What is an example how SnapMirror lag time is calculated?

Applies to

• ONTAP 9
• SnapMirror

Answer

• Scenario 1   

  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

• Scenario 2

  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

Additional Information

Frequently Asked Questions (FAQ): SnapMirror Lag