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    SubjectRe: [RFC][PATCH -mm] Freezer: Handle uninterruptible tasks
    On Mon, 9 Jul 2007, Jeremy Maitin-Shepard wrote:

    > Pavel Machek <pavel@ucw.cz> writes:
    >
    > [snip]
    >
    > > I don't know how to do that mechanism... but if we knew where to trap
    > > filesystem writes, we could simply freeze at that point, and at that
    > > point only, no?
    >
    > Any operation at all that has an external effect must not occur after
    > the snapshot is made; otherwise, there will be random hard-to-find
    > corruptions and other problems occurring as a result. Thus, for
    > example, any writes (either directly or indirectly through e.g. a
    > filesystem) to non-volatile storage, any network traffic, any
    > communication with hardware like a printer must be prevented after the
    > snapshot.

    You have forgotten one critical point: The writes to save the snapshot
    image must be allowed. That's what makes it really hard.

    Alan Stern

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