On Thu, 4 Mar 2010 17:00:10 -0500 (EST)
"Ken D'Ambrosio" [off-list ref] wrote:

On Thu, March 4, 2010 3:50 pm, Neil Brown wrote:

quoted

On Thu, 4 Mar 2010 11:12:29 -0500 (EST)
"Ken D'Ambrosio" [off-list ref] wrote:

[bad things happened to Ken's RAID-5 here]

quoted

You missed the bit where you provide concrete information rather than
vaguaries.

Humble apologies; I'm not well-enough versed with the intricacies of Linux
RAID to know what's appropriate and not.

quoted

I'm guessing that you created the array over whole-devices, and then
partitioned the array - is that correct? If fdisk shows you an
unpartitioned array, maybe just the partition table is corrupt.   Seems
strange.

Actually, no.  These were created using /dev/sd[abcd]2 -- I saved off
space on sd[abcd]1 for swap, /tmp, etc.  Done via the Ubuntu installer, if
that makes a difference.  For the record, all the /dev/sd[abcd]1 non-RAID
partitions look fine.

quoted

To so that I/we don't have to guess, please give exact commands that you
run and the exact output so we have access to the same information as you.

Well, I rebooted, and was surprised that nothing RAID-esque came up. 
Since my OS is on one of the afore-mentioned non-RAIDed partitions, the
OS, itself, booted, but none of the RAID partitions mounted.  I tried to
mount, and failed.  That's when I checked the RAID device, /dev/md0. 
fdisk showed it lacking any partitions, but the mdadm.conf file hadn't
been touched for a couple of weeks, so I was pretty sure nothing there had
changed.  On the off chance that the SCSI drives had re-ordered
themselves, I went through all 24 permutations of
mdadm --assemble /dev/md0 /dev/sda2 /dev/sdb2 /dev/sdc2 /dev/sdd2
since I wasn't sure if the drive order was significant.  All of them
"worked," inasmuch as they created /dev/md0, but in all cases it was
partitionless.  I also tried mdadm --detail --scan, to verify that it
matched UUIDs with those in the /etc/mdadm.conf file, and it did (the
array line looks thusly:
ARRAY /dev/md0 level=raid5 num-devices=4
UUID=1e89645a:7e24dcef:9e77d54f:077a6a6f  )

quoted

Too much data is much much better than not enough.

Granted... but, sometimes -- especially when learning -- there can be a
bad signal:noise ratio.  Tends to make me hesitant when I'm a (relative)
newbie to a given topic.  That being said, I *think* I've figured out what
I should be doing, but I also think I did it.  Did I miss something?

Which part of "please give exact commands that you
run and the exact output"
Did you have trouble with?

Try this:

cat /etc/fstab
cat /etc/mdadm.conf
mdadm -Esvv
mdadm -Asv
blkid -p /dev/md*

And include all the output.

(sorry if I seem grumpy, but I'm a bit tired which makes it harder to appear
polite).

NeilBrown