Re: [Xen-users] Consistent VM image backups

[Simon Hobson <linux@xxxxxxxxxxxxxxxx>]

Mike Sun wrote:
> I'm not worried about uncommitted data from the guest OS... I'm
> capturing all of the domU memory state as well, so I guess I should be
> more clear that I'm not looking for truly "consistent" disk image... I
> just want to make sure that the virtual disk state is always synced to
> the actual backing file.
As long as you pause the guest and snapshot BOTH it's disk and 
machine state at the same instant (relative to guest operations) then 
between them you will have a consistent backup. BUT the ONLY way to 
use that backup is to restore both the disk image AND machine state 
and unpause the guest from that state. Any attempt to read the 
filesystem directly will result in reading dirty data which would be 
analogous to having pulled the power plug on the guest at the instant 
the snapshot was taken.
Any uncommitted writes in the guest will be captured by the guest 
machine state, any uncommitted write in the host OS will be read as 
they would for any other write - as in a read of the file will use 
any dirty data waiting in the write cash rather than reading what is 
now invalid data from disk.
--
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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