Robert Johnston wrote:
On 4/21/05, MagicITX <[EMAIL PROTECTED]> wrote:
While the number of drives in a RAID 5 array is theoretically
unlimited, some recommend no more than 14 drives. The problem is RAID
5 is hosed if two drives fail. The more drives you have the more
statistically likely you are to suffer a two drive failure.
Incorrect. As RAID5 uses drives in sets of 3 (2 data + 1 CRC), then
you have to lose 2/3rds of the drives in the array for it to fail.
With just 3 drives, that means if 2 of the 3 drives fail, the array is
hosed. With 6 drives, that means 4 of those 6 have to fail, and so on.
Generally with RAID5 arrays that aren't a multiple of 3 (14-drive,
say), the array is configured with 12 drives in the array, and 2
drives as "Hot Spares" that are swapped in automatically if any one
drive fails.
Perhaps you are confusing RAID 3 with RAID 5. RAID 5 moves the parity
bit around (rotating parity array), no one single drive contains the
parity bits for all of the data. RAID 3 utilizes one drive to store
parity, but I'm not sure if it is in sets of 3 drives.
-Ack
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