RE: [Xen-users] high availability iscsi and xen setup

["James Harper" <james.harper@xxxxxxxxxxxxxxxx>]

> Hi All,
> 
> I'm thinking about the best way to set this up, and would greatly
> appreciate your learned opinions on it. I'm thinking of the following:
> 
> - 2 Storage servers, running LVM, heartbeat, drbd (primary/standy) and
iscsi.
>   This will protect against 1 storage server failing.
> - 2 Xen hosts, running heartbeat to ensure the domU's are available.
If
>   not, migrate all hosts on to other xen host. This will protect
against
>   1 xen host failure.
> 
> Any opinions on this arrangement of setup or links to resources
> discussing it would be much appreciated. Also any alternative ways to
> provide the same HA would be useful for comparison.

That's the way I'd do it, subject to some performance testing.

> 
> - Any Pitfalls?
> 
> - Gaps in the availability? (split-brain possibilities?)
> 

I'd have 3 networks:

1. drbd network. 2 gigabit (or 1 10G) ports bonded on each storage
server connected directly to the other - no switch.
2. storage network. 2 gigabit (or 1 10G) ports bonded on each xen server
and storage server, connected into a switch with some decent HA features
(redundant PSU's probably), or stacked switches.
3. regular networks for normal DomU traffic

So the DRBD servers would need 4 gigabit ports, bonded in pairs.

I assume from what you specified that you are running 2 completely
separate instances of HA - one to manage DRBD and iSCSI failover and the
other to manage Xen.

Split brain problems are the worst problems you'll face. If storage
server #1 goes down at 9pm (power failure and this machine is on a
circuit with a dud UPS) and everything fails over to #2, then everything
goes down at 10pm (someone forgot to fill up the generators or the UPS
runs out of battery or something), power comes back at 11pm but #2 fails
to boot and everything starts using storage server #1 which is out of
date. You'd have to be having a really bad day for that to happen
though. 

James

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