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Introduction to RAID Considerations

Last updated 2010-11-07 - Send Feedback
Related: RAID, RAID-5, RAID-6, storage
iStoragePro iT8SAE in operation

Historically, hardware RAID provided capabilities not presently possible with software RAID and than can still remain true in certain ways:

  • RAID-6 provides resilience to two drive failures;
  • Hot-spare drive(s) capability;
  • Redundant power supplies.

Hardware RAID is not necessarily a win for performance, as SoftRAID 5 proves with RAID-5.

Appropriate uses

Uses include storage for high-definition video footage, large collections of photographs, large databases, etc.

For this type of high-capacity scenario, I am loathe to use anything but RAID-6, which tolerates two drive failures with no data loss. RAID-6 requires at least six drives for efficient capacity, since two drives are used for parity.

The safest setup for critical data is RAID-6 with one hot spare, so that no human intervention is needed to restore full RAID-6 protection. With an 8-bay enclosure, use 7 drives for the RAID-6, plus one hot spare.

RAID in the enclosure

Some external enclosures build in RAID support to the enclosure itself, such as the OWC QX2. The QX2 supports RAID-5 for fault tolerance (one drive failure), and it is an outstanding choice for those on a budget, because it can be used with eSATA, Firewire 800 and USB. However, the 4-drive QX2 does not offer the dual-failure resilience of RAID-6, and it cannot match the performance of an 8-bay enclosure with a RAID card. Its appeal is its relatively low cost, and its easy connectivity.

Limitations of RAID-5 and why RAID-6 is preferred

The failure of one drive in any RAID-5 device means that the RAID is at risk of data loss until both of these happen: (1) a replacement drive is inserted, and (2) the rebuild completes (10-12 hours). The former requires human intervention, and the latter takes half a day (much longer if the drive is being heavily used).

Therefore, RAID-6 with a hot spare should be used for critical data that must remain at low risk even after one drive fails. That means an 8-bay enclosure with 8 drives, using one of them for a hot spare.

Mitigation strategies exist: a disciplined backup strategy means that it might be perfectly acceptable to use RAID-5, or even RAID-0 striping. But restoring multiple terabytes of data after a failure is an all-day process, a downtime that some professionals cannot afford.

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Mac Pro RAID card

While hardware RAID can be used internal to a Mac Pro with the Apple RAID card, the four drive bays are right at the minimum for robust hardware RAID. Furthermore, those internal bays can be put to other valuable uses at much lower cost than with hardware RAID (such as multiple striped solid state drives). However, some users will find RAID-5 perfectly appropriate for their uses.

Capabilities and cost

RAID capabilities are dependent on the type of RAID card used with a JBOD (just a bunch of disks) enclosure like the iStoragePro; it doesn’t do RAID on its own. Such cards are priced in the $600 - $1200 range.

See this handy shopping cart at OWC for all parts needed to put together a working storage setup.

Approximate cost to configure the iStoragePro with drives and RAID card
Item Cost range
iStoragePro iT8SAE SAS Expander $1300
Nine (9) 2TB drives $1350 - $2700
(enterprise-grade drives recommended)
RAID card $600 - $1200
SAS cables (2) $120
  $3070 - $ 4320

Shown below are the Atto R380 RAID card, and the iStoragePro iT8SAE tower enclosure.

Atto R380 PCIe RAID card + iStoragePro iT8SAE mini-SAS expander tower
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