I don't know nilfs2 internals, but it occurs to me that a "forward" or
"leap frog" mechanism might work too.
That is, if a file system is in a state at time T(0), and is in another
state at time T(1), then what I'm looking for is the ability to return
to a read-write file system with state T(0).
One way to do that might be to "roll back" from state T(1), thereby
discarding state T(1). But another way to accomplish this might be to
surgically construct a state T(2) which occurs after T(1) and was
identical to T(0), leaving T(1) available as a read-only snapshot.
We do this in source control. Rather than backing out a particular
change to your source tree, many source control systems require you to
commit a reverse diff. This creates a new revision, but a new revision
which is identical to an earlier revision which has the apparent affect
of removing a change.
I don't know if this would be any easier, but it's a thought.
--rich
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