[DRBD-user] Lower device already claimed error

Digimer lists at alteeve.ca
Wed Jul 11 22:27:22 CEST 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.

On 07/11/2012 04:19 PM, Arnold Krille wrote:
> On 11.07.2012 22:12, Digimer wrote:
>> On 07/11/2012 04:07 PM, Arnold Krille wrote:
>>> On 11.07.2012 21:20, Digimer wrote:
>>>> I would guess so, but you know your system better than us. I can't see
>>>> any reason though why you would want to use a single RAID member as a
>>>> DRBD device... Using the RAID device makes a lot more sense.
>>> What if someone uses one leg of an software-raid1 as a disk for drbd on
>>> machine a and mounts the physical device of the same drbd on machine b?
>>> From the don't-do-this-at-home-department,
>> You simply couldn't. The device would be a member of the array and,
>> thus, DRBD will refuse to use it, as Lin has seen. Forcing the issue
>> would be kind of like having one person pushing the gas and brakes on a
>> car while another person steered, with no real coordination between the
>> two.
>> As the kids say these day, "you'll have a bad time".
> So, once again I forgot to add the funny-markers...
> On a serious note, if I remember correctly drbd saves its "this is a
> drbd-device" information at the end of the partition (if at all), so
> when you plug the disk into a different machine and just want to access
> the data, its the same as with md (with metadata=0.99): Just mount the
> partition.
> Arnold

I figured you were kidding around, but I thought it a good chance to
throw the answer into the archives. :)

The metadata, when "internal", does get written to the end of the
backing devices. You can mount the raw device, but of course, few if any
FS will know what that metadata is, so it could well be scrogged.
Mounting a backing device read-only *should* be safe, but I haven't
tried it myself.

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