I'm halfway through setting up the root partition (and /home) to be
raid1 mirrors (thanks to help from Jeff Waugh and Jamie Wilkinson).
I've booted up from my old installation on /dev/hda6 to set up the real
/dev/hda7,/dev/sda7 to be /dev/md0 for /, and /dev/hda8,/dev/sda8 to be
/dev/md2 for /home.
I've set the partition types on hda7, sda7, hda8 and sda8 to "fd",
Linux RAID autodetect.
I've also installed grub on (hd0,6) and (hd1,6) (i.e. on /dev/hda7 and
/dev/sda7).
My two questions are: is a raid mirror device supposed to have a
partition table? If so, what should it be set to - just one partition
containing the number of blocks that the real partitions making it up
have? What should the start and end cylinders be set to? (The reason I
ask is that the raid devices are failing the filesystem check upon
boot.)
And secondly, how do you tell grub to use md0 instead of hd0 or hd1?
Is it as simple as telling it that device (hd0) is /dev/md0? (Though I
don't see a way to do that in /boot/grub/menu.lst)
How I created the raid arrays
-----------------------------
I basically unmounted the drives I wanted to turn into pieces of the
raid, used mdadm to add the (empty) partitions from sda into the raid
array, telling it that the other piece was missing, then mounted the
raid drive and copied the files from the (populated) partition, then
unmounted that populated partition and added it to the raid and watched
as the raid rebuilt itself. I could then mount /dev/md2 as /home and ls
files.
My first question is: how do you tell grub what to use as the root
drive, given that grub seems to want a physical drive spec (e.g. hd0 or
hd1)? How do you tell grub to use md0 instead of hd0 or hd1?
In my case, hd0 is /dev/hda and hd1 is /dev/sda.
I've only created md0 from hda7 and sda7 and md2 from hda8 and sda8.
I also have this in /etc/mdadm/mdadm.conf:
DEVICE /dev/hda* /dev/sda*
ARRAY /dev/md0 devices=/dev/hda7,/dev/sda7
ARRAY /dev/md2 devices=/dev/hda8,/dev/sda8
(My plan is that if I can get md0 working as /, then I'll setup md1 as
another raid array for the older system I installed initially, on hda6.
I'll use sda6 as its mirror.)
But cat /proc/mdstat though shows already md1 exists (!) and it seems to
be what I thought should be md2!
Personalities : [raid1]
md2 : active raid1 dm-5[1] dm-4[0]
121531584 blocks [2/2] [UU]
md1 : active raid1 sda8[0] hda8[1]
121531584 blocks [2/2] [UU]
md0 : active raid1 sda7[0] hda7[1]
12586816 blocks [2/2] [UU]
Where did md1 come from? What device is "dm-5" and "dm-4"?
I should mention that when trying to create the md0 raid array I got an
mdadm error something like "Invalid parameter", and an mdadm -Ds showed
that md0 had the same ID as md2. I had to do an mdadm --stop --scan
before I could get the "mdadm --create /dev/md0 --level=1
--raid-devices=2 /dev/sda7 missing" to work. After it did, an
"mdadm -Ds" showed that md0 had a different ID to md2 (and there was no
md1 at that point, either).
Though just now checking that again I see that mdadm -Ds shows that
there is a /dev/md1 array and that it has the same UUID as /dev/md2!
Also when I try to boot up now the system drops me into maintenance
mode when it attempts to check /dev/md2 (/home) and fsck reports this
error:
/dev/md2: the filesystem size according to the superblock is
30,382,923 blocks.
Physical size of the device is 30,382,896 blocks.
Either the superblock or the partition table is likely to be
corrupt!
If I run fdisk on /dev/hda or /dev/sda it tells me that hda8 and sda8
have 121,531,693+ blocks.
If I run fdisk on /dev/md2 it says it contains no partition table, and
that it has 30,382,896 cylinders.
I've previously copied /home to a spare partition, so wasn't too
frightened of running fsck on /dev/md2. It seemed to complete fine,
reporting no errors, but it always reports "the filesystem size according
to the superblock is 30,382,923 blocks. Physical size of the device is
30,382,896 blocks. Either the superblock or the partition table is likely
to be corrupt! Abort<y>? "
If I "mount /dev/md2 /home", everything looks good - all the files seem
to be there.
So my two questions are: is a raid mirror device supposed to have a
partition table? If so, what should it be set to - just one partition
containing the number of blocks that the real partitions making it up
have? What should the start and end cylinders be set to?
And secondly, how do you tell grub to use md0 instead of hd0 or hd1?
luke
------------------ For reference: ----------------------
From: Jeff Waugh <[EMAIL PROTECTED]>
Subject: Re: [SLUG] Adding RAID to Linux *after* installation?
Date: Sun, 30 Jul 2006 07:03:37 -0700
To: [email protected]
<quote who="[EMAIL PROTECTED]">
> mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/hda7 /dev/sda7
>
> it refuses, as /dev/hda7 ("/") is mounted.
Okay, so do this instead:
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda7 missing
That will create a 2 device array with one missing device. You need to bring
up the array (watch /proc/mdstat to see its status), mount it, then copy
over everything from / (using cp -a or similar). Now set up everything in
the copied partition to boot from /dev/md0 instead of /dev/hda7, and then
reboot. Boot from /dev/hda0 by using the grub menus (if you've made sure
they're changed), or edit the boot command line to do it manually. You
should find yourself running a system with / on /dev/md0. At this point, you
can use mdadm to add /dev/sda7 to the array - after checking everything is
okay, of course! :-)
This is not a wildly uncommon thing to do, but it's so easy to install with
RAID now that I think most people just do that.
> Obviously I should try this on the /home partition, not slash, until I get
> the steps worked out. At least that way if I lose any data, it'll be a
> tiny amount of junk instead of 10GB of carefully setup Linux system!
That's a good idea. :-) Running an array reliably on / or /boot requires
some extra work anyway.
- Jeff
-----------------------------
From: Jamie Wilkinson <[EMAIL PROTECTED]>
Subject: Re: [SLUG] Adding RAID to Linux *after* installation?
Date: Mon, 31 Jul 2006 15:11:56 +1000
To: [email protected]
This one time, at band camp, [EMAIL PROTECTED] wrote:
>I couldn't even find a howto for setting up a software raid (well, not
>one that talked about mdadm - only old ones talking about the older
>raidtools).
http://lists.slug.org.au/archives/slug/2004/05/msg00374.html
--
--
SLUG - Sydney Linux User's Group Mailing List - http://slug.org.au/
Subscription info and FAQs: http://slug.org.au/faq/mailinglists.html
