Preventing automatic start of ARRAY on Kubuntu

George Rapp

2014-10-16 16:19:50 UTC

Post by P. Gautschi
I'm currently experimenting with RAID5 on Kubuntu 14.10 Beta 2.
When the system boots, /dev/md127 is automatically started. This occurs
whether the line (output after creation of mdadm --detail --scan)
ARRAY /dev/md0 metadata=1.2 name=test:0 UUID=2e0d5db2:47c49e4b:75c06b8b:11bf68cf
is in /etc/mdadm/mdadm.conf or not.
I would expected that the array is only started when the line is in the file
and then as /dev/md0 and not as md127.
My final goal is to prevent the start of the array when it is degraded
(missing or previously failed disk).
I had problems with a power cable and one disk had failed. While checking
the cables a second disk failed temporarily. Even though I wasn't writing
to the fs on the array I ended with and array that could not be assembled
again, after all disk were working again. (I have tried reboots,
mdadm --assemble --force and the other tricks from
https://raid.wiki.kernel.org/index.php/RAID_Recovery
but ended with a corrupt fs.)
I'm now trying to minimize the risk of data loss. Not starting the
array when degraded (or starting it read-only) is one of my ideas
but I don't know where and how to implement this.

[re-sending this without the HTML, which the list manager doesn't care for]

Patrick -

Here's what I did (on Fedora, but I suspect the process will be
similar on any distro):
Make whatever changes you need to in /etc/mdadm.conf (for example,
removing /dev/md0 and /dev/md127);
Create a new initramfs to reflect your changes:
# cd /boot
# mv initramfs-$(uname -r).img initramfs-$(uname -r).img.backup
# dracut --mdadmconf /boot/initramfs-$(uname -r).img $(uname -r)
(The dracut command is not provided in Fedora by default, but requires
a package; in Fedora 20, the current package is
dracut-037-11.git20140402.fc20.i686.)

This creates a new initramfs with your updated mdadm.conf. After you
reboot with the new initramfs image, you will have to start whatever
MD array(s) you want manually. At this point, for example, you could
try to start the array without --force, and it would refuse to start
if degraded.

Post by P. Gautschi
Another idea is, that the array could switch to read only when
degraded during runtime. Ideally this would be delayed until the
fs does a flush/sync to be in a recoverable state. Of course file
accesses would fail is such a situation but it prefer this to
the rick of loosing data.
Is it possible to do this?

My Fedora 20 kernel (3.16.4-200.fc20.i686+PAE) seems to do this
automatically for me. My RAID 5 suffered a disk failure that dropped
it down to one (of three) component devices, and the kernel stopped
writes immediately to prevent data loss. After powering off and
reassembling my array with --force to sync up the event counters, I
didn't lose anything.

Good luck.
--
George Rapp (Pataskala, OH) Home: george.rapp -- at -- gmail.com
Work: george.rapp -- at -- hp.com (or) george.rapp.ctr -- at -- dfas.mil

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