Re: [Xen-users] Consistent VM image backups

[Bart Coninckx <bart.coninckx@xxxxxxxxxx>]

On 01/28/11 09:36, Simon Hobson wrote:
> 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.
>
isn't pausing and saving the machine state actually the same as saving
the machine (xm save)?

B.



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