Xen users - Mar 2006 - evms-ha + Xen

Hi,

Curious if anyone here has tried evms-ha plugins with Xen and heartbeat.

My concept is to slice shared disks (SAN) into volumes and use volumes 
as VBDs for domUs.  I am hoping that EVMS can offer some assurance that 
no 2 domUs will simultaneously access a given area of disk at the same time.

Has anyone here attempted this or is anyone aware of fundamental flaws 
in the idea?

I will certainly post HOWTOs if things work out.

Thanks,
Fraser

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Fraser Campbell <fraser@georgetown.wehave.net> on 03/17/2006 04:03:02 PM:
> Hi,
>
> Curious if anyone here has tried evms-ha plugins with Xen and heartbeat.
>
> My concept is to slice shared disks (SAN) into volumes and use volumes
> as VBDs for domUs.  I am hoping that EVMS can offer some assurance that
> no 2 domUs will simultaneously access a given area of disk at the same
time.
>
> Has anyone here attempted this or is anyone aware of fundamental flaws
> in the idea?
>
> I will certainly post HOWTOs if things work out.
>
> Thanks,
> Fraser
If you use separate volumes for the VBDs for the domUs then no two domUs
will access the same area of the disk, regardless of whether the disk space
comes from a SAN or whether you are running Linux-HA or not.

It would be helpful to know how you are using Xen and Linux-HA together.  I
can think of a couple different setups.  Are you running the domUs as
separate nodes in a cluster?  Are you running the whole physical machine as
a node in a cluster, for example, with another machine standing by as a
backup?  Some other setup?

Steve D.


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Steve Dobbelstein wrote:
> If you use separate volumes for the VBDs for the domUs then no two domUs
> will access the same area of the disk, regardless of whether the disk space
> comes from a SAN or whether you are running Linux-HA or not.
You are correct but I need to be sure that the same domU cannot be started 
twice ...

The idea is to have 2 or more dom0s connected to common disk(s), I''m
not a
big fan of nfs root solution so we''re looking at SAN.

Not being familiar with EVMS terminology yet so I''ll revert to use of
LVM
terminology for now ...

If 2 dom0s try to bring up the same domU then we have a problem. I presume 
a cluster aware volume manager (evms-ha) would mark the LV as busy and 
there would be no fear of the LV ever being double mounted.

Also with evms-ha I should be able to create an LV from one node and have 
all other connected nodes immediately see it (to make live migration 
possible without manually rescanning other nodes for new LVs).

> It would be helpful to know how you are using Xen and Linux-HA together.  I
> can think of a couple different setups.  Are you running the domUs as
> separate nodes in a cluster?  Are you running the whole physical machine as
> a node in a cluster, for example, with another machine standing by as a
> backup?  Some other setup?
Multiple physical nodes (dom0s) sharing some disk(s). I like the idea of 
evms-ha for the already mentioned (presumed) benefits - extra safety 
measures preventing split-brain (starting same domU in 2 places).

Heartbeat''s exact role within the solution (if any) I haven''t
determined,
mostly I''m looking at it because I believed it was required for
evms-ha.
Some other uses for heartbeat do come to mind of course:

* it would have a list of all domUs within cluster and possibly a preferred
   dom0 for each domU
* it could be used as a framework for migrating domUs (perhaps triggered by
   utilization, hardware issues, shutdown, etc.)

Thanks for the reply
-- 
Fraser Campbell <fraser@wehave.net>                 http://www.wehave.net/
Georgetown, Ontario, Canada                               Debian GNU/Linux

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