[mythtv-users] INFO: RAID comparison for MythTV

[Ijaaz A. Ullah]

-----Original Message-----
From: mythtv-users-bounces at mythtv.org
[mailto:mythtv-users-bounces at mythtv.org] On Behalf Of Robert Johnston
Sent: Thursday, April 21, 2005 7:14 PM
To: MagicITX; Discussion about mythtv
Subject: Re: [mythtv-users] INFO: RAID comparison for MythTV

> 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.

RAID5 means striped data with striped parity.  It is not based on a 2 data,
1 CRC system. This is just the minimum amount of drive you need to create
one.  You can have any amount of drives greater than 2 in a RAID5.  If you
have a 14 disk RAID5, data is striped across all 14 disks.  If you lose any
drive, then the system will recover the missing information from the drive
through parity.  If you lose a 2nd drive, there is no parity information to
recover the data from.

The idea of 2 data + 1 CRC may work for a 3 disk RAID5, but does not work
for any other.  

If a RAID5 loses a disk when it is reduced (1 drive already failed), then
you've lost all of your data.

In general you'd have to have 2 complete physical failures to lose all of
your data.  Because it's always possible to re-assemble the raid and recover
data from the parts of the disk that are not destroyed (bad sectors, etc).