Dell PERC 4/SI - Manual (Page 15)

When a physical drive in a RAID array fails, you can rebuild the drive by recreating the data that was stored on the drive before it failed. The RAID controller
uses hot spares to rebuild failed drives automatically and transparently, at user-defined rebuild rates. If a hot spare is available, the rebuild can start
automatically when a drive fails. If a hot spare is not available, the failed drive must be replaced with a new drive so the data on the failed drive can be rebuilt.
Rebuilding can be done only in arrays with data redundancy, which includes RAID 1, 5, 10, and 50.
The failed physical drive is removed from the logical drive and marked ready awaiting removal once the rebuild to a hotspare begins. If the system goes down
during a rebuild, the RAID controller automatically restarts the rebuild after the system reboots.
An automatic drive rebuild will not start if you replace a drive during an online capacity expansion or RAID level migration. The rebuild must be started manually
after the expansion or migration procedure is complete.
Rebuild Checkpoint
The Dell PERC firmware has a feature to resume a rebuild on a physical drive in case of an abrupt power loss or if the server rebooted in the middle of a
rebuild operation. In any of the following cases, however, a rebuild will not resume:
l A configuration mismatch is detected on the controller.
l A reconstruction is also currently in progress.
l The logical drive is now owned by the peer node.
Rebuild Rate
The rebuild rate is the percentage of the compute cycles dedicated to rebuilding failed drives. A rebuild rate of 100 percent means the system gives priority to
rebuilding the failed drives.
The rebuild rate can be configured between 0 percent and 100 percent. At 0 percent, the rebuild is done only if the system is not doing anything else. At 100
percent, the rebuild has a higher priority than any other system activity. Using 0 or 100 percent is not recommended. The default rebuild rate is 30 percent.
Hot Swap
A hot swap is the manual replacement of a defective physical disk unit while the computer is still running. When a new drive has been installed, a rebuild will
occur automatically if:
l The newly inserted drive is the same size as or larger than the failed drive
l It is placed in the same drive bay as the failed drive it is replacing
The RAID controller can be configured to detect the new disks and rebuild the contents of the disk drive automatically.
SCSI Physical Drive States
The Physical SCSI drive states are described in Table 2 -3 .
Table 2-3. SCSI Physical Drive States
NOTE: When the rebuild to a hotspare begins, the failed drive is often removed from the logical drive before management applications, such as Dell
OpenManage Storage Management, or Dell OpenManage Storage Management, detect the failed drive. When this occurs, the events logs show the
drive rebuilding to the hotspare without showing the failed drive. The formerly failed drive will be marked as "ready" after a rebuild begins to a
hotspare.
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".
State Description
Online The physical drive is working normally and is a part of a configured logical drive.
Ready The physical drive is functioning normally but is not part of a configured logical drive and is not designated as a hot spare.