how do versioning filesystems take snapshot of opened files?

[Xin Zhao]

Hi,


If a file is already opened when snapshot command is issued,  the file
itself could be in an inconsistent state already. Before the file is
closed, maybe part of the file contains old data, the rest contains
new data.
How does a versioning filesystem guarantee that the file snapshot is
in a consistent state in this case?

I googled it but didn't find any answer. Can someone explain it a little bit?

Thanks
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