intensive applications such as video data storage). Access speeds have improved
at a much slower rate (around a factor of 3 over 10 years), while the number of
I/O operations required per second has increased tremendously, particularly for
Web application servers.
RAID level 5, which increases the number of I/O operations needed to write a
single logical block, pays a significant time penalty in terms of write performance.
RAID level 1 is therefore the RAID level of choice for many applications with
moderate storage requirements and high I/O requirements.
RAID system designers have to make several other decisions as well. For
example, how many disks should there be in an array? How many bits should
be protected by each parity bit? If there are more disks in an array, data-transfer
rates are higher, but the system will be more expensive. If there are more bits
protected by a parity bit, the space overhead due to parity bits is lower, but there
is an increased chance that a second disk will fail before the first failed disk is
repaired, and that will result in data loss.