RAID: Not Such a Clever Idea for Your Home PC - Risky Array of Independent Disks
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RAID 1 does not protect against the unlikely eventuality of both drives failing together. What are the odds of that happening? Modern disks are very reliable; wouldn't it be almost unheard of for two drives to die at the same time? No, it's not! Hard drive failure results not just from faulty manufacturing or wear and tear. Drives can fail as a result of other components being faulty. Such disasters can and often will take both drives.
Further, the reliability of the modern hard disk is exaggerated. The quoted Mean Time Between Failure (MTBF) for the average modern IDE drive is about 1,000,000 hours. The MTBF for a system with 2 disks, A and B, striped is 1/(1/MTBF A + 1/MTBF B), or 500,000 hours. That's almost invincible! In the real world however, we see approximately 2% of disks go faulty in the first 24 months. That would give the two drive user a 4% chance of a disk failure. The extra risks from the RAID controller failing, external faults like defective PSUs, power surges, shock damages etc can be added up. If scientifically done this may push the chances of a failure up to 10.27% or 8.43% or some other "exact" figure depending on how the stats are compiled -- but it will be higher than 4%.
So far we've seen that the risks of a drive failing are a lot higher than MTBF figures suggest. But the biggest risks are not hardware failures.
By far the largest number of PCs (using RAID) that are returned as faulty have perfectly working disks, controllers with no fault, PSUs pumping out the right voltages to the right places etc. Yet the user has lost all data and the Windows installation to boot. (Not another pun!)
Why? From our survey of a sample of our customers here's how it tends to happen:
The first and foremost risk is that the RAID BIOS loses the information it stores to track the allocation of the drives. We've seen this caused by all manner of software particularly anti-virus programs. Caught in time a simple recreation of the array (see last page) resolves the problem in over 90% of the cases.
BIOS changes, flashing the BIOS, resetting the BIOS, updating firmware etc can cause an array to fail. BIOS changes happen not just by hitting delete to enter setup. Software can make changes to the BIOS.
Disk managers, hard disk utilities, imaging and partitioning software etc. can often confuse a RAID array.
Reinstalling operating systems on top of existing installations or trying to repair a Windows installation by reinstalling the OS can cause problems.
And the #1 cause of data loss is (drum roll, please)...
...user error!
Very often users panic at the "insert boot disk" message. Panic causes users to make errors in recovering their PC to a fully working state. Staying cool is the key.
Next: Protection and Recovery >>
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