Xen users - Dec 2005 - Sharing a partition between dom0 and domU

Hi!

I''m currently preparing to xen installation. In the system that
I''m
going to set up, there will be a need to copy large amounts of data
(therefore I want to avoid using network) from domUs to the dom0 (one
way only).

Mounting a filesystem RO, when a domU has it mounted RW is probably
asking for trouble. Therefore I was thinking about using some remounting
protocol similar to what''s described in thread starting at
http://lists.xensource.com/archives/html/xen-users/2005-05/msg00309.html

But then I thought about making LVM snapshots of the domU partition, and
mounting it RO in dom0. Has anyone tried this?

What I''m worried about is that the snapshot would have to be taken in
an
instant at which the filesystem is in a consistent state. IIRC the
kernel enforces consistency when one is using LVM in a single system, by
telling the filesystem driver to make the filesystem consistent for the
moment the snapshot is being taken. But I''m not sure how to tell the
kernel in domU that dom0 is about to take a snapshot of its filesystem.

Please note that I have no desire to _completly_ eliminate network
usage. Using it for some kind of synchronization between the domains is
fine with me. I just want to avoid copying the large data across the
network.

Any ideas?

regards,

Marcin
-- 
Marcin Owsiany
porridge@expro.pl

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Marcin Owsiany wrote:
> I''m currently preparing to xen installation. In the system that
I''m
> going to set up, there will be a need to copy large amounts of data
> (therefore I want to avoid using network) from domUs to the dom0 (one
> way only).
Considered using a filesystem like GFS which can be safely mounted by 
multiple machines?


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On Thu, Dec 15, 2005 at 02:01:53PM -0600, Charles Duffy
wrote:
> Marcin Owsiany wrote:
> >I''m currently preparing to xen installation. In the system
that I''m
> >going to set up, there will be a need to copy large amounts of data
> >(therefore I want to avoid using network) from domUs to the dom0 (one
> >way only).
> 
> Considered using a filesystem like GFS which can be safely mounted by 
> multiple machines?
Good idea. Before I start to look at it: are there more filesystems like
this?

Marcin
-- 
Marcin Owsiany
porridge@expro.pl

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On Friday 16 December 2005 09:15, Marcin Owsiany wrote:
> On Thu, Dec 15, 2005 at 02:01:53PM -0600, Charles Duffy wrote:
> > Marcin Owsiany wrote:
> > >I''m currently preparing to xen installation. In the
system that I''m
> > >going to set up, there will be a need to copy large amounts of
data
> > >(therefore I want to avoid using network) from domUs to the dom0
(one
> > >way only).
> >
> > Considered using a filesystem like GFS which can be safely mounted by
> > multiple machines?
>
> Good idea. Before I start to look at it: are there more filesystems like
> this?
There is also OCFS2 from Oracle or you can try PeerFS (commercial) but has 
nice features.
OCFS2 compared to GFS is a bit simpler to install I would say.


Michael

-- 
----------------------------------------------------------------------------------------
Michael Mey                                  
Thinking Objects Software GmbH    |   mailto: michael.mey@to.com 
Lilienthalstrasse 2/1                         |   phone: +49 711 88770-147
70825 Stuttgart-Korntal, Germany  |   fax: +49 711 88770-449
----------------------------------------------------------------------------------------


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On Fri, Dec 16, 2005 at 10:27:19AM +0100, Michael Mey
wrote:
> On Friday 16 December 2005 09:15, Marcin Owsiany wrote:
> > On Thu, Dec 15, 2005 at 02:01:53PM -0600, Charles Duffy wrote:
> > > Marcin Owsiany wrote:
> > > >I''m currently preparing to xen installation. In the
system that I''m
> > > >going to set up, there will be a need to copy large amounts
of data
> > > >(therefore I want to avoid using network) from domUs to the
dom0 (one
> > > >way only).
> > >
> > > Considered using a filesystem like GFS which can be safely
mounted by
> > > multiple machines?
> >
> > Good idea. Before I start to look at it: are there more filesystems
like
> > this?
> There is also OCFS2 from Oracle or you can try PeerFS (commercial) but has 
> nice features.
Thanks for the pointers. Have you tried any of these with Xen, i.e. not
by accessing the FS from separate nodes over network, but by directly
mounting the same device twice?
> OCFS2 compared to GFS is a bit simpler to install I would say.
Any more comments on maturity, stability, features?

regards,

Marcin
-- 
Marcin Owsiany
porridge@expro.pl

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On Friday 16 December 2005 11:11, Marcin Owsiany wrote:
> On Fri, Dec 16, 2005 at 10:27:19AM +0100, Michael Mey wrote:
> > On Friday 16 December 2005 09:15, Marcin Owsiany wrote:
> > > On Thu, Dec 15, 2005 at 02:01:53PM -0600, Charles Duffy wrote:
> > > > Marcin Owsiany wrote:
> > > > >I''m currently preparing to xen installation. In
the system that I''m
> > > > >going to set up, there will be a need to copy large
amounts of data
> > > > >(therefore I want to avoid using network) from domUs to
the dom0
> > > > > (one way only).
> > > >
> > > > Considered using a filesystem like GFS which can be safely
mounted by
> > > > multiple machines?
> > >
> > > Good idea. Before I start to look at it: are there more
filesystems
> > > like this?
> >
> > There is also OCFS2 from Oracle or you can try PeerFS (commercial) but
> > has nice features.
>
> Thanks for the pointers. Have you tried any of these with Xen, i.e. not
> by accessing the FS from separate nodes over network, but by directly
> mounting the same device twice?
No, I haven''t, but it should work the same way as it does with a
cluster-fs on
a san being mounted from several nodes at the same time.
> > OCFS2 compared to GFS is a bit simpler to install I would say.
>
> Any more comments on maturity, stability, features?
The purpose I was testing GFS and OCFS2 was for storing domU-fileimages on it 
with the result that it works fine with OCFS2 and doesn''t work with GFS
due
to some loopback-mount limitations of GFS.


-- 
----------------------------------------------------------------------------------------
Michael Mey                                  
Thinking Objects Software GmbH    |   mailto: michael.mey@to.com 
Lilienthalstrasse 2/1                         |   phone: +49 711 88770-147
70825 Stuttgart-Korntal, Germany  |   fax: +49 711 88770-449
----------------------------------------------------------------------------------------


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Michael Mey wrote:
> The purpose I was testing GFS and OCFS2 was for storing domU-fileimages on
it
> with the result that it works fine with OCFS2 and doesn''t work
with GFS due
> to some loopback-mount limitations of GFS.
Would that limitation impact folks using phy: to access LVM volumes from 
the DomU (and mounting them directly out of /dev/mapper in the Dom0), or 
only those using some mechanism other than LVM/phy: for managing storage?


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