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Description of RAID 5 - Independent Access Arrays

In RAID 5, the redundancy offered in RAID 3 by a single parity disk, is distributed across all the disks in the array. Data and relative parity are never stored on the same disk. One user may be writing a chunk to disk 0 and the corresponding parity to disk 3, another user may be writing to chunk 4 of disk 1 and updating parity on disk 2. There is a clear dividend in terms of performance and the speed of transactions. During disk writes, RAID 5 cannot produce a write performance comparable to that of straight disk striping because other operations have to be undertaken to make and store parity codes. The I/O performance of the array depends very much on the relative levels of reads and writes requested. When a stripe is modified, unmodified portions must also be read to re-generate the parity for the entire stripe. Once the parity has been generated, the modified data and parity information must be written to disk. This is commonly know as Read/Modify/Write strategy. It reflects that, though RAID 5 is superior to RAID 0 because it offers redundancy, it is not able to perform as well as RAID 0 in terms of write performance. Because RAID 5 has distributed parity, two reads and two writes must be performed for every write operation. However, the write penalty can be overcome by the use of write caching which allows write data to be stored in the memory prior to writing to the disk, so freeing the host processor for other tasks. Summary: RAID 5 is ideal for organizations running databases and other transaction-based applications such as: banks, airline and railway reservation systems, government departments, utilities and telecommunications.*