When you get to RAID levels 4 and 5, you are dealing with extremely sophisticated arrays that employ a number of techniques to increase read/write performance and redundancy in order to protect mission critical data. Let's take a look at these RAID levels, which offer performance benefits over RAID 2 and RAID 3 arrays, and redundancy benefits over a RAID 1 array.
Level 4 RAID Arrays
Level 4 RAID is one of the more sophisticated of the original 5 RAID levels originally devised by technology experts at Berkeley. Level 5 RAID, though, with distributed parity, has it beat in terms of write performance and data redundancy.
It's important to remember that, if you suffer hard disk failure in any RAID array, the other drives in the array may not be far behind. Make sure you back-up your RAID data and consider transferring all your data to a newer array with hard drives that are not so close to the end of their useful life spans.
Let's take an in-depth look at how level 4 and 5 RAID arrays work and the differences between them.
About RAID 4 Arrays
A RAID 4 array offers block level striping, which means that data can be striped in blocks of a variety of sizes depending on the applications and data to be stored.
Each drive in a RAID 4 array operates independently so I/O requests take place in parallel, speeding up performance over previous RAID levels.
However, a RAID 4 array will not operate as quickly as RAID 5, because parity relies on a single disk drive, rather than parity data being distributed across the disks in the array. Dedicated parity slows down write speeds but has no affect on read speeds in a RAID 4 array.
A RAID 4 array requires at least two disk drives: two for striping data and one for parity.
About RAID 5 Arrays
Similarly, a RAID 5 array also uses block striping. The major difference is in the parity data, which is distributed across all drives in the RAID array, rather than confined to one disk. This makes a RAID 5 array better protected against data loss, since a RAID 5 array can tolerate the loss of any single disk drive in the array. There is no single parity drive that may fail.
A RAID 5 array is also faster than level 4 RAID, because there is no single parity disk that will create a data input bottleneck. In a RAID 4 array, the array can only write as fast as the parity disk.
A single drive failure will, however, reduce performance across the array. It's important to replace a failed drive as quickly as possible to avoid placing added strain on the other disks in the array and to keep performance levels up to speed.