How Does the Resynchronization Process Work? Page 2

How Does the Resynchronization Process Work?

A resynchronization process goes through the following steps:

1. A resynchronization event takes place, either manually or automatically.
2. Primary and Replica virtual machine VHD sizes are checked.
3. Primary Server starts tracking changes on the VHD while the resynchronization is in progress.
4. The writes are tracked in a HRL file and are only replicated once the resynchronization process has been completed.
5. Event ID 29242 is logged, which records the Virtual Machine Name, VHD files, VHD file’s start and end block.
6. A differencing disk is created for VHDs. This is because if the resync operation needs to be cancelled, it can be cancelled easily.
7. The differencing disk is merged into the VHD file at the end of the resync operation.
8. At this stage, VHDs are compared and then synced. This comparison is done block by block and only the blocks that differ are sent across the network. This reduces the amount of data sent over the network.
9. Event ID 29244 is logged in the Primary Server when the resynchronization process is over. The Event ID 29244 includes information about the VHD being replicated, the virtual machine name, blocks sent, time taken and the result of the operation.
10. At this stage, changes are successfully merged to the VHD file and therefore operation cannot be cancelled.
11. All the recovery points and HRL files are deleted.

As you know, resynchronization is a process in which all recovery points are deleted and the Primary Virtual Machine is prepared for a fresh replication. This is very similar to when you enable the replication for a virtual machine.

The overall purpose of resynchronization process is to make sure the Primary Server starts tracking the changes on the VHD files once again and replication can resume as usual. So, proceed with the “manual” resynchronization process only:

• If the Primary Server is no longer replicating to the Replica Server and a signifcant number of changes have taken place at Primary virtual machine.
• If you suspect that the contents of Primary and Replica VMs are not matching somehow.

Nirmal Sharma is a MCSEx3, MCITP and Microsoft MVP in Directory Services. He has specialized in Microsoft Technologies since 1994 and has followed the progression of Microsoft Operating System and software. In his spare time, he likes to help others and share some of his knowledge by writing tips and articles on various sites and contributing to Solution IDs for www.Dynamic-SpotAction.com. Nirmal can be reached at nirmal_sharma@mvps.org.

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