Dell H800, H700 manual RAID 1 Mirroring, Example of RAID Advantages of RAID

Dell

For example, a four-drive virtual disk may be configured with 16 stripes (four stripes of designated space per drive). Stripes A, B, C and D are located on corresponding hard drives 0, 1, 2, and 3. Stripe E, however, appears on a segment of drive 0 in a different location than stripe A; stripes F through H appear accordingly on drives 1, 2 and 3. The remaining eight stripes are allocated in the same even fashion across the drives.

RAID 0 provides improved performance because each drive in the virtual disk needs to handle only part of a read or write request. However, because none of the data is mirrored or backed up on parity drives, one drive failure makes the virtual disk inaccessible and the data is lost permanently.

Data 1	Data 2	Data 3	Data 4
Data 5	Data 6	Data 7	Data 8
Data 9	Data 10	Data 11	Data 12
Data 13	Data 14	Data 15	Data 16
Drive 0	Drive 1	Drive 2	Drive 3

Figure 4. Example of RAID 0

Advantages of RAID 0

∙I/O performance is greatly improved by spreading the I/O load across many channels and drives (best performance is achieved when data is striped across multiple channels with only one drive per channel)

∙No parity calculation overhead is involved

∙Very simple design

∙Easy to implement

Disadvantages of RAID 0

∙Not a "true" RAID because the failure of just one drive will result in all data in a virtual disk being lost

∙Should not be used for critical data unless another form of data redundancy is deployed

5.3.2RAID 1 (Mirroring)

RAID 1 is achieved through disk mirroring to ensure data reliability or a high degree of fault tolerance. In a RAID 1 configuration, the RAID management software instructs the subsystem's controller to store data redundantly across a number of the drives (mirrored set) in the virtual disk. See Figure 5.

In other words, if a disk fails, the mirrored drive takes over and functions as the primary drive.

DELL PERC H700 and H800 Technical Guide 22