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Creating a snapshot in OneFS is a relatively instantaneous process. It typically takes substantially less than a second to perform, depending on the amount of snapshot prep work that has to happen first.
First, OneFS’ write-back cache (coalescer) is paused, and any uncommitted writes are flushed to allow the file system to be quiesced for the short period of time required to create the snapshot. Next, a marker is placed at the top-level directory inode for a particular snapshot and a unique snapshot ID is assigned.
Once this is done, the coalescer resumes and writes continue as normal, and any changes to HEAD (current version) are recorded in the snapshot inodes when any of the logical blocks they reference are altered - until another snapshot is taken.
The moment a snapshot is created, it essentially consumes zero space until file creates, deletes, modifies, and truncates start occurring in the structure underneath the marked top-level directory.