vSAN requires capacity for its internal operations. For a cluster to be able to tolerate a single host failure, vSAN requires free space to restore the data of the failed host. The capacity required to restore a host failure matches the total capacity of the largest host in the cluster.
- Operations threshold - Displays the space vSAN requires to run its internal operations in the cluster. If the used space exceeds the operations threshold, vSAN might not operate properly.
- Host rebuild threshold - Displays the space vSAN requires to tolerate one host failure. If the used space exceeds the host rebuild threshold and the host fails, vSAN might not successfully restore all data from the failed host.
For more information on the capacity thresholds, see Monitor vSAN Capacity.
vSAN provides you the option to reserve the capacity in advance so that it has enough free space available to perform internal operations and to repair data back to compliance following a single host failure. By enabling reserve capacity in advance, vSAN prevents you from using the space to create workloads and intends to save the capacity available in a cluster. By default, the reserved capacity is disabled.
- Operations reserve - Reserved space in the cluster for vSAN internal operations.
- Host rebuild reserve - Reserved space for vSAN to be able to repair in case of a single host failure.
To enable reserved capacity for the host rebuild, you must first enable the operations reserve. When you enable operations reserve, vSAN reserves 5% additional capacity in the operations reserve as a buffer to ensure you have time to react to the capacity fullness before the actual threshold is reached.
Capacity Reservation Considerations
- When you enable reserved capacity with the host rebuild reserve and a host is put into maintenance mode, the host might not come back online. In this case, vSAN continues to reserve capacity for another host failure. This host failure is in addition to the host that is already in the maintenance mode. This might cause the failure of operations if the capacity usage is above the host rebuild threshold.
- When you enable reserved capacity with the host rebuild reserve and a host fails, vSAN might not start repairing the affected objects until the repair timer expires. During this time, vSAN continues to reserve capacity for another host failure. This can cause failure of operations if the capacity usage is above the current host rebuild threshold, after the first host failure. After the repairs are complete, you can disable the reserved capacity for the host rebuild reserve if the cluster does not have the capacity for another host failure.