Journal considerations are different between on-premise recovery sites and Public Cloud recovery sites. Zerto's Public Cloud architecture requires applying journal data to the recovery disk to happen before the recovery VM can be powered-on. This behaviour is different than the Hypervisor-based recovery sites where Zerto can instantly create a view of the recovery disk without applying any data. Consequently, public cloud RTO is longer than on-premise solutions.
For more details about Journal Sizing, please see the ZVR Journal Sizing and Best Practices document for initial sizing. Additionally, the Journal reports in Zerto Analytics can be used to monitor actual Journal usage by a VPG.
VMware and Hyper-V On-Premise Recovery Sites - Journal Sizing
For on-premise solutions, the general formula to calculate Journal size is:
Journal size = History_in_seconds*Average_throughput_of_all_VM_volumes*Average_compression_ratio
For example, if the history is 3 days (259,200 seconds) and:
• the average throughput is 10 MBPS and
• the data is 50% compressible on average
Then the journal size for this VM would be:
259,200*10*0.5 = 1,296,000 MBPS (~1266 GB)
Public Cloud Recovery Sites - Journal Sizing
Due to the difference in the architecture of Public Cloud solutions, configured history should be as short as possible to achieve the fastest RTOs.