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RE: [Xen-users] unionfs with xen

To: "'Simon Hobson'" <linux@xxxxxxxxxxxxxxxx>
Subject: RE: [Xen-users] unionfs with xen
From: "Pallab Chakrabarty" <pchakrabarty@xxxxxxxxxxxxxxx>
Date: Mon, 20 Apr 2009 09:12:46 -0500 (CDT)
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I tried with rhel5.1 to create a guest using the following config:


disk=[ 'file:/opt/img-store/shared-disk,hdb,r', 'file:/var/lib/xen/images/rhel5-1/disk0,hda,w', ]


where shared-disk is used as read-only and a separate disk0 is used to capture the user defined guest changes as RW.. Note, shared-disk is pre-existing image meant to share among the guests..


but seems, all the changes made in the new guest is still writing on the read-only shared-disk, instead disk0.


Even if I change chmod 400 of shared-disk file, still it writes there..


I was hoping to capture the delta changes with disk0, per guest basis to serve my purpose..



Kindly guide as required for the same ..




- regards




-----Original Message-----
From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of Simon Hobson
Sent: Friday, April 17, 2009 3:14 PM
To: Xen-users@xxxxxxxxxxxxxxxxxxx
Subject: RE: [Xen-users] unionfs with xen


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

Christmas stocking fillers. Some available as e-books.



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