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| As the Hard Disc Spins RAID I: The Lesser Levels | ||
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(Review by MS, February 23, 2004) | ||
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WD Raptor WD360GD |
RAID Level1 Mirroring / Duplexing
RAID Level1 mirrors or duplexes the information of any drive by storing an identical copy on a secondary drive. The advantage is full redundancy and fault tolerance, however, there is no performance increase, at least not with respect to write speed. The situation is somewhat different when it comes to reads since data can be accessed in a “zipper-like fashion” from both drives, meaning that alternating blocks of information are transferred from each drive to the controller. This does result in a performance increase, especially in the case of sequential read transfers.
Mirroring clones an entire drive and all its operations to a second copy. Note that both drives are controlled by the same host bus adapter. Drawing in adaptation of a similar schematic on the PCGuide.
Duplexing vs. Mirroring
In a duplexing instead of mirroring, also the controller itself is duplicated, to offer additional fail-safe protection against controller failure. The main drawback of any RAID Level1 system is that it is rather uneconomic since only ˝ of the total storage capacity is useable. The latter issue and the associated cost overhead also somewhat disqualifies the Level1 strategy from being listed as true RAID.
Duplexing uses a separate drive controller in addition to a separate mirror drive, and sometimes there are failover mechanisms in place that can switch both drives to a single controller, meaning that each controller can talk to each drive in a crossbar configuration. This way, even if one drive experiences a catastrophic failure, while simultaneously the second controller breaks down, there will still be full functionality until replacements are installed. Drawing in adaptation of a similar schematic on the PCGuide.
RAID Level 0+1 / Level 10
RAID Level 10 describes striping of mirrors whereas RAID 0/1 refers to mirroring of a striped array to add fault tolerance to performance increase. If any single device fails, it is still not possible to “rebuild” the array from the second “Partner in Stripe”, rather, a copy of the stripe-set has to be created from the mirrored backup. RAID Level 10 and 0/1 double the number of devices required for any storage array and is neither space- nor cost-effective. In both cases the performance is approximately the same as in a RAID Level0 setup.
Detailed differences between RAID Level 10 and 0/1 are described on the PC-Guide and on J. Heiss' site.
Intermission
So far we have described some storage techniques that fall under the general umbrella of RAID, at least as far as this term is used in colloquial parlance. However, none of the methods discussed so far effectively utilizes the resources available for both performance (including storage capacity) increase and fault tolerance. However, none of the methods discussed so far resorts to utilizing parity as a simple, et very effective means of increasing both performance and redundancy. As a sneak preview of the next articles: Adding parity can reduce the number of drives by as much as ~50 % through better resource utilization while providing the same redundancy and fault tolerance as RAID 1, combined with the speed advantage of RAID 0.
As a summary of where we are and what is about to come in the next few articles, currently used levels of RAID are:
Lesser RAID Levels:
