Managing RAID on Linux

Hybrid Arrays

One of the most important features of software RAID is its ability to use existing arrays as member disks. This property allows you to not only create extremely large arrays, but to combine different RAID levels to achieve different degrees of performance and redundancy. The major benefit of using hybrid arrays is their ability to withstand multiple disk failures.

As I mentioned in Chapter 2, it’s advisable to spread member disks across multiple I/O channels whenever possible. This not only increases array performance but, in the case of a mirror, also helps prevent array failure if an I/O channel becomes unavailable because of a disk controller failure or faulty cabling. For example, imagine that /dev/sda and /dev/sdb are member disks of /dev/md0, a RAID-1, and that they are connected to the same controller A. If controller A fails, then all of /dev/md0 becomes unavailable.

Now imagine that each disk is connected to its own controller. For instance, suppose /dev/sda is connected to controller A and /dev/sdb is connected to controller B. In this case, either controller A or B can fail without crashing /dev/md0 (see Figure 3-5), because the other controller’s disk is still operational.

Figure 3-5. Using multiple controllers help prevent downtime.

Dispersing disks among multiple controllers is equally important when working with hybrid arrays. For example, take the case of ...