C[omp]ute

From: "andrew cooke" <andrew@...>

Date: Sun, 4 Mar 2007 21:12:44 -0300 (CLST)

In retrospect, software RAID in Linux is easy.  If you're doing a clean
install there's nothing to it.  However, I couldn't (for a long time) find
anything that said this.  So I read explanations about RAID technology and
wondered how on earth I could boot from a software RAID - it seemed like a
chicken and egg situation unless you went with simple mirroring, which was
inefficient.

So I started by trying to install the OS on a separate PATA disk (the
P5LD2 has a single parallel IDE controller that is intended for CD/DVD). 
Luckily this failed (Grub crashed) and when I went to the SuSE IRC channel
some jerk knew enough to imply I was an idiot, but not enough to answer
any questions.

Still, it was useful to know I was wrong.  Searching some more I found
this page -
http://en.opensuse.org/How_to_install_SUSE_Linux_on_software_RAID -
(obvious in retrospect, but I turned it up only when I switched googling
from "suse" to "opensuse").  That was enough to make me realise I had
missed a "RAID button" on the installer interface.

In fact, I hadn't missed it - I just didn't realise how powerful it was.

So here's what I did - it uses just the 4 identical SATA disks I bought
and it gives you a system completely on RAID that makes efficient use of
disk space.  All the below is via the "expert" partitioning section of the
install (from scratch).

1 - Partition each disk the same.  I used the following scheme for all:
  sd?1:  100MB  (destined to be /boot)
  sd?2:    4GB  (destined to be swap)
  sd?3   290GB  (destined to be root)
(Incidentally, I don't quite see how 320GB drives end up being 290GB in
size).

2 - Instead of formatting these partitions, mark them to be used as RAID
(in the create or edit dialogue, select "do not format" and then "RAID").

3 - Use the drop-down RAID button/menu to join these into three different
RAID devides:
  md0:   100MB RAID 1  /boot  (ext3, mirrored across all disks)
  md1:   3x4GB RAID 5  swap
  md2: 3x290GB RAID 5  /      (ext3)
Note that /boot is RAID 1, which means each disk is the same.  That lets
you bootstrap the system (RAID5 would be unreadable because it's striped).
 This is a waste of space, but it's very small so doesn't matter.  Next,
swap is RAID just so that the system doesn't crash if a disk dies (it
would be faster to have swap without RAID, distributed across all disks). 
Finally, I use a single partition for everything else.  You can, of
course, do something more complex.

Once you do this "everything just works".  The md0... devices "replace"
sda1..., sdb1.. and SuSE does all the right things.  The only difference
from normal is that when it boots you see lots of exciting messages as the
kernel puts everything together.

One final note - it seems that it's best to use SMART for disk monitoring
(or you may not realise when a disk dies).

For the record this was with SuSE 10.2, x86 64bit, and I finally
downloaded the CDs since it was faster overall than repeated experiments
with network install.

Summary: once you work up the courage (and use the RAID button/menu) this
is easy.

Andrew