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What does "lag" mean for SnapMirror/SnapVault?

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Applies to

  • ONTAP 9
  • Data ONTAP 8 7-Mode

Answer

  • A 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
  • 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

 

 

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