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Re: [Xen-users] remus vs. pacemaker/drbd?

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Subject: Re: [Xen-users] remus vs. pacemaker/drbd?
From: Greg Woods <woods@xxxxxxxx>
Date: Tue, 07 Sep 2010 09:55:19 -0600
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On Tue, 2010-09-07 at 09:28 -0600, Nick Couchman wrote:
> Pacemaker/DRBD, on the other hand, just synchronizes the data, 
> and if it detects that one of the domUs has died, starts it up somewhere 
> else. 
>  So, with Remus, the theory/goal is 0 downtime of your domU, whereas 
> Pacemaker 
> simply minimizes downtime to a certain point - the time it takes to detect 
> failure
>  and boot the new domU.

Actually, pacemaker can do live migrations with some limitations. If the
server on which the domU is running suddenly crashes, then domU does
have to be booted from scratch on the other server. But if one server is
taken down gracefully, Pacemaker can live migrate the domU (not
instantaneous but only takes a few seconds). This does require that the
domU disk images reside on shared storage though (NFS or a clustered
file system like GFS or OCFS).

I have no experience with Remus, but from Nick's description of it, it
sounds like Remus might be a whole lot easier to set up and may be a
good way to go if all you care about is failover for your domU's, or if
you really need instantaneous failover in the event of a server crash.

Pacemaker provides more flexibility in that you can have other things
like DHCP/DNS servers, NFS mounts, etc. also fail over. But Pacemaker
also has a very steep learning curve. Starting from scratch and never
having used either Xen or Pacemaker before, it took me almost six months
to build a fully functioning high availability setup for running Xen
domU's. I don't know how long it would take to learn Remus but it sounds
like it would be considerably easier.


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