Managing RAID on Linux

Redundancy

Redundancy is a feature that allows an array to survive a disk failure. Not all RAID levels support this feature. In fact, although the term RAID is used to describe certain types of non-redundant arrays, these arrays are not, in fact, RAID because they do not support any data redundancy.

Warning

Despite its redundant capabilities, RAID should never be used as a replacement for reliable backups. RAID does not protect your data in the event of a fire, natural disaster, or user error.

Mirroring

Two basic forms of redundancy appear throughout the RAID specification. The first is accomplished with a process called disk mirroring, shown in Figure 1-1. Mirroring replicates data onto every disk in the array. Each member disk contains the same data and has an equal role in the array. In the event of a disk failure, data can be read from the remaining disks.

Figure 1-1. Disk mirroring writes a copy of all data to each disk.

Improved read performance is a by-product of disk mirroring. When the array is operating normally, meaning that no disks have failed, data can be read in parallel from each disk in the mirror. The result is that reads can yield a linear performance based on the number of disks in the array. A two-disk mirror could yield read speeds up to two times that of a single disk. However, in practice, you probably won’t see a read performance increase that’s quite this dramatic. ...