[mythtv-users] INFO: RAID comparison for MythTV

[Ackster]

Robert Johnston wrote:

>On 4/21/05, MagicITX <magicitx at gmail.com> 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