The Role of the Journal During Protection

After defining a VPG, the protected virtual machine disks are synced with the recovery site. After initial synchronization, every write to a protected virtual machine is copied by Zerto to the recovery site. The write continues to be processed normally on the protected site and the copy is sent asynchronously to the recovery site and written to a journal in a storage account managed by a Virtual Replication Appliance (VRA). Each protected virtual machine has its own journal.

In addition to the writes, every few seconds all journals are updated with a checkpoint time-stamp. Checkpoints are used to ensure write order fidelity and crash-consistency. A recovery can be done to the last checkpoint or to a user-selected, crash-consistent checkpoint. This enables recovering the virtual machines, either to the last crash-consistent point-in-time or, for example, when the virtual machine is attacked by a virus, to a point-in-time before the virus attack.

Data and checkpoints are written to the journal until the specified journal history size is reached, which is the optimum situation. At this point, as new writes and checkpoints are written to a journal, the older writes are written to the virtual machine recovery virtual disks. When specifying a checkpoint to recover to, the checkpoint must still be in the journal. For example, if the value specified is 24 hours then recovery can be specified to any checkpoint up to 24 hours. After the time specified, the mirror virtual disk volumes maintained by the VRA are updated.

During recovery, the virtual machines at the recovery site are created and the recovery disks for each instance, managed by the VRA, are copied to the recovered virtual machine. Information in the journal is promoted to the virtual instances to bring them up to the date and time of the selected checkpoint.