Previously, I’ve presented you with a look at many RAID levels, including some hybrid RAID levels, such as
RAID-10 and RAID-50. In a series of articles on data protection, I will provide
you with a look at some non-standard RAID levels. Some of them are available
only from specific manufacturers, but some are actually very slowly working
their way into more general use.
RAID 1E (striped mirroring, enhanced mirroring, hybrid mirroring)
RAID 1E — which, depending on the vendor, is also called striped mirroring, enhanced mirroring, and hybrid
mirroring — is a RAID level that combines RAID 0’s striping capabilities
with RAID 1’s mirroring protection. If you’re thinking that this RAID method
sounds a lot like RAID 10, you should understand one critical difference
between RAID 1E and RAID
10. RAID 1E uses an odd number of disks to achieve your data protection
goals while RAID 10 requires the use of an even number of disks.
Whereas RAID levels 0 and 1 each require a minimum of two
disks, RAID 1E requires a minimum of three disks. Keep in mind that RAID 10 requires
at least four disks. As is the case under RAID 1, RAID 1E has a 50 percent disk
capacity overhead. In other words, only half of the total capacity of the array
is available for use.
RAID 1E works by striping data across all of the disks in
the array a la RAID 0. As you know,
however, RAID 0 is less than ideal for enterprise environments since the loss
of any disk in the array results in
the loss of all data stored on the array. Where RAID 1E becomes viable is in
the next step, in which a copy of the data is then striped across all of the
disks as well. Simply striping the data in the exact same way as the initial
stripe would not be any good since copies of the data would still reside on the
same disk as the original copy. RAID 1E therefore, shifts the second copy of
the data over on each physical disk. Take a look at Figure A. Each number refers to a block of data. A number with an M refers to a mirrored block of that
data.
|
Figure A |
 |
| A simplistic look at how RAID 1E works |
Now, imagine that disk five suffers a failure. Blocks three
and eight are stored on that disk, along with the mirrored copies of blocks five
and ten. However, the array can suffer this failure since mirrored copies of blocks three and eight are stored on disk one. In theory, you can lose multiple disks in a RAID 1E array as long as the disks are not adjacent. In Figure A, for example, you can lose disks one and three, with disk one’s blocks one and six mirrored to disk two, and disk three’s blocks two and seven mirrored to disk four.”
RAID 1E can potentially provide more performance than a
traditional RAID 1 array. With an odd number of disks, RAID 1E provides more
spindles (in many RAID 1E cases, three disks/spindles instead of two). Like
RAID 1, RAID 1E’s primary disadvantage is its 50 percent disk overhead. Another
significant RAID 1E disadvantage is its relatively low support from controller manufacturers.
Summary
RAID 1E looks to be an interesting alternative to RAID 1 when
somewhat better performance is necessary, and you don’t want to go the RAID 10
route. Are any of you running systems with RAID 1E? If so, leave a comment and
let us know your reasoning and experience.
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