OP Peter and I exchanged a few Emails and I recommended he start with
a flavor of Ubuntu on a spare hard drive, and loop devices to learn
mdadm. He found it helpful and thought it might help someone else, and
despite this mailing list
""" not really really suitable for "how to get started with Linux"
information. """" the following is our EMail:
I would advise setting up Xubuntu on your spare drive, and leaving
your RAID disks completely disconnected while you learn mdadm.
On that drive, create a few blank 1GB files, and loop devices:
fallocate -l 1G file1.img
losetup /dev/loop0 file1.img
fallocate -l 1G file2.img
losetup /dev/loop1 file2.img
fallocate -l 1G file3.img
losetup /dev/loop2 file3.img
fallocate -l 1G file4.img
losetup /dev/loop3 file4.img
Then you can create a raid array with these fake hard drives
(/dev/loop0, /dev/loop1, etc...)
mdadm --create -n4 -l5 /dev/md0 /dev/loop0 /dev/loop1 /dev/loop2 /dev/loop3
Check rebuild status with:
cat /proc/mdstat
Create a file system:
mkfs.ext4 /dev/md0
Mount the filesystem
mkdir /mnt/testraid
mount /dev/md0 /mnt/testraid
Copy some files to it, maybe a movie, episode of SVU, etc, then:
mplayer somemovie.mkv
Then, while watching the movie, fail a disk
mdadm --fail /dev/md0 /dev/loop3
mdadm --remove /dev/md0 /dev/loop3
Check status, delete the loop device, delete the file:
cat /proc/mdstat
losetup -d /dev/loop3
rm file4.img
And I'll leave it to you to figure out how to create a new loop disk,
readd it to the raid, and resync while before your movie completes..
Once you are familiar and want to tackle your real drives.. From the
command line you can usu mdadm commands to attempt to --assemble the
array in degraded mode. When using the mdadm commands I believe there
are some special options for running in readonly mode, and/or not starting
it unless all devices are available. You may even need to use the
--force command if your drives are out of sync but you trust the data
on them.
When you start deleting superblocks and using the --create flag is
when you have to be careful.
-----------------------------------------------------------------------------------------
Hi Scot - excellent email - thanks a million...
Various hiccups getting the hardware ready but those were not
insurmountable. Install went OK and I remember how excited I used to
get about using the UNIX OS - various things are coming back to me.
I considered writing up the various tweaks to your instructions on the
mail server - do you think that would be a valid exercise that someone
else might gain from?
In case your feeling sceptical...
unused devices: <none>
peter@peter-MS-7374:~$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 loop3[4] loop2[2] loop1[1] loop0[0]
3142656 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/3] [UUU_]
[================>....] recovery = 82.4% (864464/1047552)
finish=0.1min speed=27014K/sec
unused devices: <none>
peter@peter-MS-7374:~$ cat /proc/mdstat
Personalities : [raid6] [raid5] [raid4]
md0 : active raid5 loop3[4] loop2[2] loop1[1] loop0[0]
3142656 blocks super 1.2 level 5, 512k chunk, algorithm 2 [4/4] [UUUU]
The film was an episode of Homelands - it didn't even hiccup!
If you have any other exercise suggestions to help me get up to speed
then I am all ears - off to bed now - somehow it got late!
Thanks for you assistance.
Peter.
---------------------------------------------------------
Peter,
For further exercises (to familiarize yourself with creating,
breaking, and rebuilding a raid), I recommend the following additional
scenarios:
a) With a working raid up and running, unmount the filesystem, stop
the array, then stop one of your loop devices. Try to assemble the
array with the missing disk, start and stop the array a few times, and
also familiarize yourself with the --run and --no-degraded options, as
well as the --examine features for understanding superblocks. Remember
just mounting filesystems may change metadata on the raid disks, so
this and this will impact the data integrity on the raid, even if you
don't manipulate any files.
b) After you have messed around a bit, maybe even changed some data in
degraded mode, stop the array, restart your 'missing' loop device and
attempt to restart the raid array with all the devices. After the
array starts degraded, you'll likely have to --add the disk again for
the rebuild to start.
c) Try to --create an array with your existing loop devices and check
out all the warnings you'll get about existing memberships in raid
arrays. You'll find that, with the exception of the --zero-superblock
command, it is usually pretty difficult to break things. If you
somehow convince mdadm to start or recreate an array with questionable
disks (like with the --assume-clean) option, familiarize yourself with
the various filesystem check tools.
--Scott
That leads me to the following general questions about mdadm and linux raid...
I have certainly RTFM and learned many things in the past dozen years
or so from internet examples, broken arrays, kernel panics on suspend,
bad drive cabling, mistypes using dd, blowing away the first gig of a
partition, growing, shrinking, migrating, etc. Are there formal test
cases and scenarios for mdadm and linux-raid?
Also many of the emails I have seen pass through this mailing list
involve some interesting combinations of raid device superblock
mismatches that beg the question.. How could you have possibly gotten
your raid components into *that* state...
In addition to the typical use cases covered in the manual (creating
an array, growing, shrinking, replacing disks, etc) it might be
interesting to have a list of misuse cases for folks to try and work
out.. (Ooops, I accidentally blew away my superblock, what can I do
without a full rebuild)
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