Bug#567468: md homehost
Neil Brown <firstname.lastname@example.org> writes:
> On Wed, 24 Feb 2010 14:41:16 +0100
> Goswin von Brederlow <email@example.com> wrote:
>> Neil Brown <firstname.lastname@example.org> writes:
>> > On Tue, 23 Feb 2010 07:27:00 +0100
>> > martin f krafft <email@example.com> 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.
Yes you can do something about it: Only start the raid arrays with the
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?