[mythtv-users] INFO: RAID comparison for MythTV

Robert Johnston anaerin at gmail.com
Thu Apr 21 23:13:47 UTC 2005


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.

Generally, the reason no more than 14 drives per RAID5 is recommended
is that the maximum number of devices you can have on a Wide SCSI (UW,
U2W, U160, U320) bus is 15, which taking one off for the controller
card, leaves a maximum possible of 14 drives per channel. RAIDing
across channel on SCSI is less than optimal.

> I have seen the argument that you shouldn't get all of your drives at
> the same time since they are more likely to fail at about the same
> time.  As mentioned above that is bad with RAID 5.

This one is true, however. A company I was working for got bitten by
this when a caseload of IBM U320 SCSI HDD's all failed within 90 hours
of being installed into the new Domain Controllers, Mail Servers and
live Web Servers, as the case had been dropped at the distributors.
-- 
Robert "Anaerin" Johnston


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