. The failed drive is
removed from the logical drive and marked ready awaiting removal once the rebuild to a hotspare begins. See
Table 4 -12 in Assigning RAID Levels for detailed
information about the minimum and maximum number of hard drives supported by each RAID level for each RAID controller. You can make hot spares of the
physical drives that are not in a RAID logical drive.
There are two types of hot spares:
l Global Hot Spare
l Dedicated Hot Spare
Global Hot Spare
A global hot spare drive can be used to replace any failed drive in a redundant array as long as its capacity is equal to or larger than the coerced capacity of
the failed drive. A global hot spare defined on any channel should be available to replace a failed drive on both channels.
Dedicated Hot Spare
A dedicated hot spare can be used to replace a failed drive only in a selected array. One or more drives can be designated as member of a spare drive pool;
the most suitable drive from the pool is selected for fail over. A dedicated hot spare is used before one from the global hot spare pool.
Hot spare drives can be located on any RAID channel. Standby hot spares (not being used in RAID array) are polled every 60 seconds at a minimum, and their
status made available in the array management software. PERC 4/Di/Si and 4e/Di/Si offer the ability to rebuild with a disk that is in a system, but not initially
set to be a hot spare.
Observe the following parameters when using hot spares:
l Hot spares are used only in arrays with redundancy, for example, RAID levels 1, 5, 10, and 50.
l A hot spare connected to a specific RAID controller can be used to rebuild a drive that is connected to the same controller only.
l You must assign the hot spare to one or more drives through the controller's BIOS or use array management software to place it in the hot spare pool.
l A hot spare must have free space equal to or greater than the drive it would replace. For example, to replace an 18 GB drive, the hot spare must be 18
GB or larger.
Disk Rebuilds
Level Description
10 Configure RAID 10 by spanning two contiguous RAID 1 logical drives. The RAID 1 logical drives must have the same stripe size.
50 Configure RAID 50 by spanning two contiguous RAID 5 logical drives. The RAID 5 logical drives must have the same stripe size.
NOTE: When running RAID 0 and RAID 5 logical drives on the same set of physical drives (a sliced configuration), a rebuild to a hotspare will not occur
after a drive failure until the RAID 0 logical drive is deleted.
NOTE: If a rebuild to a hotspare fails for any reason, the hotspare drive will be marked as "failed". If the source drive fails, both the source drive and
the hot spare drive will be marked as "failed".