If you have some disks in a system that you decide not to configure into a RAID array,
what do you do with them? Traditionally, they are left to act as independent drive volumes
within the system, and that's how many people in fact use two, three or more drives in a
PC. In some applications, however, it is desirable to be able to use all these disks as if
they were one single volume. The proper term for this is spanning; the
pseudo-cutesy term for it, clearly chosen to contrast against "redundant array of
inexpensive disks", is Just A Bunch Of Disks or JBOD. How
frightfully clever.
JBOD isn't really RAID at all, but I discuss it here since it is sort of a "third
cousin" of RAID... JBOD can be thought of as the opposite of partitioning: while
partitioning chops single drives up into smaller logical volumes, JBOD combines drives
into larger logical volumes. It provides no fault tolerance, nor does it provide any
improvements in performance compared to the independent use of its constituent drives. (In
fact, it arguably hurts performance, by making it more difficult to use the underlying
drives concurrently, or to optimize different drives for different uses.)
When you look at it, JBOD doesn't really have a lot to recommend it. It still requires
a controller card or software driver, which means that almost any system that can do JBOD
can also do RAID 0, and RAID 0 has significant performance
advantages over JBOD. Neither provide fault tolerance, so that's a wash. There are only
two possible advantages of JBOD over RAID 0:
- Avoiding Drive Waste: If you have a number of odd-sized drives, JBOD
will let you combine them into a single unit without loss of any capacity; a 10 GB drive
and 30 GB would combine to make a 40 GB JBOD volume but only a 20 GB RAID 0 array. This
may be an issue for those expanding an existing system, though with drives so cheap these
days it's a relatively small advantage.
Easier Disaster Recovery: If a disk in a RAID 0 volume dies, the data
on every disk in the array is essentially destroyed because all the files are striped; if
a drive in a JBOD set dies then it may be easier to recover the files on the other drives
(but then again, it might not, depending on how the operating system manages the disks.)
Considering that you should be doing regular backups regardless, and that even under JBOD
recovery can be difficult, this too is a minor advantage.
Note: Some companies
use the term "spanning" when they really mean striping, so watch out for that!
Next: Summary Comparison of RAID Levels
