Setting up CTDB on OCFS2 and VMs ...

[Chan Min Wai]

Dear Michael,

Thank for the mail with explanation. 

I'm using CTDB with OCFS2 and without Drbd. (worst then Steve situation I believes)

But base on you explanation I briefly understand the risk and also what am I missing. 

I really believes that I've the CTDB running by accident. 

As per your description due to the locking problem I run CTDB in one mode and join the AD. 

Thank for Steven suggestion I turn off  locking and everything work. 

So I'm in that situation. 

I'll try to add in peacemaker and cman in future and change Gentoo wiki guide. 

I'm really wondering why is with all the screaming for and now I know why. 

In Steve and mine situation the splits brain issue on CTDB are not really well handled, we totally dependent on the recovery script on that. 

But Drbd would have handle the ocfs2 issue. (As for me who share one network storage over 2 server that might not be an issue.). 

It will be a big issue if we are using CTDB with PDC or classical domain. But as a member server, I'm not too sure how much problem we will have as all the information coming from AD DC. 

Thank for the explanation again. 

Regard,
Min Wai


> Michael Adam <obnox at samba.org> 於 2014年12月7日 07:48 寫道：
> 
>> On 2014-12-06 at 20:59 +0100, steve wrote:
>> 
>> Now please let us talk about Setting up CTDB on OCFS2 and VMs ...
>> 
>> One way to set up an active:active 2 node cluster is to use ocfs2, drbd and
>> ctdb. We have documented it and 3 independent domains are using it. It is
>> available now. It has been tested in the field since September. That's what
>> we offer. We thought we'd mention it. It may save you a lot of time getting
>> your cluster into production. Take it or leave it. But please do not
>> misrepresent it.
> 
> From reading your various posts here on the list,
> which were pretty vague, one could get the impression that
> 
>  you are using drbd in parallel/addition to ocfs2
>  to provide a split brain prevention mechanism
>  as a substitute for the recovery lock file
>  placed on ocfs2.
> 
> When asked, you never detailed how you accomplished
> this (to use drbd for split brain), though.
> (Neither did you elaborate in which way the recovery
> lock file did not work in your setup.)
> 
> Now I read your blog post, and it sheds some light...
> I am not an expert of ocfs2 or drbd, and you don't
> really explain it but merely give the instructions
> (and do some amount of bashing of available documentation..).
> But here is what I understand from your post:
> 
> - You use drbd to replicate the block storage for ocfs
>  between the two nodes.
> - You format the drbd block device with ocfs2.
>  This gives you a clustered ocfs2 active on both nodes.
> - You configure ctdb on the two node cluster
>  with mangement of public addresses and Samba,
>  but *without* split brain protection.
> - Samba is run in all-active clustered mode using ctdb
>  for databases and failover and the ocfs2 for shares.
> 
> 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.
> 
> Note: notification of a split brain does
> not imply protection of the damages it can do!
> So the question is really: what apart from
> notifying steve is your cluster doing when
> a split brain occurs?
> 
> 
> One more comment to your blog post:
> 
> According to your instructions, you call "net ads join"
> before starting ctdb. This can not work. net ads join
> needs ctdb running to operate, because it needs to
> store the join information in the clustered secrets.tdb
> handled by ctdb.
> 
> A procedure I'd recommend is this:
> 
> - configure ctdb without MANAGES_SAMBA and MANAGES_WINBIND
> - start ctdb
> - do net ads join
> - configure ctdb to use MANAGES_SAMBA and MANAGES_WINBIND
> - restart ctdb
> 
> 
> Cheers - Michael
>