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Re: [Xen-users] UnionFS + Xen

> Anyway, this may not really be a xen issue, but I thought I'd ask. I
> was trying to get several domU's to share a base LVM-backed
> filesystem, but when I fire up a second domain using the same fs, it
> says that the vbd is already in use. I take it isn't possible to have
> multiple mounts of an LVM-backed volume? That's one reason why I had
> to use a file-backed root fs above.

The Xen tools won't let you have multiple mounts to a filesystem unless 
they're all read only.  This is just a "safety catch" to avoid people 
shooting themselves in the foot.  If you're Really Sure you want writeable 
sharing, stick "w!" as the permission in the config file.

> Also... As I'm not too sure about this, is it *safe* for a file-backed
> root fs to be shared across multiple domUs?

Not if they're mounting it writeable...

> I have the initrd mount it 
> as read-only BUT the xen config has to make it writable. Making it
> read only in the config causes the mount process to spit an error
> (EXT3 INFO requires write access ...).

If you mount it read only in the guest, it's probably OK.  It's a bit odd that 
exporting it read only doesn't work though - we export /usr read only and 
then mount it read only in the guest, which seems to work fine.


> -gino
> On 5/25/05, Gino LV. Ledesma <gledesma@xxxxxxxxx> wrote:
> > On 5/25/05, Anthony Liguori <aliguori@xxxxxxxxxx> wrote:
> > > Gino LV. Ledesma wrote:
> > > >Hi, list
> > > >
> > > >Has anyone gotten something similar to work? Any ideas / tips /
> > > >comments / suggestions in doing so? I'm tweaking my xen RPM spec file
> > > >to support unionfs and right now just looking at the boot up process
> > > >of getting the domain to mount something else on top of the exported
> > > >root file system (either a file-backed VBD or yet another NFS export).
> > >
> > > This is actually a large part of the paper for this presentation at
> > > OLS:
> > >
> > > http://www.linuxsymposium.org/2005/view_abstract.php?content_key=117
> > >
> > > There's a number of approaches to solving this problem.  unionfs would
> > > be ideal but it's a bit unstable.  Another approach is to keep certain
> > > directories read only (like /usr, /bin, /lib, /sbin, etc.) and others
> > > read write (/etc, /var/, etc.).  This will get you pretty far.
> >
> > Thanks for the reply. I guess this is one of the most oftens suggested
> > approaches. The two goals that I'd like to meet are:
> > 1. Storage flexibility (resize as necessary) -- LVM looks good to go,
> > though in our setup where we can use NetApps, NFS would be another
> > approach.
> > 2. Ease of administration / maintenance -- Some of the issues I'm wary
> > about when going unionfs is that if we make changes to the underlying
> > filesystem (e.g. OS upgrade) and there are changes on the overlay,
> > conflicts might occur.
> >
> > I'll look to doing both -- I'd like to keep things simpler, as there'd
> > be less chances of breaking things.
> >
> > Thanks again.
> >
> > gino ledesma
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