A Rapid Recovery core running on an ESXi virtual machine is configured to use HotAdd for improved backup performance.
Upon investigation of VDDK logs some backup jobs may be reverting to slower transport mechanisms such as NBD or NBDSSL
This circumstance may relate to the known VMware issue -
HotAdd fails with more than five concurrent backup operations.
When a backup application uses more than five (5) concurrent processes to back up or restore virtual machines using the HotAdd transport mode, one or more of the operations may fail. Logs will contain errors such as “The directory is not empty” and “Error acquiring instance lock” then “HotAdd ManagerLoop caught an exception.” The workaround is to reduce the number of concurrent backup or restore processes to five (5) or less.
Allow the alternate transport mechanisms to be used on the backup job at times when the HotAdd threshold of more than 5 is reached
Increase the snapshot interval to decrease the likelihood of more than 5 HotAdd jobs running simultaneously and a slower transport mechanism then being called