Installation guide
RAID 5
RAID 5 uses parity for fault tolerance. The advantage of using parity is that instead of having to double the number of disk drives in
the system, you only have to add one disk drive to store the parity. RAID 5 uses parity, but distributes the parity among all of the disk
drives in the RAID volume.
RAID 5 is very popular because it provides a fault tolerant soluon at a relavely low cost. For the cost of one addional disk drive
fault tolerance is achieved, but this is at a relavely high performance cost. In order to maintain the parity, when a logical write (a
write to the logical volume) occurs a number of steps are required:
1. The parity and data disks must be read.
2. The new data is compared to the data already on the disk drive and changes are noted.
3. A new parity is calculated based on step 2.
4. Both the party and data disks are wrien to.
So, for a single logical write, four physical I/Os must take place. When calculang the number of disk drives that are needed in your
system, you must take into account the addional overhead due to RAID 5.
RAID Comparison
Each RAID level has its own aributes and performance characteriscs as described above. The following table shows a comparison
of those aributes and characteriscs. As you can see, there are vast dierences among the most popular RAID levels.