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RE: [Xen-users] unionfs with xen

To: Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] unionfs with xen
From: Simon Hobson <linux@xxxxxxxxxxxxxxxx>
Date: Fri, 17 Apr 2009 10:43:46 +0100
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Pallab Chakrabarty wrote:

Are you trying to have a common filesystem image
shared by many DomUs, and use unionfs to allow
each DomU to have it's own modified version ?


kindly guide me..

I don't know - not something I've ever worked with.

A quick look in the Xen manual (http://tx.downloads.xensource.com/downloads/docs/user/) comes up with this :
Block devices should typically only be shared between domains in a read-only fashion otherwise the Linux kernel's file systems will get very confused as the file system structure may change underneath them (having the same ext3 partition mounted rw twice is a sure fire way to cause irreparable damage)! Xend will attempt to prevent you from doing this by checking that the device is not mounted read-write in domain 0, and hasn't already been exported read-write to another domain.

Now, if you are prepared to do the unionfs stuff in the DomU then I'd guess you could export the shared block device read-only to each guest, plus a separate read-write volume per guest to union with it.

Looking at the live CD systems (like Knoppix) might give you some ideas.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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