CHAPTER 10 n HARDWARE CONSIDERATIONS 250
RAID 6
Generally used only with SATA drives (where the number of devices is limited), RAID 6 is
for those who are ultraparanoid about data loss. RAID 6 involves not one but two parity
stripes. That means it would take three discrete disk failures before any data would be
lost (see Figure 10-6).
Figure 10-6.With RAID 6, two drives can be lost, and the array remains available.
The drawback with RAID 6 is that essentially two entire disks are wasted on storing
parity information. If you had four 1TB drives arranged in a RAID 6 array, you would be
left with only 2TB of usable storage.
MEAN TIME TO FAILURE
Disk drives include a specification, usually entitled ???mean time to failure??? (MTTF), which lists the
amount of time that a drive should function before it fails. The key word here is failure. A disk drive has
mechanical parts, and it will fail eventually. A drive won??™t necessarily fail when it has passed its MTTF
rating (I??™ve got a 20MB disk drive that is a decade old that still runs), but assuming that the drive will
run through the life cycle of the server is a dangerous game to play.
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