Re: [Xen-devel] sharing file between running domU (windowxp) and dom0

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

Hi there,

> > You should never, ever do that.  You *must always* shut down a domain
> > before modifying its filesystem from the outside.  Otherwise you can
> > create corruption that will destroy the domain's disk data.  Just
> > pausing the domain or saving it is not enough to protect you, and
> > neither will using the guest's "hibernate" feature.  The domain
> > *really* has to be properly shut down.
>
> Mark,I guess i understood your point here.
> My question is what about cluster filesystems like GFS or OCFS2.

I left that bit out because you mentioned just modifying a WinXP filesystem 
disk.  Cluster filesystems should be OK, provided all the domains accessing 
them are taking part in the locking protocol (i.e. as long as domanis are set 
up like you would do for separate hosts then it's fine).

It's also fine for multiple domains to mount a shared network filesystem such 
as NFS or CIFS because those are designed to handle multiple access.

Most other filesystems have no concept of writes happening "behind their back" 
so they can't be used in this way.  In additon to what I said earlier, please 
note that it's not just having multiple writers to an FS that's dangerous.  
Even if you have the disk mounted read-only in one domain when you write to 
it in another, you're liable to cause that domain to read corrupted data and 
possibly crash.

I think you can get away with not shutting down a domU if you are able to 
properly unmount the filesystem before you modify it in dom0, then mount it 
again in the domU.  But that does mean that you can never modify the / 
filesystem in this way, since it can't be unmounted.

> Does virtual environment outs some extra constraints on the filesystems?

Shouldn't add constraints beyond those of a normal host.  It's just that in 
usual operation you're not in a position to accidentally mount a filesystem 
on multiple hosts so many people haven't come across this before.

> FWIW how is XenFS different from OCFS2 and GFS?

XenFS is more like a network filesystem than a disk filesystem.  The data is 
stored by a "normal" filesystem and XenFS exports that filesystem.  The 
difference from a network filesystem is that it uses shared memory instead of 
IP.

> What i mean is how is a HVM domain(e.g linux) and domain0 different
> from two machines on a network from the perspective of cluster
> filesystems?

Should be equivalent to the multiple host case, so long as you're running a 
true cluster filesystem and both domains are configured correctly to access 
it.

Cheers,
Mark

> PS - I am a rookie as far as distributed/cluster fs are
> concerned.Please spare my ignorance and kindly enlighten me on this.
>
> Thanks
>       Pradeep
>
> > I'm sorry to emphasize that so harshly but it really is very very
> > important.
> >
> > This is because filesystems are written with the assumption that they
> > "own" the disk and that nothing will ever change "underneath" them.
> > If you modify the filesystem from the outside then you're doing stuff
> > without the guest OS knowing about it.  It won't be looking for this
> > and will get confused.  This could seriously damage your filesystem.
> >
> > If you do it by accident, you could try xm destroy-ing the domain
> > immediately in order to prevent it getting confused by the changes to
> > the underlying disk - before it has time to corrupt anything.  xm
> > destroying a domain which is modifying the filesystem has corruption
> > risks of its own though (and will lose any in-memory data), so it's
> > better just to avoid this situation.
> >
> > I hope that helps.  Again, sorry to be so harsh, but this is one
> > thing about virtual machines that is *really* important to watch out
> > for, or you will have major problems at some stage.
> >
> > Cheers,
> > Mark



-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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