Dell PERC 4/DC - Manual (Page 25)

Figure 4 -2 shows the example of a RAID 50 span with three RAID 5 arrays of different sizes. (Each array can have from three to 14 hard disks.) Data is striped
across the three RAID 5 arrays until the smallest array is full. The data is striped across the remaining two RAID 5 arrays until the smaller of the two arrays is
full. Finally, data is stored in the additional space in the largest array.
Figure 4-2. Storage in a RAID 50 Array
Performance Considerations
The system performance improves as the number of spans increases. 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.
Optimizing Storage
Data Access Requirements
Each type of data stored in the disk subsystem has a different frequency of read and write activity. If you know the data access requirements, you can more
successfully determine a strategy for optimizing the disk subsystem capacity, availability, and performance.
Servers that support video on demand typically read the data often, but write data infrequently. Both the read and write operations tend to be long. Data
stored on a general-purpose file server involves relatively short read and write operations with relatively small files.
Array Functions
Define the major purpose of the disk array by answering questions such as the following, which are followed by suggested RAID levels for each situation:
l Will this disk array increase the system storage capacity for general-purpose file and print servers? Use RAID 5, 10, or 50.
l Does this disk array support any software system that must be available 24 hours per day? Use RAID 1, 5, 10, or 50.
l Will the information stored in this disk array contain large audio or video files that must be available on demand? Use RAID 0.
l Will this disk array contain data from an imaging system? Use RAID 0 or 10.
Planning the Array Configuration
Fill out Table 4 -8 to help you plan the array configuration. Rank the requirements for your array, such as storage space and data redundancy, in order of
importance, then review the suggested RAID levels. Refer to Table 4 -7 for the minimum and maximum number of drives allowed per RAID level.
Table 4-8. Factors to Consider for Array Configuration
Requirement Rank Suggested RAID Level(s)