[Linux-cluster] Re: Linux-cluster Digest, Preventing LVM from concurrent access

[Edson Marquezani Filho]

>
> You have some interesting and different ideas here:
>
> 1.- Shared storage with non-clustered filesystem: i have no experience
> with HA-LVM, i have just checked its knowledge base on RHEL to refresh
> my ideas.
>
> If i'm not wrong, what you want is to have a shared storage with
> protection from concurrent mounts for your non-clustered filesystems,
> but EVEN for admins doing, by hand, erratic mounts, am i right?

Yes, that's it.

> You can use a different approach to HA-LVM with CLVMD and a resource
> script i have developed called "lvm-cluster". You can find it here:
>
> https://www.redhat.com/archives/cluster-devel/2009-June/msg00065.html
>
> It's not included in the main project yet, but some people in the list
> are testing it successfully.
>
> Actually it would not avoid an admin doing, by hand, an erratic mount
> but a bugzilla ticket has been opened by Brem Belguebli to fix an LVM
> issue that is causing that behaviour. Apart from this admin issue, the
> script would protect the concurrent mount of your filesystem by the
> usage of the LVM layer.
>
> I do like more this approach because you don't need to generate new
> initrd images for your nodes every time you configure new volumes to be
> into the HA-LVM.

Hum... that's good too. By the way, I was concerned about that
requisite, which is absolutely not much pleaseant to do.

Anyway, I'm trying to set up your script here, but I realized that
actualy I still don't know how to do that. =/
(I'm using that tool called Luci, which looks good, specialy to
someone that knows almost nothing about cluster. =)

Ok, I have defined a script resource with the path of your script, and
I guess that now I have to create a service, that's it?
But how, if I don't have a single service to be watched. (Actualy, my
service is my whole server, we could say.)

Could you give me a configuration example? I'm really confused here yet.

Wait, should I write a script for this task and use it on service
configuration? I have tought about a script that always returns OK to
verify-all, and status, and lauch VMs for start. Does it make any
sense?

> 2.- Communications interruption and split-brain: you should use some
> fencing system to make the 2nd node "kill" the main one and take the
> resources.

Ok, I have understood that there's no way to avoid that problem unless
turning off a server. Anyway, this is acceptable, once I don't expect
such a situation happening oftenly.

> Having the main node booted back again AND the comms not reestablished
> will cause you a split-brain again, with a new fencing action. I usually
> do not set cluster services (CMAN, RGMANAGER, etc.) to be automatically
> started on the nodes. I mean they must be re-run and activated by an
> admin.
>
> In the other hand you can use a Quorum Disk, provided my CMAN too, as
> the 3rd vote on your cluster and keep the quorum. I use both approaches
> at the same time (no auto-start of cluster services and qdisk).
>
> 3.- Communications channels: there's no way to set on CMAN more than one
> comm channel, but you can set more than one network interface to be part
> of that channel, via bonding. I think this is one of the lacks of CMAN
> actually.
>
> I hope this helps. Just ask whatever you want.
>

Thank you, very much.