Say I have a vSAN cluster with 5 hosts for a total of 50TB vSAN datastore (10TB x 5) and there is 30TB used.
When I place a host in Maintenance with FDM I see that the vSAN datastore total size decreases to 40TB but the used disk space actually increases to say 37TB
Then once the hosts goes in maintenance (data sync completes) the used disk space reduces to the expected level i.e. 30TB
Is this expected as to me it seems that whilst the FDM is in progress there is more disk space being used?
Yes, this is expected behaviour as when we migrate the data here we temporarily have an extra data-set/components where it is being moved to and the original data on the host being put in MM is not removed until the new set has been completely synced. The host entering MM is marked as not usable for data placement as soon as it has started entering MM and thus the reduced total capacity.