Skyline/vSAN Health is the first place we go when we troubleshoot a vSAN cluster. But in case we have a problem that appears from time to time, it can happen that an updated vSAN Health report will not indicate any problems.
One of the simplest ways to check the historical vSAN Health data is to check them in your vCenter’s log file:
Some time ago we looked into a rare case where a vSAN object (VMDK) with FTT-1 mirror SPBM policy didn’t require a witness component.
So usually with FTT-1 mirror policy we have two components of the object and a witness component. What do you think happens with a witness component when instead of FTT-1, FTT-2 SPBM policy is assigned to an object ? FTT-2 means our object can survive a failure of its two components. For a VMDK object it means there will be three copies of this VMDK and two can be inaccessible without affecting VM’s I/O traffic.
For FTT-2 there will be more than just one witness component…
A VMDK will have three copies + 2 witness components. That is why FTT-2 policy requires minimum of 2n+1 = 5 nodes / ESXi hosts. And any two out of 5 can be inaccessible without affecting the service.
How about FTT-3? FTT-3 means our object can survive a failure of its three components. For a VMDK object it means there will be four copies of this VMDK and three can be inaccessible without affecting VM’s I/O traffic.
A VMDK will have four copies + 3 witness components. That is why FTT-3 policy requires minimum of 2n+1 = 7 nodes / ESXi hosts. And any three out of 7 can be inaccessible without affecting the service.
esxcli vsan debug object list is a command that can be used to list the components of the object , here is how it looks for a VMDK with FTT-3 SPBM policy:
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