In ESXi 7.0, a new partition scheme was introduced which also brings along a new set of storage requirements. These changes are explained in the official documentation here and the following VMware KB 77009 also contains some additional info which can be helpful. Storage changes are not easy but this was necessary to not only better support some of the current capabilities but more importantly, it setups the foundation for future ESXi capabilities.
The biggest change to the partition layout is the consolidation of VMware Tools Locker, Core Dump and Scratch partitions into a new ESX-OSData volume (based on VMFS-L). This new volume can vary in size (up to 138GB) depending on a number of factors including the current ESXi boot media (USB SD-Card, Local Disk) but also the size of the device itself, which is explained in the official documentation.
From some of the comments on Twitter, Reddit and the direct inquiries that I have received, this new behavior seems to be most impactful to smaller homelabs where a fresh install of ESXi 7.0 has been performed. Folks have shared that their ESX-OSData volume has taken up 120GB which can be quite significant if you have a smaller disk which can be quite common. I normally install ESXi on a USB device and I also use vSAN, which has a different behavior and I have also not upgraded my physical ESXi host (E200-8D) to 7.0 yet.
I performed a fresh installation of ESXi 7.0 (running as Nested ESXi VM) that was configured with 1TB of storage and here is what the filesystem layout now looks:
We can see that the ESX-OSData volume takes up ~119.75GB, which is not too bad for 1TB volume but I can understand this may not be ideal if you have something smaller such as 250GB to 512GB disk. Due to the size of the local device, the boot options mentioned in the KB would not be helpful and I was curious myself if this ESX-OSData volume size could be configurable. In doing some research it looks like the size of the ESX-OSData can be specified using the following ESXi boot option (SHIFT+O during the initial boot) called autoPartitionOSDataSize
Disclaimer: This may not be officially supported by VMware as it deviates from the system defaults and can have other unintended behaviors. Use at your own risk.
I performed another fresh installation of ESXi 7.0 but now passing in autoPartitionOSDataSize=4096 (MB) we can now see our ESX OS-Data volume is no longer using 120GB like before.
You should ensure that size your ESX-OSData greater than 4GB, to ensure that coredump files can be created as you can see my example, 50% has already been used up. Since this new volume will store some pretty important files, you should really give yourself a buffer if you decided to deviate from the system defaults that has been selected.
Interestingly, I also ran another experiment where I had upgraded from ESXi 6.7 Update 3 to ESXi 7.0 and here is the partition layout before the upgrade
and here is the partition layout after the upgrade and for the exact same 1TB disk which was completely empty. I suspect the size of ESX-OSData was due to my selection of preserving the existing VMFS volume.