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How to Troubleshoot Stuck Delta Syncs

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An administrator may experience a VPG enter a delta sync and get stuck. This article explains how to perform basic troubleshooting for such an issue.

Root Cause

There are two main reasons for Delta syncs to become stuck:

  1. Production VM powered off.

  2. Network issues between the production and recovery VRAs.

  3. Recovery/Production storage delay higher than the configured limit.


  • The VPG is in a continuous Delta sync that never completes.
  • RPO continuously rises.
  • New checkpoints are not created.


Bandwidth Throttling

If Bandwidth or time-based Throttling are enabled, the throttling values might not be sufficient for the environment to perform replication and delay in the Delta sync. Disabling the throttling feature may assist the sync to be completed.
Navigate to the site settings and check rather or not you have Bandwidth Throttling enabled (For more information on Bandwidth regulation click here). If it is enabled, check rather or or not the value meets the minimum required bandwidth (for more information see how to identify the minimum required bandwidth).

VRA resources

If the VRA's resources are congested, the sync may not progress. For more information, please see how to validate VRA resources. If you identified that additional resources are required you can follow the KB Adding resources in Vmware.

Network Resources

To check if the network resources are causing the bottlenecks:

  1. Use iPerf tool to verify the bitrate available between the 2 sites. For more information on how to run iPerf please see - How to check bandwidth using iperf. 

  2. Compare the results by:

    1. Use the bandwidth output to verify if the bandwidth observed matches the configuration.

    2. If the values match, login to the ZVM UI and go to the dashboard page. There you can find the WAN graph which you can compare to the results of iPerf. 

Network Troubleshooting

To properly replicate, the VRAs must be able to communicate over ports 4007 and 4008 in a bidirectional fashion.
Please review the following KB for some network troubleshooting steps:
VRA Network troubleshooting using Plink


Storage delay

By default, Zerto will start throttling I/Os whenever the storage latency goes above 40 ms. This configuration can be viewed in the Site Settings menu -> Throttling -> Show Advanced Settings.
Do not change this configuration without consulting with Zerto support.
Make sure that the storage latency does not go above the configured limit.


Contact Support

If the issue persists, contact Zerto support and include the following information:

  1. Reference the KB

  2. Attach screenshots of the results of all of the steps that were taken (including Iperf, VRA resources, Screenshots, Etc.)

  3. Name of the the affected VPGs and their VRAs

  4. The following logs will be needed 

    1. Time frame - 8 Hours

    2. Relevant VPGs

    3. Relevant Sites

    4. Relevant VRAs

    5. Relevant hosts logs and hypervisor logs.

    6. If VCD is being used, VCD logs will be required as well.

  5. Collect the logs only after you’ve opened a case and have the case number.

** for more information please see How to collect Zerto logs**