Setting up CTDB on OCFS2 and VMs ...

[Richard Sharpe]

On Sat, Dec 6, 2014 at 4:21 PM, Michael Adam <obnox at samba.org> wrote:
> On 2014-12-07 at 00:48 +0100, Michael Adam wrote:
>>
>> So the important bit is that in your case ctdb
>> is running unprotected from split brain.
>> The only reference to split brain is a notification
>> of user steve in case drbd detects a split brain.
>> If I get it right (there are no details about this
>> in the blog post), this means that until user steve
>> reacts to that notification the ctdb/samba cluster
>> runs happily in the split brain situation and
>> corrupts the users' data.
>
> Ok, maybe it is not quite as bad. The config snippet
>
> net {
>   allow-two-primaries;
>   after-sb-0pri discard-zero-changes;
>   after-sb-1pri discard-secondary;
>   after-sb-2pri disconnect;
> }
>
> Which is explained to some extent in
>
> http://www.drbd.org/users-guide/s-configure-split-brain-behavior.html
>
> seems to indicate that in case of split brain
> certain measures are potentially taken.
>
> Also read the explanations about DRBD split brain here:
>
> http://www.drbd.org/users-guide/s-split-brain-notification-and-recovery.html
>
> This states that DRDB split brain is different from
> cluster split brain (also called cluster partition).
>
> So I'd really like to know what happens in your
> setup in a split brain situation.

Well, it turns out that drbd has this thing called dual-master mode,
which turns it into shared storage for two nodes only.

So, as long as the OCFS2 DLM is also running, there should not be any
split-brain events.

Making sure that the DLM was running was why I put so much effort into
getting the ocfs2-tools code running.

The disadvantage of using DRBD is that you cannot run more than a
2-node cluster.

-- 
Regards,
Richard Sharpe
(何以解憂？唯有杜康。--曹操)