Re: [Xen-users] Backup a DomU

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Stefan Becker wrote:

> I copy the disk, swap and cfg files.... What do you mean with full system?
"full system" would mean the entire guest - not just the filesystem 
images, but the machine state as well (which means the entire memory 
image plus processor state).
When you backup the disk image files, what you are getting is roughly 
the same as you'd have on the disks of a bare-metal machine after 
yanking the power cord ! Just telling the OS to sync is **NOT** 
enough in the general case - that will only sync data which 
applications have asked the OS to write to disk, anything cached by 
the application (particularly the case with databases) will not be 
included.
I agree with Niels, it's an ugly way to do it which relies on taking 
a dirty snapshot of the filesystem and hoping that it will clean up 
when the guest boots with it.
It's a topic that comes up every now and then (try searching the 
archives), and there are many points of view. Personally, I treat 
each guest as a machine in it's own right and back it up from within 
the guest in exactly the same way as I handle a bare-metal machine.
> Ok but I ask me, why the author of the book writes this solutuion?
No idea, you haven't given any clue what book you've been reading.
It's a method that will "work well enough" for some use cases - 
typically low write frequency/volume, no applications which cache 
data, "robust" filesystems. Ignore these provisos and it gets less 
reliable, and in extreme can be virtually guaranteed to result in 
corrupt data for some use cases. The author should have pointed out 
these limitations.
--
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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