VMware requires that 30% unused space (“slack space”) be maintained in the vSAN datastore within the Service Offering, to support operation of the SDDC. Adequate slack space is required for use of the vSAN datastore. If storage free space reaches (or falls below)25%, it is possible that the customer could lose the ability to utilize the SDDC, and the environment could become inoperable. If unused space in an SDDC vSAN datastore drops reaches (or falls below)25%, VMware will automatically add hosts to the SDDC to prevent damage to the SDDC.
vSAN “slack space” is simply free space that is set aside for operations such as host maintenance mode data evacuation, component rebuilds, rebalancing operations, and VM snapshots. Activities such as rebuilds and rebalancing can temporarily consume additional raw capacity. Host maintenance mode temporarily reduces the total amount of raw capacity a cluster has. This is because the local drives on a host that is in maintenance mode do not contribute to vSAN datastore capacity until the host exits maintenance mode.
There are a number of sources that recommend 25-30% slack space when designing and running a vSAN cluster. For example, a vSAN datastore with 20TB of raw capacity should always have 5-6TB of free space available for use as slack space. This recommendation is not exclusive to vSAN. Most other HCI storage solutions follow similar recommendations to allow for fluctuations in capacity utilization. Storage policy changes is a good reason to maintain that slack space.
There are a couple of cases where storage policy changes can temporarily consume more capacity. One scenario is when a new policy that requires a change in component number and/or layout is assigned to a VM. Another scenario is when an existing storage policy that is assigned to one or more VMs is modified. In both cases, vSAN will use additional capacity to make the necessary changes to components to comply with the assigned storage policy.