Re: [linux-lvm] LVM and fault tolerance

[Russell Coker <bofh@coker.com.au>]

On Wednesday 28 March 2001 08:29, Anders Widman wrote:
> ok. great... so if I make say 20GB partitions on all disks and put them
> in a RAID-5 array. Can I add extra disks and rebuild the array? I have
> seen some expensive RAID cards that does that.

RAID-4 involves having N data disks (N >= 2) and 1 parity disk.  The parity 
disk contains the XOR of the blocks on the N data disks.  If one of the N 
disks dies then it's contents can easily be regenerated by the XOR of the 
surviving N-1 disks and the parity disk.
RAID-5 is the same but has the parity data spread across all the disks for 
best performance.  Thus RAID-5 has 3 or more disks.

In RAID-4 or RAID-5 if you lose two disks at the same time then the XOR won't 
get your data back and you are comprehensively stuffed.

If you create a RAID-5 with two partitions on the same disk then you may as 
well use a bulk eraser.

The scheme I mentioned in my previous message is the simplest way of doing 
this with such disks.  But really if your time is worth more than about $10 
per hour you should just buy some more 80G disks.

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