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

[Miles Fidelman <mfidelman@xxxxxxxxxxxxxxxx>]

Nick Couchman wrote:
> It's worth noting that these are slightly different solutions.  Remus keeps a 
> running (but paused), completely synchronized version of your domU(s), and, 
> when it senses that the original domU is not available, unpauses the standby 
> domU from it's last synchronized state.  This is really any extension of live 
> migration, as remus is just continually live migrating your memory and disk 
> data, but never shuts down the original or unpauses the standby version unless 
> something happens to the original.  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.
>
>    
Understood.

In my case, I'm running Pacemaker/DRBD and can live with short outages.  
But... it's pretty tricky to keep all the pieces configured and running 
properly, and I'm not looking forward to the gotchas when it's time to 
migrate from Debian Lenny to Debian Squeeze - what with all different 
versions, dependencies, and such as one updates first one machine, then 
another, while trying to stay live.
Remus looks a lot simpler, at the cost of the resources associated with 
running backup VMs.  But then again, Pacemaker/DRBD requires that you 
have the resources to migrate to - so you need a hot spare machine 
anyway.  Whether the backup VM is running or not, you need the same 
resources - and I assume the network load is similar whether its 
generated by Remus or by DRBD.
Which comes back to the question of whether Remus is mature enough to 
rely on or not.
Thanks,

Miles

--
In theory, there is no difference between theory and practice.
In<fnord>  practice, there is.   .... Yogi Berra



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