a customer is running a small two node vSphere 6 Standard cluster. Each server has three local datastores, two with ~400 GB and one datastore with 18 TB.
ESX1-Datastore1: 400 GB
ESX1-Datastore2: 400 GB
ESX1-Datastore3: 18 TB
ESX2-Datastore1: 400 GB
ESX2-Datastore2: 400 GB
ESX2-Datastore3: 18 TB
Each ESXi runs a HP StoreVirtual VSA, which resides in Datastore2 and each VSA has has a 300 GB VMDK attached. The VSA are used to provide a small 250 GB shared VMFS. All VMFS are version 5.61 and they aren't upgraded from earlier VMFS versions.
Today I tried to protect a VM using VMware FT. The config of this VM, let's call it VM1, is stored in a shared VMFS datastore. Data VMDKs are stored in ESX1-Datastore3. During the FT config dialog of the vSphere Web Client, I chose the shared VMFS datastore for the secondary VMs VMX and the Tie Breaker file. The data VMDKs should be stored in ESX2-Datastore3. I finished the configuration dialog and everything was fine. Some seconds later the VM was protected by VMware FT. I removed the FT protection and added VMDKs to provide 16 TB storage (8x 2 TB VMDKs, SCSI0:3 up to SCSI0:10). I went through the FT config dialog of the vSphere Web Client, I chose again the shared VMFS for the secondary VMs VMX and the Tie Breaker file. The data VMDKs should be stored in ESX2-Datastore3. Some seconds later, an error popped up that ESX2-Datastore3 doesn't have enough disk space, and therefore the secondary VM on ESX2 was unable to start. Just to be clear: 16 TB were allocated in ESX2-Datastore3, and 2TB were free. And only data VMDKs were stored in ESX2-Datastore3, no swap file or similar. All other datastores had also sufficient disk space. I disabled FT, removed one of the 2 TB VMDK and tried it again. And again I was informed that ESX2-Datastore3 doesn't provide enough free disk space, to fulfill my request to start the secondary VM. With 4x 2 TB VMDKs (and two VMDKs for OS and apps, so in sum 6 VMDKs) and in sum 8 TB, the secondary VM was able to start. I added a 5th 2 TB VMDK and again the secondary VM was unable to start.
My question is: Why? I know that I can only use 2 TB VMKDs. No problem. But why can't I add more than 4x 2 TB VMDKs? Configuration maximums for ESXi 6 told me, that I can have up to 16 disks with VMware FT. Is there a limit for max. storage that can be added to a VM (e. g. 8 TB)
Thanks for advice!