Dell PERC 4/SI - Manual (Page 42)

l Maximum number of hard disk drives that you can use in each RAID level
l Array configuration
l Logical drive properties
l Clearing physical drives
l Designating physical drives as hot spares
l Rebuilding failed physical drives
l Checking data consistency
l Reconstructing logical drives
l Performing an online capacity expansion
l Performing drive roaming or drive migration
Guidelines for SCSI Devices in a RAID Array
Observe the following guidelines when connecting and configuring SCSI devices in a RAID array:
l Consider the number of hard disk drives in the array when deciding on the RAID level to use. See RAID Levels for the number of drives supported for
each array level.
l Use drives of the same size and speed to maximize the effectiveness of the controller.
l When replacing a failed drive in a redundant array, make sure that the replacement drive has the same or larger capacity than the smallest drive in the
array (RAID 1, 5, 10, and 50).
When implementing RAID 1 or RAID 5, disk space is spanned to create the stripes and mirrors. The span size can vary to accommodate the different disk sizes.
There is, however, the possibility that a portion of the largest disk in the array will be unusable, resulting in wasted disk space. For example, consider a RAID 1
array that has the following disks, as shown in
Table 4 -10 .
Table 4-10. Storage Space in a RAID 1 Array
In the RAID 1 example, data is mirrored across the two disks until 20 GB on Disk A and B are completely full. This leaves 10 GB of disk space on Disk B. Data
cannot be written to this remaining disk space, as there is no corresponding disk space available in the array to create redundant data.
Table 4 -11 provides an example of a RAID 5 array.
Table 4-11. Storage Space in a RAID 5 Array
In the RAID 5 example, data is striped across the disks until 40 GB on Disks A, B, and C are completely full. This leaves 20 GB of disk space on Disk C. Data
cannot be written to this remaining disk space, as there is no corresponding disk space available in the array to create redundant data.
RAID levels 10 and 50 span RAID 1 and RAID 5 arrays, respectively. When one array fills its available storage space, the other array(s) may have additional
storage space available. You can still use fill the additional available space in the larger array(s). Because there is additional storage space in the larger array
(s), you can use arrays of different sizes without having to leave storage space unused. See
Storage in RAID 10 and RAID 50 Arrays for more information
about storage space in RAID 10 and 50 arrays.
Assigning RAID Levels
Disk Disk Size Storage Space Used in Logical Drive for RAID 1 Array Storage Space Left Unused
A 20 GB 20 GB 0 GB
B 30 GB 20 GB 10 GB
Disk Disk Size Storage Space Used in Logical Drive for RAID 5 Array Storage Space Left Unused
A 40 GB 40 GB 0 GB
B 40 GB 40 GB 0 GB
C 60 GB 40 GB 20 GB