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Re: [Xen-users] Re: Xen, LVM, DRBD, Linux-HA

To: Tomasz Nowak <tnowak-p@xxxxxxxxxxxxx>
Subject: Re: [Xen-users] Re: Xen, LVM, DRBD, Linux-HA
From: Nico Kadel-Garcia <nkadel@xxxxxxxxx>
Date: Tue, 22 Apr 2008 11:08:12 +0100
Cc: xen-users@xxxxxxxxxxxxxxxxxxx
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Tomasz Nowak wrote:
Rainer Sokoll <rainer@xxxxxxxxxx> wrote:

I want to put it all together.
So far, I have a running machine with multiple domUs, and I am very
excited. But now I want to go a step ahead:
I have two identical machines with a big amount of RAM and disk space.
My goal is to hve all the domUs almost redundant - and want to ask you
for your advice.
The plan is to have every VM in its own logical volume and have DRBD
replicate this volume to the other machine. Assumed Linux with
Linux-HA runs in a VM, here comes my question:
If the complete Linux VM runs in its own logical volume, Linux-HA
cannot work, since the configuration must not be the same on both
sides. One solution would be to have a minor file based backend that
holds /etc/, /proc/, /var/, /boot/ (and maybe more?). Or maybe there
is a more sophisticated solution?
How would you start?

Overall, what you're really trying to create is something like a live CD or NFS boot environment, both of which are fairly well documented and worked out for you. Why not look into these as a starting point?

I've developing quite the same solution :)
Machine A will have two separate disks - one for 'local' domUs, second
for machine B domUs replicas). Machine B will also have two disk
drives - one for local domUs and second as A domUs secondary device.

Right now I'm probably in the same place you are (I'm also very
excited :]) but I hope all this is possible due to the fact,
that block device names are exported to domU as virtual names.

So if Dom1a uses /dev/vg0/dom1vda as 'xvda' on machine A,
there is no problem that the same Dom1a would use
/dev/vg1/dom1vda as 'xvda' on machine B.
Hold it. The contents of /dev/ are not usually part of a local file system these days. They're usually 'udev' provided, at least on Linux, and get auto-regenerated and mounted as a special type of file system at boot time.

That said, distinguishing between different file systems to mount is often vastly simplified by using NFS mounting over a local network file system, rather than playing around in the Xen configurations. That makes it easier to reconfigure, dynamically, as well. And if you run autofs, you can symlink from /net/[server]/[exportdir] to your local designated target, such as /var/log/httpd or /usr/local or such. Backup needs to be done with caution for various reasons, but it's workable.

That's just
a difference of /etc/xen/dom1a.cfg on dom0 @A and dom0 @ B,
which probaby should be sligtly different anyway (percheps
less vcpsu and less memeory). Some script that notices
crased Dom1a @ boxA could not only re-create Dom1A @ boxB,
but also execute some other commads like:
xm vcpu-set Dom1b ... and xm mem-set Dom1b ...
to temporarly lower regular recource usage of Dom1b @ boxB.

The only issue I worry about right now is a disk performance,
i.e. how much do I drop of my current ~100MB/s decent speed
(ext3 on LVM2 on S/W Raid10,f2 on 2xSAS SFF 10K)
putting somewhere into all that one more layer - DRDB.

Good question. I've not played with DRBD, and would also welcome information on it.

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