Re: [Xen-users] Xen 4.0 feature request

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Enrico Valsecchi wrote:

> I think that xen must have a tools that allow create a consistent 
> snapshot, because
> are a unique possibility to have this features in any environment.
As I understand it, you want to be able to take a snapshot of a 
running guest - and then modify it's file system by deleting files 
etc.
Without VERY tight integration between hypervisor/tools/guest OS then 
this isn't going to happen. Even a snapshot of the volume (eg LVM 
snapshot) isn't going to help you from outside of the guest.
The reason ?

Your guest has mounted filesystems. Unless you are prepared to run 
clustered filesystems then you cannot modify the filesystem from 
outside of the guest without corrupting the filesystem. Whatever you 
read (or modify) from outside of the guest cannot take into account 
what the guest may have cached. To get around this would require 
tools that could "look inside" the guest - both it's internal memory 
structures and it's disks to work out what it's view of the 
filesystem is, and then be able to update both the disk and any in 
memory structures relating to it.
The level of integration required to cope with the OS, the filesystem 
in use, etc is such that I very much doubt if anyone will put in the 
work required.
I think there are three practical ways to modify the guest filesystem :

1) Do the mods from within the guest.
2) Shutdown the guest and mount it's filesystem elsewhere (eg in Dom0)
3) Use a clustered filesystem that you can mount on two systems.

--
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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