Virtual machine disk files (VMDKs) can be deployed in three different formats—thin, thick, or eagerzeroedthick. Both thin and thick disk files use lazy zeroing, where the initial zeroing of the disk blocks is delayed until the first write. Eagerzeroedthick disk blocks are pre-allocated with zeros at the time of disk provisioning, making it unnecessary to zero the disk on a first write basis during normal running operations. This provides approximately a 10–20 percent performance improvement over the other disk formats.

Most Microsoft Exchange 2013 high availability features are highly sensitive to system response time. The additional overhead of disk zeroing during normal operations might cause unnecessary disk latency and potentially cause a false cluster failover event. If you are deploying DAGs on a vSphere platform, VMware highly recommends defining an operational standard to only use eagerzeroedthick disks for Microsoft Exchange mailbox databases and log files.