Dell PERC 4/DC - Manual (Page 67)

Assigning RAID Levels
Only one RAID level can be assigned to each logical drive. Table 4 -5 shows the minimum and maximum number of drives required.
Table 4-5. Physical Drives Required for Each RAID Level
Summary of RAID Levels
RAID 0 uses striping to provide high data throughput, especially for large files in an environment that does not require fault tolerance.
RAID 1 uses mirroring and is good for small databases or other applications that require small capacity, but complete data redundancy.
RAID 5 provides high data throughput, especially for small random access. Use this level for any application that requires high read request rates, but low
write request rates, such as transaction processing applications. Write performance is significantly lower for RAID 5 than for RAID 0 and RAID 1.
RAID 10 consists of striped data across mirrored spans. It provides high data throughput and complete data redundancy, but uses a larger number of spans.
RAID 50 uses parity and disk striping and works best with data that requires high reliability, high request rates, high data transfers, and medium-to-large
capacity. Write performance is limited to the same as RAID 5.
Storage in RAID 10 and RAID 50 Arrays of Different Sizes
For RAID levels 10 and 50, the additional space in larger arrays can store data, so you can use arrays of different sizes. Figure 4 -1 shows the example of a
RAID 50 array with three RAID 5 arrays of different sizes. Data is striped across the three arrays until the smallest drive is full. The data is then striped across
the larger two arrays until the smaller of those two arrays is full. Finally, data is stored in the additional space in the largest of the three arrays.
Performance Considerations
Performance is better the more spans there are. As the storage space in the spans is filled, the system stripes data over fewer and fewer spans and RAID
performance degrades to that of a RAID 1 or RAID 5 array.
Figure 4-1. Storage in a RAID 50 Array
RAID Level Minimum # of Physical Drives Maximum # of Physical Drives for
PERC 4/SC
Maximum # of Physical Drives for
PERC 4/DC
0 1 14 28
1 2 2 2
5 3 14 28
10 4 14 28
50 6 14 28