Dell Adaptec U320 SCSI Raid 0 or 1 Controller Card - Manual (Page 14)

Optimizing Performance
RAID Performance Characteristics
0 Highest Read and Write Performance, no fault tolerance.
1 Good Read Performance and Lower Write Performance.
Maximizing Capacity
RAID Level Description Drives Supported Capacity
0 Striping 0 - 4 maximum drives supported (Number of disks) x (smallest disk's capacity)
1 Mirroring 2 Smallest disk's capacity
RAID Availability
RAID availability depends not only on the RAID level and number of attached hard drives, but also on whether spare
drives are available.
Spare Drives and Data Recovery
Spare drives can be utilized by physically replacing drives following a hard drive failure. Hot Spares are drives that are
attached to the controller, but are not actively available for data storage. Upon a hard drive failure the hot spare
replaces the failed drive and fault tolerance is restored by rebuilding data onto the spare drive.
NOTE: A spare can only be used by the system to rebuild an array if it is the same size as, or larger than, the
failed drive.
The following types of spares are supported by the Adaptec U320 SCSI RAID 0 or 1 controller:
Global Hot Spares —Protects every array that the spare drive has sufficient available capacity to protect.
Global Hot Spares
When an array member (a drive) fails, a global hot spare of sufficient capacity is automatically used to restore the data
contained on the failed drive. The behavior of the system after a failure depends on the size of the spare relative to the
drive it is replacing:
If the global hot spare is the same size, or less than 100 MB larger than the drive it is replacing, it becomes a
member of the array with the failed drive and ceases to be marked as a global hot spare.
If the global hot spare is larger than the drive it is replacing by 100 MB or more, the spare replaces the failed
drive, yet remains a global hot spare. The unused portion of the global hot spare is available for use in the event
of future failure.