Re: [Xen-users] modifying file-systems in Dom0 while mounted read-only in DomU

[Reinhard BrandstÃdter <reinhard.brandstaedter@xxxxxx>]

Hi,

> > Although this is a nice feature  - in my case these two DomUs are forming
> > a cluster and the common configuration resides on the read-only disk - it
> > seems dangerous to me.
> >
> > Although with this feature I could seamless change the configuration of
> > the cluster :-) :
> > - mount the partitition in Dom0
> > - restart one DomU and let it gain the service with changed configuration
> > - restart the second DomU with the new configuration
>
> You should be able to see new data without rebooting by droping cache:
> echo 3 > /proc/sys/vm/drop_caches

For the purposes I'd use this feature that works great. I understand the 
dangers imposed by this procedure (as Luciano mentioned them earlier). But 
I'd only use it if the services are shut down on the VMs, flush the cache and 
force a "reload" of the contents. This saves me from rebooting the VMs

>
> But I suggest using a cluster filesystem instead (OCFS2, GFS, etc.). Then
> the changes will be visible everywhere you mount the filesystem.

Well I thought of that, but a cluster filesystem would mean more complexity in 
the VMs, which I want to avoid. And after all my VMs don't need to write to 
that partition in any case, and it would be quite a overhead setting up a CFS 
if all participants except one only have read-only access.

> IIRC, you'll have to specify the virtual disk to xen with '...,w!' to
> allow multiple mounting of the same partition in read/write mode.

Thats exactly what I want to avoid :-)

Reinhard

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