[clug] forget RAID?

[Michael Cohen]

On Wed, Feb 21, 2007 at 04:56:47PM +1100, Chris Smart wrote:
> I'm no expert but in my opinion, the thing to realise is that hard 
> drives die. They die quicker and more easily when they are not operating 
> within their spec in regards to things like temperature. They are also 
> unreliable if they aren't connected to a decent power source.  So if you 
> have two hard drives in a computer and one dies, then you have a high 
> possibility that the other drive, operating in the same conditions, will 
> also die at some stage. But in reality all hard drives die. All, every 
> one, every single one ;)

Thats my experience too - I have seen too many raid setups which all die at
once to assume its merely bad luck. I think the worst thing is that usually a
raid array will have all the disks from the same initial batch - because they
all got bought together - which means that their MTBF will be very very
similar.

> I agree that raid5 is not really a great level of protection. If you 
> lose one hard drive, then you cannot afford for another to die, yet to 
> re-build the array you need to 'stress' the rest of the drives to 
> re-build the newly inserted drive from parity information. That means 
> you have a higher chance that one more hard drive will die and if it 
> does then you lose all your data. Of course you have the same problem in 
> a 2 x drive raid1 system, but in a 4 x drive raid5 you triple your 
> chances of losing your data in a rebuild because any one of the 
> remaining three could die.

Thats true, but dont discount raid altogether - usually people use raid because
it gives you much better performance for some types of disk access patterns.
For reading raid 1 is great, not so good for writing.

> And remember it's not just about hard drives dying, it's also about hard 
> drive corruptions, bad sectors and things that like that which affect 
> the integrity of your array.

Again this is very good advice - raid is never a substitute for your backups. I
prefer having backups at the file level though because bad sectors/ corruptions
can get detected much faster that way.

> Raid 1 is the best option then. Why would you get rid of raid1 and have 
> a single drive running and then backing up to another? Just use raid1 

Because if there is a filesystem corruption or bad sectors on the disk, raid 1
will just propegate the garbage around and trash your "backup" drive
automatically. Raid 1 is not a backup either - better to use a seperate disk
and implement proper generational/incremental backup.

RAID 1 is totally useless for cases where you wrote the wrong thing on the
disk, and need to recover the data - as in a user calling you telling you they
did an rm -rf / on your system (hopefully they were not root at the time).
RAID1 will just propagate the new data (zeros everywhere) to _all_ your disks
almost immediately.

Thats what backups are for.

Michael.