Information Technology Reference
In-Depth Information
performance (since all disks can be accessed in parallel), but if an
error occurs on any disk, the entire RAID will fail.
•
Mirrored:
A
mirrored
RAID (or RAID 1) writes the same data
simultaneously to two or more disks. (So, the RAID capacity equals
the capacity of any member disk.) If any one drive fails, another can
take over instantly and seamlessly with no loss of data and no down
time; you can then replace the faulty drive at your leisure.
RAIDs with more than two disks can have other configurations—
including RAID levels 2, 3, 4, 5, and 6, and several combinations of
levels. (You can read about the various forms of RAID in
Wikipedia
.)
Before I explain why you might care about a striped or mirrored RAID,
I want to mention a few technologies that look superficially like RAID
but are in fact quite different underneath:
•
JBOD:
Some manufacturers sell enclosures with multiple disks
inside that share a power supply, controller, and interface(s). But,
each of these disks is independently accessible from your computer.
Although you could use software to combine them into a RAID, in
their native state they're
JBOD
—Just a Bunch of Disks.
•
Concatenation:
You can also use either hardware or software to
combine two or more independent disks into a single logical volume
whose size is the total of all the disks combined. That might sound
like a striped RAID, but the data doesn't alternate between disks—
it's stored sequentially, so there's neither a performance benefit
as there is with RAID 0, nor data redundancy as there is with
RAID 1. Concatenated disks sometimes go by the names
BIG
or
SPAN
(which don't stand for anything even though they're in all
caps), and are sometimes incorrectly referred to as RAIDs.
•
BeyondRAID:
Data Robotics uses the trademarked term
BeyondRAID to refer to a method of combining disks into a larger
volume that provides data redundancy while maintaining the
capability to use disks of different sizes and to dynamically change
the array's configuration, things you can't ordinarily do with a
RAID. I say more about this ahead, in
Drobo Storage Devices
.
