Re: [Xen-users] Re: Re: Redundant server setup

[Matthew Wild <M.Wild@xxxxxxxx>]

On Friday 12 May 2006 13:03, Matthew Palmer wrote:
> On Fri, May 12, 2006 at 09:38:09AM +0100, Matthew Wild wrote:
> > What I've been working on is using a separate box to provide the storage
> > for the domUs through iSCSI. So at the moment I have two machines set up
> > as dom0s and one NAS box using iscsitarget to provide storage. As long as
> > I use the /dev/disk/by-id names for the disks, I can run xm migrate
> > --live and everything switches over imperceptibly.
>
> Shared storage is the best if you can swing it.  Not necessarily the
> cheapest option in the world, though if you want real reliability -- I'd
> never just use another box as my NAS, because that's a new and possibly
> even more dangerous single point of failure.  And fully-redundant
> channel-bonded everything can get costly and complex quickly.  DRBD has
> it's quirks, but it's nicely redundant and *cheap*.
>
But as far as I can see, DRBD only provides twin machine shared storage. I 
have a few servers I want to use as dom0s and since they are generally simple 
1U boxes I want them as clean from local storage as possible. The NAS box is 
bigger and generally better protected with redundant PSU, hardware RAID with 
hotswap spares etc. If I could pair two machines like that and then offer 
storage to the dom0s from that redundant pair that would be better.

Otherwise I'd just put a FC HBA in each box and connect them to our SAN 
directly, but since these are meant to be cheap replaceable server boxes I 
don't really want to do that as that'll also mean buying another SAN 
switch :-(. Or stay with the AlphaServer cluster, which is nicely redundant, 
but getting long in the tooth.

> > All we need now is to use heartbeat to check if domUs or a dom0 has
> > failed and start up on the other as appropriate.
>
> That's easy enough to do -- you just specify domU::<hostname> in the
> haresources line for each of your domUs, and write a quick domU script for
> resources.d to up/down the domains.  Bonus points are available for
> checking if we're doing a gentle move (hb_takeover instead of a DR event)
> and use live migration to minimise downtime.
>
I've yet to take a proper look at heartbeat yet so these hints are helpful.

Matthew
-- 
Matthew Wild                       Tel.: +44 (0)1235 445173
M.Wild@xxxxxxxx                    URL http://www.ukssdc.ac.uk/
UK Solar System Data Centre and
World Data Centre - Solar-Terrestrial Physics, Chilton
Rutherford Appleton Laboratory, Chilton, Didcot, Oxon, OX11 0QX

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