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When the data protection service is enabled on a dataset, snapshots are actually only taken when there is a change made after the last snapshot was created. Depending upon the retention schedule this may lead to instances where one expects to see a snapshot, but a snapshot was not created because there was no change in the data, leading to confusion. If for example based on the schedule one expects hourly snapshots, yet some appear to be missing resulting in fewer than 24 snapshots created during last 24 hours, it is important to remember that unless there is change to warrant a new snapshot to be created, snapshots will be skipped, because they would end-up being empty, i.e. there will be gaps. An empty snapshot is still a point of reference however, and it is possible to create snapshots manually, on a filesystem that is not changing, which will result in one or multiple empty snapshots. This can only happen manually however, and not considered to be desirable for the purposes of data protection.

If you are wondering what this really means in terms of recovery, here's a brief summary. When you attempt to recover a file which may have had multiple modifications over last 24 hours, for example, you should be able to identify modifications made during that period with a 1 hour recovery point objective (RPO), because modifications will be captured by one snapshot, assuming hourly snapshots were enabled. However, if for example a snapshot was taken some number of hours prior to you attempting to recover the data, even if hourly snapshots are enabled, if there were no changes since that snapshot was taken, you won't see any subsequent snapshots. The latest version of the file will be in the last snapshot. Daily snapshots and weekly snapshots should capture at least some of the changes that were captured by the hourly snapshots, but some of those changes may be lost between two intra-day snapshots.