I have been playing around with the snapshot target with dmsetup, and I
can set up a snapshot that diverts writes to a backing file, so that
modifications to the snapshot device are discarded when I remove the
snapshot device and go back to using the origin device. What I can't
figure out is how to keep the origin device mounted and have the
snapshot device appear to contain the origin's data at the time it was
created, even though the origin continues to be modified.
I thought one of the main ideas of the snapshot was to be able to backup
a consistent filesystem image even though the filesystem continues to be
written to, but it seems that having the origin device mounted gets an
exclusive lock on it so that the snapshot device can not be targeted at
it, so how do you do this?
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