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What does "lag" mean for SnapMirror/SnapVault?

Last Updated:


Applies to

  • ONTAP 9
  • SnapMirror
  • SnapVault


  • SnapMirror or SnapVault relationship's lag time is calculated using:
    • The snapshot timestamp
    • The time on the destination system
    • The amount of time needed to transfer the snapshot from source to destination
  • The term 'lag' is typically associated with performance, with the common perception that lag is the elapsed time since the last successful update
  • While this is not completely incorrect, it does not account for 2 other factors:
    • The time, based on the clock and timezone, on the source and destination storage controllers
    • The duration of the transfer
  • The time on the source and destination is important because this determines the timestamps on the file system and snapshots.
    • If time is configured incorrectly, timestamps will be inaccurate  
    • Because lag is calculated based on snapshot timestamps, if the time is not correct, lag will not be correct  
  • The duration of the transfer is also overlooked because of the nature of replication
    • Lag is not measured based only on the time a transfer starts and completes
    • Lag is measured from the time the snapshot is created on the source, plus the duration of the transfer
      • Transfer can be schedule update or manual update transfers. 
  • Consider the following SnapMirror scenario:
    Source  Destination
    ControllerA:vol_1 ControllerB:vol_1_mir
  1. A scheduled update starts at 12:00pm
  2. A SnapMirror snapshot is created on the Source volume, and a transfer is started 
  3. The transfer takes 45 minutes to complete
  4. The time on the destination system is now 12:46pm
  5. The transfer completed 1 minute ago

If measured during step 5, the lag is 46 minutes, because:

  • 46 minutes have elapsed since the snapshot was created on the source
  • 46 minutes elapsed since the snapshot was successfully transferred to the destination
  • On the Destination, Lag is calculated by finding the difference between:
    • The snapshot creation timestamp
    • The time on the destination, based on the destination storage controller's clock 
    • If the time is not configured correctly on the destination or source, the lag time will be incorrect
  • Consider the following scenario:
    Primary Secondary
    CIFS_SVM:vol_1 CIFS_DR:vol_1_dr
  1. Based on the snapshot policy on vol_1, a snapshot is created at 5pm
  2. The snapshot is created with the snapmirror-label sv_daily
  3. At 1 AM the following morning, a scheduled snapmirror update is triggered, configured to replicate any snapshot labeled sv_daily
  4. The transfer takes 30 minutes to complete

The lag in this scenario would be 8 hours 30 minutes, because:

  • At the time of the scheduled snapmirror update, eight hours had elapsed since the snapshot was created and labeled sv_daily
  • Transferring the snapshot from the source to the destination took 30 minutes

In summary

  • Lag is the difference between the snapshot timestamp and the time on the destination system
  • Lag includes the amount of time needed to transfer a snapshot from source to destination
  • When examined in the context of snapshot timestamp, and the duration of the transfer, "long" lag times are often found to be normal

Additional Information

See What is SnapMirror lag time?


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