Jonathan Gazeley wrote:

Dear all,

This is hopefully a simple question for you to answer, but I am fairly 
new to RAID and don't want to risk losing my data!

My setup is as follows:
- I have four 500GB disks. Each disk is split into a 5GB partition, 
and a 495GB partition.
- The four 5GB partitions are in a RAID-5 array, md0. CentOS is 
installed on this 15GB partition.
- The four 495GB partiions are in a RAID-5 array, md1. This partition 
holds my user data.

However, I have decided I no longer wish to two arrays across the 
disks. I've added a fifth disk on which I have installed the OS 
without RAID, meaning the old md0 is currently unused. Can I simply 
remove the four 5GB partitions, and resize the four 495GB partitions 
to fill the entire disk? Will this break anything?

Not unless you make a mistake in typing, or your hardware or power fails 
during the process. Of course in that case you will possibly lose 
everything.

Before anybody tells me off for having the OS on a non-RAID disk, this 
is a home server and therefore high availability is not an issue. But 
keeping my data safe against disk failures is important to me.

Having your o/s and swap on RAID reduces your chances of losing your 
data. If it were me and reliability were the goal, I would have the o/s 
mirrored on the first two drives (as seen by the BIOS) so you boot if 
one fails hard, then put swap RAID-10 in the little 5GB partition on the 
other three drives, then convert the raid-5 to raid-6 using the rest of 
the added fifth drive.

Anything which reduces the chances of an unclean shutdown improves the 
chances of keeping your data. A decent UPS is a big help in that regard. 
Disk failures on the data drives are not the only threat to your data!

-- 
bill davidsen [off-list ref]
  CTO TMR Associates, Inc
  Doing interesting things with small computers since 1979