WARNING - OLD ARCHIVES

This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
   
 
 
Xen 
 
Home Products Support Community News
 
   
 

xen-users

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.

HTH,
Mark

> -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
>
> _______________________________________________
> Xen-users mailing list
> Xen-users@xxxxxxxxxxxxxxxxxxx
> http://lists.xensource.com/xen-users

_______________________________________________
Xen-users mailing list
Xen-users@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-users

<Prev in Thread] Current Thread [Next in Thread>