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Bug#567468: md homehost

Neil Brown <neilb@suse.de> writes:

> On Wed, 24 Feb 2010 14:41:16 +0100
> Goswin von Brederlow <goswin-v-b@web.de> wrote:
>> Neil Brown <neilb@suse.de> writes:
>> > On Tue, 23 Feb 2010 07:27:00 +0100
>> > martin f krafft <madduck@madduck.net> wrote:
>> >> The only issue homehost protects against, I think, is machines that
>> >> use /dev/md0 directly from grub.conf or fstab.
>> >
>> > That is exactly correct.  If no code or config file depends on a name like
>> > /dev/mdX or /dev/md/foo, then you don't need to be concerned about the whole
>> > homehost thing.
>> > You can either mount by fs-uuid, or mount e.g.
>> >    /dev/disk/by-id/md-uuid-8fd0af3f:4fbb94ea:12cc2127:f9855db5 
>> What if you have two raids (one local, one from the other hosts that
>> broke down) and both have LVM on it with /dev/vg/root?
>> Shouldn't it only assemble the local raid (as md0 or whatever) and then
>> only start the local volume group? If it assembles the remote raid as
>> /dev/md127 as well then lvm will have problems and the boot will likely
>> (even randomly) go wrong since only one VG can be activated.
>> I think it is pretty common for admins to configure LVM to the same
>> volume group name on different systems. So if you consider raids being
>> pluged into other systems please keep this in mind.
> You are entirely correct.  However lvm problems are not my problems.
> It has always been my position that the best way to configure md is to
> explicitly list your arrays in mdadm.conf.  But people seem to not like this
> and want it to all be 'automatic'.  So I do my best to make it as automatic
> as possible but still remove as many of the possible confusion that this can
> cause as possible.  But I cannot remove them all.
> If you move disks around and boot and lvm gets confused because there are two
> things call "/dev/vg/root", then I'm sorry but there is nothing I can do
> about that.  If you had an mdadm.conf which listed you md arrays, and had
>    auto -all
> then you can be sure that mdadm would not be contributing to this problem.
> NeilBrown

Yes you can do something about it: Only start the raid arrays with the
correct homehost.

If the homehost is only used to decide wether the prefered minor in the
metadata is used for the device name then I feel the feature is entirely
useless. It would only help in "stupid" configurations, i.e. when you
use the device name directly.

Another scenario where starting a raid with the wrong homehost would be
bad is when the raid is degraded and you have a global spare. You
probably wouldn't want the global spare of one host to be used to repair
a raid of another host.


PS: If a raid is not listed in mdadm.conf doesn't it currently start too
but the name can be random?

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