Previously, I've presented you with a look at many RAID levels, including some hybrid RAID levels, such asRAID-10 and RAID-50. In a series of articles on data protection, I will provideyou with a look at some non-standard RAID levels. Some of them are availableonly from specific manufacturers, but some are actually very slowly workingtheir 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 hybridmirroring -- is a RAID level that combines RAID 0's striping capabilitieswith RAID 1’s mirroring protection. If you’re thinking that this RAID methodsounds a lot like RAID 10, you should understand one critical differencebetween RAID 1E and RAID10. RAID 1E uses an odd number of disks to achieve your data protectiongoals while RAID 10 requires the use of an even number of disks.
Whereas RAID levels 0 and 1 each require a minimum of twodisks, RAID 1E requires a minimum of three disks. Keep in mind that RAID 10 requiresat least four disks. As is the case under RAID 1, RAID 1E has a 50 percent diskcapacity overhead. In other words, only half of the total capacity of the arrayis available for use.
RAID 1E works by striping data across all of the disks inthe array a la RAID 0. As you know,however, RAID 0 is less than ideal for enterprise environments since the lossof any disk in the array results inthe loss of all data stored on the array. Where RAID 1E becomes viable is inthe next step, in which a copy of the data is then striped across all of thedisks as well. Simply striping the data in the exact same way as the initialstripe would not be any good since copies of the data would still reside on thesame disk as the original copy. RAID 1E therefore, shifts the second copy ofthe 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 thatdata.
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Figure A |
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| A simplistic look at howRAID 1E works |
Now, imagine that disk five suffers a failure. Blocks threeand eight are stored on that disk, along with the mirrored copies of blocks fiveand 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 atraditional RAID 1 array. With an odd number of disks, RAID 1E provides morespindles (in many RAID 1E cases, three disks/spindles instead of two). LikeRAID 1, RAID 1E’s primary disadvantage is its 50 percent disk overhead. Anothersignificant RAID 1E disadvantage is its relatively low support from controller manufacturers.
Summary
RAID 1E looks to be an interesting alternative to RAID 1 whensomewhat better performance is necessary, and you don’t want to go the RAID 10route. Are any of you running systems with RAID 1E? If so, leave a comment andlet us know your reasoning and experience.
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