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Re: [Xen-users] Consistent VM image backups

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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