What happens with VSAN when a host is put into Maintenence Mode?

[ejward]

I have a cluster of 4 VSAN 5.5 hosts. in addition to the SSD in each host, each host has 3x1TB drives.  So, 12TB total.

I have about 4TB of VMs out there and I'm out of disk space.  Does that seem right?

One of the hosts is in Maintenance Mode right now, does that take away from storage capacity?

What is the default Failures to Tolerate if you never set a storage policy?

If I try to set FTT to 2 now, will take anything down?  I know I may not have enough space to do that right now but I was wondering if there was any downtime in setting this on the fly.

[elerium]

When a host is put into maintenance mode, you do lose the storage capacity provided by it. There is a 1 hour delay/timer before the objects on your maintenance mode host are then rebuilt on the remaining capacity of your cluster, so nothing really happens until that 1 hour passes.

If you have 4TB of VMs, assuming FTT=1, it's using 8TB of storage. If you also have one host down > 1 hr, you will have 9TB total capacity with 8TB used so yes you may be near full/out of space.

The default failure to tolerate (FTT) is 1. If you set FTT=2, it will  fail because you don't have enough hosts to accommodate that. FTT=2 has a minimum requirement of 5 nodes with 6 being recommended. More info here: 4 is the minimum number of hosts for VSAN if you ask me

[virtualg_uk]

Yes your calculations are correct.

Yes putting a host in maintenance mode will reduce the storage available to the vSAN datastore

FTT=1 is the default FTT setting on the Default Storage Policy

Keep in mind that it's possible to put a host into maintenance mode with full evacuation or maintain availability