[linux-lvm] LVM and fault tolerance
Russell Coker
bofh at coker.com.au
Wed Mar 28 06:43:40 UTC 2001
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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