RE: [Xen-users] xm save

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: xen-users-bounces@xxxxxxxxxxxxxxxxxxx 
> [mailto:xen-users-bounces@xxxxxxxxxxxxxxxxxxx] On Behalf Of 
> Devraj Mukherjee
> Sent: 03 May 2007 06:02
> To: xen-users@xxxxxxxxxxxxxxxxxxx
> Subject: [Xen-users] xm save
> 
> Hi everyone,
> 
> I am trying to understand the utility of xm save? Is it meant for
> backing up domainU images or is it meant for only backing up state of
> the image?

"xm save" will pause the domain, save it's internal state and shut down
the domain. You can then make a copy of the guest image, and save both
the save-file from "xm save" and the guest-image backup somewhere safe
(DVD, Tape or such). Once you have stored the backups, you can restart
the guest where it was with "xm restore". If you then have a crash, you
can restart from the backup with "xm restore" again. 

In xen 3.0.4, this only works for para-virtual guest. 3.1.0 (previously
called 3.0.5) will have HVM (fully virtualized) supprot for save/restore
functionality. 

The same code is also used for migration, where it's essentially just
passing the guest image over to another machine. Live migration also
uses the same bits of code, with the difference that the guest isn't
paused initially, instead it runs on and the page-table management is
put into "log-dirty" mode. The log-dirty mode allows changes to guest
memory to be tracked, and the changes are then transmitted to the new
machine. Once the number of changes are small enough, the guest is
paused and the last few changes sent to the new machine, and the new
machine performs the "restore" part of save/restore. This allows for a
very short break in the operations - under good circumstances in the
range of a few dozen milliseconds of down-time. 

--
Mats
> 
> Thanks
> 
> -- 
> "I never look back darling, it distracts from the now", Edna Mode (The
> Incredibles)
> 
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