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

[Nico Kadel-Garcia <nkadel@xxxxxxxxx>]

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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