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SnapshotIQ has several fundamental differences as compared to most snapshot implementations. The most significant of these are:
The process of capturing a snapshot in OneFS is near instantaneous. However, there is a small amount of snapshot preparation work that has to occur. The moment a snapshot is taken, it consumes zero space until any file writes occur to the data it contains.
Any changes to a dataset are then recorded in the pertinent snapshot inodes, which contain only referral (‘ditto’) records, until any of the logical blocks they reference are altered or another snapshot is taken. In order to reconstruct data from a particular snapshot, OneFS will look though all of the more recent versions snapshot tracking files (STFs) until it reaches HEAD (current version). In so doing, it will systematically find all the changes and recreate the point-in-time view of that dataset.
Figure 3. Snapshot read chain
In this example, to reconstruct file 1-2-3-4 as it appeared at 08:00 SnapshotIQ would need to read snapshots 12:00, 16:00 (4PM) and 20:00 (8PM).
In extreme cases, OneFS might have to ‘paint’ through hundreds or thousands of snapshots to reconstruct the correct older version.