Information Technology Reference
In-Depth Information
Why You Might Care about a Striped RAID,
JBOD, or Concatenated Storage
The highest-capacity individual drive mechanism available in
mid-2013 is 4 TB. But you might have more than 4 TB of data to
back up! Several companies sell devices that appear to be external
hard drives but hold more than 4 TB, because their cases contain
multiple drive mechanisms (for example, two 3 TB disks). The
enclosure's circuitry combines the two disks into a larger volume—
sometimes it's a striped RAID 0 volume, sometimes it's merely
concatenated, and sometimes it's a JBOD but with hardware or
software features that let you reconfigure it as a RAID (perhaps even
a mirrored RAID 1, for more fault tolerance but half the capacity).
The good news is that such devices offer the benefit of a large, fast
disk without making you cobble together your own hardware- or
software-based RAID and dealing with lots of boxes and cables. The
bad news is that if you use RAID 0 and a mechanism in your enclosure
dies, you could lose all the data on both drives. Even in the best case,
you'll have to send the device back to the manufacturer for repair.
And, if one of two or more concatenated disks fails, you may or
may not be able to get the data on the remaining disk(s) without the
manufacturer's intervention.
I'm not saying you should avoid such devices; I'm saying you should
know what you're getting into if you rely on them. Mathematically,
your chances of losing the data on RAID 0 or concatenated disks
are double that of a single mechanism of the same type. So if you
get a multi-drive enclosure and have the choice, I suggest that you
use RAID 1 or higher, because all levels of RAID beyond 0 offer
redundancy to protect data in case of drive failure (along with a
reduction in capacity).
Why You Might Care about a Mirrored RAID
Mirrored RAIDs have at least two copies of your data, so, unlike
striped RAIDs, they protect you against drive failure. The downside
is that you need twice as many disks for a given amount of capacity.
Some people believe that a mirrored RAID consisting of their Mac's
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