When you enable deduplication and compression, in certain cases, you might need to reduce the level of protection for your virtual machines.
Enabling deduplication and compression requires a format change for disk groups. To accomplish this change, vSAN evacuates data from the disk group, removes the disk group, and recreates it with a new format that supports deduplication and compression.
In certain environments, your vSAN cluster might not have enough resources for the disk group to be fully evacuated. Examples for such deployments include a three-node cluster with no resources to evacuate the replica or witness while maintaining full protection. Or a four-node cluster with RAID-5 objects already deployed. In the latter case, you have no place to move part of the RAID-5 stripe, since RAID-5 objects require a minimum of four nodes.
You can still enable deduplication and compression and use the Allow Reduced Redundancy option. This option keeps the VMs running, but the VMs might be unable to tolerate the full level of failures defined in the VM storage policy. As a result, temporarily during the format change for deduplication and compression, your virtual machines might be at risk of experiencing data loss. vSAN restores full compliance and redundancy after the format conversion is completed.