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Data Robotics DROBO 5D
The Data Robotics Drobo 5D is a 5-bay enclosure (3.5" drives) supporting functionality similar to RAID 5 or RAID-6, allowing drive failure of one drive without data loss, or failure of two drives when configured appropriately.
Its main appeal is for large amounts of fault-tolerant storage using 3TB or 4TB drives without any need for the user to understand the internal workings of RAID-5/6.
With five 4TB drives, the Drobo 5D contains 20TB of hard drive storage, but the equivalent of one drive is used for parity data by default, or the equivalent of two drives for double fault tolerant mode.
Configured for the equivalent of RAID-5 (tolerates one drive failure), this leaves 16TB available, which the OS X Finder reports as a 17.59TB volume, Drobo Dashboard reports it as a 14.49TB volume, and diglloydTools DiskTester reports as a 16.0TB volume (DiskTester defines TB as 1024^4).
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Additional fault tolerance for failure of up to two drives reduces the capacity available for use to less than 12TB out of the 20TB “pool”.
The following applies to any RAID-5 setup: users are well advised to keep a “cold spare” on hand in case of drive failures so that the RAID rebuild process can begin immediately— this could take a day or even several days for a unit with a lot of data on it. In RAID-5 mode (tolerance for one drive failure), failure of a 2nd drive before that rebuild process completes means a total loss. Hence waiting a day or two or three for a spare drive to show up is penny-wise and dollar-foolish.
Furthmore, fault tolerance is not protection against other types of failures.
A bit more
The appeal of the Drobo is that (a) a drive failure or failures (depending on configuration) can be tolerated without loss of data, and features in this reviewer’s view that are best avoided: (b) drives of different capacities and brands can be mixed and matched, and (c) more capacity means replacing any drive with one of larger capacity.
The Drobo 5D can be configured to use the equivalent space of one drive for single-drive parity (like RAID-5) or two drives (RAID-6), though Data Robotics does not use this terminology, and the proprietary format isn’t actually RAID-5 or RAID-6 (which raises some concerns about recovery modes).
According to Data Robotics, the Drobo is not RAID 5 (striping + parity), but it’s the same idea: some data plus some redundant information is stored on each drive such that failure of any particular drive can be tolerated without data loss. What Data Robotics has done is to generalize a well-understood technology (RAID) such that dissimilar drives can be used together, truly a Redundant Array of Inexpensive Disks, or RAID.