Converting single non-raid to raid1

[mark-OGhnF3Lt4opAfugRpC6u6w at public.gmane.org (Mark J Dulcey)]

On 2/17/2010 11:06 AM, Jerry Feldman wrote:

> Does the new physical drive need to be the same geometry as the older
> drive. Let's say I have a Seagate 1TB, and buy a new WD 1TB drive, will
> that cause a problem as long as the raid partition on both are the same
> size.

No special requirements for geometry. The only constraint is that the 
RAID can only be as large as the smaller of the two drives -- drives 
from different manufacturers and even different manufacturing runs vary 
slightly in size.

There is an argument in favor of using non-identical drives. Drives from 
the same manufacturing run tend to have correlated failures (that is, 
they're more likely than normal to fail at about the same time -- that's 
one reason that RAID 5 setups have been far less reliable in the real 
world than statistical analysis predicted), so it's better to make your 
RAID 1 out of drives from different manufacturers or at least different 
manufacturing lots.

> Secondly, is there any advantage or disadvantage to allocating the boot
> partition as a raid1. I think in a prior discussion, there was an
> overwhelming opinion that swap should be on the raid1.

Using RAID1 for the boot partition gives you a built-in backup, though 
GRUB won't take advantage of it automatically (it loads the OS from the 
boot partition as an ordinary non-RAID ext3 or whatever) so you might 
have to manually reconfigure your boot setup to recover from a drive 
failure.

Swap on RAID1 gives you higher read performance (because the reads can 
be divided), slightly lower write performance (because the data has to 
be written to both drives, and there is more CPU overhead), and data 
integrity. All in all a good deal. But with RAM as cheap as it is 
nowadays, why is your system swapping?