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RAID 5 for performance and reliability
Please read the RAID page for background.
RAID 5 (striping + parity) offers the following benefits:
- high performance approaching the speed of RAID 0 stripe;
- redundant parity information which allows one drive to fail with no data loss.
In general, RAID 5 requires hardware support, though that might change soon.
How to get RAID 5
The easiest way to get RAID 5 is via an external enclosure like the Other World Computing QX2, see the review. With such enclosures, RAID 5 is built-in, and the unit appears like one large hard drive to the computer. There are many RAID enclosures on the market, and many are quite costly.
Another way to get RAID 5 is via a hardware RAID PCI-Express card, installed in the expansion slot of the Mac Pro. Apple sells such a card, but some users complain that it won’t allow the machine to sleep properly, and that the battery on it can drain, and cause nagging failure reminders. I have used the Apple card.
RAID 5 requires a minimum of three drives. Two drives would be a RAID 0 stripe or RAID 1 mirror. For a 4-bay unit like the QX2, choose five identical drives (for installed, plus a fifth for a spare, should a failure occur).
Capacity
RAID 5 uses one drive for parity. With 3 drives, you get usable capacity of 2 drives (66% utilization), so three drives is not very space efficient. With 4 divs, you get the capacity of 3, which 75% utilization.
Using 2TB drives, a 4-drive RAID 5 offers 6TB of usable space.
Performance
RAID 5 offers performance very close to RAID 0 striping, the fastest RAID option.
When a drive fails
When a drive in a RAID 5 fails, a replacement can be inserted (a “cold” spare), or an already present “hot spare” will take over for the failed drive. In either case, the redundant information is then regenerated to bring the array back to its pre-failure state, which can then tolerate a subsequent failure.
Rebuilding can take some hours, somewhere between 7 and 12 hours for 2TB drives, but the unit can continue to be used during the rebuild. If a second failure should occur before the required information can be regenerated, then the data is lost.
Misleading claims about RAID 5 Permalink
Not long ago, an article by Robin Harris appeared on ZDNet (Why RAID 5 Stops Working in 2009), rebutted by various articles, including this one.
The article claims that RAID 5 is fatally flawed because a rebuild after a drive failure would be all but guaranteed to fail from a read error (in the context of 2TB drives). It makes other tenuous assumptions about failure rates that don’t reflect my experience, or even how most people use their computers. The article lacks any real-world substantiation, and uses statistics incorrectly to come to a flawed conclusion.
The article also misses a key point: a RAID 5 remains perfectly usable after a drive has failed. Data can be copied or backed-up. In fact, the unit could then continue to works for months or years, though it could not tolerate another drive failure.
I’ve been running a 4-way RAID 0 stripe for four years now, and the only failure I had was a Maxtor 500GB back in 2006 and that was a quick failure, a bad drive to start with. My Mac Pro runs 10-14 hours a day, so that’s a darn good track record.
Don’t assume, test!
But it’s always good to test, so I set about determining if I could produce even a single error from my Hitachi 2TB 7K2000 hard drives, reviewed here. I actually have twelve of these drives, and I’ve never had a problem of any kind with them.
I installed four of the 7K2000 drives into my Mac Pro Nehalem, configuring them as a RAID 0 stripe, my interest being maximum speed to detect read errors.
Next I ran SoftRAID 4.0 “Certify” for well over one full pass. There were zero errors on all four drives. That represents testing 10-12TB of data access with no errors (I ran about 1.4 passes before stopping it).
After that, I filled the array with huge files, then ran IntegrityChecker to compute SHA1 hashes on all files. I ran four complete passes (and a partial fifth pass), having it verify all files, which means that more than 32TB of data was read without a single error.
In total, my testing showed the ability to read more than 40TB of data on 4 drives with zero errors. That’s 20 times as much data as the alleged size issue in the ZDNet article. If the analysis in that article were correct, then I ought to go buy lottery tickets right now.
Conclusions
Don’t believe everything you read, especially without data the substantiates the claim.
RAID 5 is a very good thing, and well worth considering for some applications, where drive failure is a serious inconvenience. But stick to a backup plan, RAID 5 is a reliability solution, not a backup.
Your most cost-effective and convenient plug-and-play path to RAID 5 is the Other World Computing QX2.
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