In the VSAN 6.0 Design & Sizing Guide, a caveat was placed around the size of a VMDK, and the Number of Failures to Tolerate (FTT) number. It reads like this:
“If the VMDK size is greater than 16TB, then the maximum value for NumberOfFailuresToTolerate is 1.”
I’m pleased to say that this restriction has been lifted in VSAN 6.2.
That limit was due to the metadata size. You might recall if you are a regular reader of this blog that each component can only grow to a maximum size of 255GB. With a 62TB VMDK, that a lot of components. Now if you factor in the Number of Failures to Tolerate, with a value of 1, you double the components, with 2 you treble it and with 3 (the maximum) with quadruple it. These all need to be tracked in metadata.
If the object configuration is too large, then we run the risk of the object configuration size being larger than the assigned metadata. That is why we added that Number of Failures to Tolerate limit restriction for large objects in VSAN 6.0 and 6.1.
However, VSAN engineering have implemented a compression method for metadata in VSAN 6.2. This now allows us to remove the limit. Now you can have very large VMDK with higher Number of Failures to Tolerate values, if you so wish.