When you use auto-partitioning boot option when installing or auto-deploying ESXi, the auto-partitioning option creates a VMFS datastore on your host's local storage. You have several options to keep your local storage flash disks unformatted.
By default, auto-partitioning deploys VMFS file systems on any unused local storage disks on your host, including flash disks.
However, a flash disk formatted with VMFS becomes unavailable for such features as virtual flash and Virtual SAN. Both features require an unformatted flash disk and neither can share the disk with any other file system.
To ensure that auto-partitioning does not format the flash disk with VMFS, use the following boot options when you install ESXi or boot the ESXi host for the first time:
If you use Auto Deploy, set these parameters on a reference host.
In the vSphere Web Client, select the host to use as a reference host and click Manage.
Click System to open the system options, and click Advanced System Settings.
Scroll to VMkernel.Boot.autoPartition and set the value to true.
Scroll to VMkernel.Boot.skipPartitioningSsds and set the value to true.
Reboot the host.
If flash disks that you plan to use with Flash Read Cache and Virtual SAN already have VMFS datastores, remove the datastores.