[mythtv-users] INFO: RAID comparison for MythTV

Kevin Kuphal kuphal at dls.net
Fri Apr 22 00:21:00 UTC 2005


Robert Johnston wrote:

>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.
>  
>
This is incorrect (see the wikipedia entry linked above).  RAID 5 writes 
one set of parity across all drives.  If 2 drives are lost, no matter 
the number in the set, all data is lost.  Additional hot spares can 
mitigate the risk at the expense of spare disks.  If you think about it, 
how could 2 disks (of your six disk set) possibly contain all the data 
from the failed 4?  That would mean 200GB (for 100GB disks) storing all 
the data for a 600GB RAID set or at least all the parity data needed to 
rebuild that data.

Kevin


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