Dell PERC 4/SI - Manual (Page 13)

Parity generates a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets.
Parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives or stripes across all disk drives in an array. The types
of parity are shown in
Table 2 -1 .
Table 2-1. Types of Parity
If a single disk drive fails, it can be rebuilt from the parity and the data on the remaining drives. RAID level 5 combines distributed parity with disk striping, as
shown in
Figure 2 -3 . Parity provides redundancy for one drive failure without duplicating the contents of entire disk drives, but parity generation can slow the
write process.
Figure 2-3. Example of Distributed Parity (RAID 5)
Disk Spanning
Disk spanning allows multiple physical drives to function like one big drive. Spanning overcomes lack of disk space and simplifies storage management by
combining existing resources or adding relatively inexpensive resources. For example, four 20 GB drives can be combined to appear to the operating system as
a single 80 GB drive.
Spanning alone does not provide reliability or performance enhancements. Spanned logical drives must have the same stripe size and must be contiguous. In
Figure 2 -4 , RAID 1 arrays are turned into a RAID 10 array.
Figure 2-4. Example of Disk Spanning
Spanning for RAID 10 or RAID 50
Table 2 -2 describes how to configure RAID 10 and RAID 50 by spanning. The PERC 4/Di/Si and 4e/Di/Si family supports spanning for RAID 1 and RAID 5 only.
The logical drives must have the same stripe size and the maximum number of spans is eight. The full drive size is used when you span logical drives; you
cannot specify a smaller drive size.
Parity Type Description
Dedicated The parity of the data on two or more disk drives is stored on an additional disk.
Distributed The parity data is distributed across more than one drive in the system.
NOTE: Make sure that the spans in a RAID 10 array are in different backplanes, so that if one span fails, you won't lose the whole array.
NOTE: Spanning two contiguous RAID 0 logical drives does not produce a new RAID level or add fault tolerance. It does increase the size of the logical
volume and improves performance by doubling the number of spindles.