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When snapshots are manually deleted, OneFS marks the appropriate snapshot IDs and queue a job engine job to affect their removal. The SnapshotDelete job is queued immediately, but the job engine will typically wait a minute or so to actually start running it. During this interval, the snapshot will be listed as ‘delete pending’.
A similar procedure occurs with expired snapshots. Here, the snapshot daemon is responsible for checking expiration of snapshots and marking them for deletion. The daemon performs the check every 10 seconds. The job is queued to delete a snapshot completely. It is then up to the job engine to schedule it. The SnapshotDelete job might run immediately (after a minute or so of waiting) if the job engine determines that the job is executable and there are no other contending jobs with higher priority running at that moment. For SnapshotDelete, it is only run if the cluster is in a fully available state (no drives or nodes are down).
The most efficient method for deleting multiple snapshots simultaneously is to process older through newer, and SnapshotIQ will automatically attempt to orchestrate deletes in this manner. A SnapshotDelete job engine schedule can also be defined so that snapshot deletes only occur during wanted times.
On deletion of a snapshot, OneFS immediately simply modifies some of the tracking data and the snapshot disappears from view. However, the actual behind-the-scenes clean-up of the snapshot can involve a fair amount of work, which is performed in the SnapshotDelete job.
In the following example, snapshot ID 100 is being deleted. To accomplish this, any changes will likely need to be moved to the prior snapshot (ID 98), because that snapshot will no longer be able to read forward.
Figure 12. Deleting a snapshot – Part 1
Snapshot 100 has two changed blocks: block 0 and block 4. Snapshot 98 was changed after snapshot 98 was taken, so block 4 can be deleted, but block 0 needs to be moved over to snapshot 98.
Figure 13. Deleting a snapshot – Part 2
The oldest snapshot can be deleted very quickly. An ordered deletion is the deletion of the oldest snapshot of a directory and is a recommended best practice for snapshot management. An unordered deletion is the removal of a snapshot that is not the oldest in a directory. This can often take approximately twice as long to complete and consume more cluster resources than ordered deletions.