Data Domain: Snapshots and behavior with MTree Replication | FAQ

APPLIES TO:

• All Data Domain systems

• All supported DDOS versions

This article provides answers to frequently asked questions about snapshot behavior in the context of MTree replication on Dell EMC Data Domain systems. 

For a more in-depth explanation of how MTree replication works see: Data Domain - How MTree Replication Works

1. Question: Is it possible to tune the frequency of the system-generated snapshots to trigger frequent replication of the MTree?
   Answer: No. The system automatically creates snapshots when no replication is in progress and the replication lag exceeds 15 minutes. This interval is not configurable. 
2. Question: How many snapshots can be created on any MTree?
   Answer: Each MTree supports a maximum of 750 snapshots. This includes all user-created snapshots and the couple of system-generated snapshots for MTree replication. There will always be at most two system-generated snapshots at the source and destination when MTree replication is set up.
3. Question: If a MTree already has 750 snapshots, then will MTree replication no longer work?
   Answer: No, MTree replication will be able to create the necessary snapshots to replicate the data without impacting any of the existing snapshots on the MTree. However when the 750 snapshot limit is reached, new user-generated snapshots cannot be created without first deleting some of the user-generated snapshots.
4. Question: If I have a MTree with user-created snapshots and then setup MTree replication, will the existing snapshots on the MTree get replicated to the destination?
   Answer: When MTree replication is initialized, then all existing snapshots are replicated in-order from the source to the destination. Initialization of MTree replication causes a system-generated snapshot to be created on the source MTree which also gets replicated to the destination. Once this system-generated snapshot has finished replicating to the destination, then the initialization process is completed.
5. Question: Can I view the system-generated snapshots?
   Answer: Yes, the snapshot list for MTrees contains the system-generated snapshots along with the user-created snapshots. The system-generated snapshots have a format of REPL-MTREE-AUTO-<YEAR>-<MONTH>-<DAY>-<HOUR>-<MIN>-<SEC>.
6. Question: Does the user need to clean up the system-generated snapshots?
   Answer: No, MTree replication takes care of cleaning up the system-generated snapshots.
7. Question: Do system-generated snapshots consume space until cleaning is run?
   Answer: Yes, the system-generated snapshots are deleted but the space associated with them is not reclaimed until GC is run.
8. Question: Does MTree replication delete user-created snapshots?
   Answer: MTree replication only deletes the system-generated snapshots identified by the format used for naming them (REPL-MTREE-AUTO-<YEAR>-<MONTH>-<DAY>-<HOUR>-<MIN>-<SEC>). If a user-created snapshot uses the same format as that used by system-generated snapshot (not recommended), then that snapshot gets deleted by MTree replication.
9. Question: Does MTree replication create system-generated snapshots on the source when the destination Data Domain is unreachable?
   Answer: MTree replication creates the system-generated snapshots when no snapshot is being replicated (or in the queue for replication) and the replication lag is more than 15 minutes. So, even if replication is interrupted for hours, MTree replication only has a maximum of two system-generated snapshots on the MTree.
10. Question: What is the maximum number of system-generated snapshots that is created on each MTree?
    Answer: At any time, there is a maximum of two system-generated snapshots within a MTree.
    On the source MTree, there can be two system-generated snapshots. One snapshot that is being replicated and the other which is the previous snapshot (exists on both source and destination). Once the latest snapshot is completely replicated and available at the destination, then the previous snapshot is deleted from the list at both the source and destination.
11. Question: Can I create snapshots on the source and destination of the MTree replication context?
    Answer: For a MTree replication context, the destination MTree is read-only and it is not possible to create snapshots on this read-only MTree.
    MTree replication is designed to automatically replicate snapshots created on the source to the destination. Once the snapshot is replicated to the destination, the retention period of the snapshot can be configured without impacting the retention period set on the source.
12. Question: Does MTree replication replicate the retention period for a snapshot to the destination?
    Answer: From DD OS 5.1 and above, the retention period for a MTree snapshot is not replicated to the destination. Rather, all snapshots at the destination have a default retention period of 1 year.
    Once the snapshot has been replicated to the destination, a user may choose to change this retention period to an appropriate value.
13. Question: Can I configure separate retention periods on the source and destination?
    Answer: From DD OS 5.1 and above, the user can specify the retention period for the snapshot at the source. The default retention period for all snapshots at the destination MTree is 1 year. At any time, the user may choose to independently modify the retention period of the snapshot on the destination.
14. Question: Do MTree snapshots expire simultaneously on the source and destination of a MTree replication context?
    Answer: There are two types of snapshots associated with MTree replication:
      User-created snapshots
      System-generated snapshots
    The system-generated snapshots are deleted from the source and destination simultaneously. The snapshot list displays a 1 year retention time for the system-generated snapshot at the destination, but it is deleted once it is no longer required (a new snapshot has been replicated to the destination).
    User-created snapshot has independent expiry times on the source and destination. This can be viewed by listing the snapshots on the source and destination.
15. Question: The system-generated snapshots show a retention period of 1 year at the destination. Does this mean that this snapshot is retained on the destination for 1 year?
    Answer: No, system-generated snapshots are automatically deleted once they are no longer required when a new snapshot has been replicated to the destination. No manual intervention is required to change the retention period of these snapshots.
16. Question: Why are there appended digits to the replicated MTree snapshot names?
    Answer: If you create a MTree snapshot and later replicate it to another Data Domain system. You will notice that the name now has an appended underscore and digits. What happens is that the name of the snapshot on the source gets the snap id appended to the name.  The reason for doing this is because the source and destination may have different retention times.  For example: If a user creates a snapshot called MtreeTest1 on the source, and it then gets replicated and retained on the destination.  Now let us say that the user later deletes the MtreeTest1 snapshot on the source and re-creates a new snapshot also named MtreeTest1.  If the destination snapshot had a longer retention time than the source, then the new snapshot name when replicated would collide with the destination MtreeTest1 snapshot.  To prevent this unexpected behavior the snap id is appended to the destination snapshot name this prevents the likelihood of creating an unexpected collision. 

REFERENCE: Data Domain: MTree Replication FAQs | Frequently Asked Questions