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

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Subject: Re: [Xen-users] remus vs. pacemaker/drbd?
From: Miles Fidelman <mfidelman@xxxxxxxxxxxxxxxx>
Date: Tue, 07 Sep 2010 11:49:26 -0400
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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.


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.



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

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