Network Servers: Everything You Wanted to Know but Were Afraid to Ask
If you think about a simple RAID array with data spread across many disks, you'll
probably notice that, while it improves performance, it also increases the chance of a
disk failure. Using five disks to do the work of one means that five times more chances
exist for a disk failure. Because the data is spread among all the disks, if one fails, you
might as well throw away all the data on all the remaining disks because it's useless if a
big chunk is missing. Fortunately, different RAID schemes address this problem.
There are many different ways to use multiple disks together in some sort of RAID
scheme and, accordingly, a number of RAID levels are defined, each of which describes
a different technique, as follows:
This scheme is a configuration whereby data is spread (striped) across
multiple disks, although with no redundancy. Losing one drive in a RAID 0
array results in the loss of data on all the disks. RAID 0 is appropriate only for
improving performance and should be used only with nonessential data. RAID
0 arrays can stripe data across two or more disks, as shown in Figure 13-1.
This type of array doesn't stripe data across multiple disks. Instead,
it defines a standard whereby data is mirrored between disks. Two disks are
used instead of one, and the data is kept synchronized between the two disks.
If one of the disks fails, the remaining disk continues working just fine, until
the failed drive can be replaced. RAID 1 is often simply referred to as mirroring.
An enhancement to RAID 1 is called duplexing; the data is still duplicated
between two disks, but each disk has its own disk controller, adding another
level of redundancy (because you can lose either a disk or a controller and
still keep operating). Duplexing can also improve performance somewhat,
compared to straight mirroring. Some RAID 1 implementations are also
intelligent enough to read data from either disk in such a way that whichever
disk has its drive head closest to the data performs the read request, while the
other one sits idle. However, all writes must occur simultaneously for both
disks. Figure 13-2 shows a typical RAID 1 array layout.
A RAID 0 array stripes data across multiple disks