Before you attempt to upgrade vSAN, verify that your environment meets the vSphere hardware and software requirements.
1: Verify that vSAN 6.6 supports the software and hardware components, drivers, firmware, and storage I/O controllers that you plan on using. Supported items are listed on the VMware Compatibility Guide website at http://www.vmware.com/resources/compatibility/search.php.
2: Verify that you have enough space available to complete the software version upgrade. The amount of disk storage needed for the vCenter Server installation depends on your vCenter Server configuration.
3: Verify that you have enough capacity available to upgrade the disk format. If free space equal to the consumed capacity of the largest disk group is not available, with the space available on disk groups other than the disk groups that are being converted, you must choose Allow reduced redundancy as the data migration option.
4: Verify that you have placed the vSAN hosts in maintenance mode and selected the Ensure data accessibility or Evacuate all data option.
5: Verify that you have backed up your virtual machines.
6: All vSAN disks should be healthy
>> No disk should be failed or absent
>> This can be determined via the vSAN Disk Management view in the vSphere Web Client.
7: There should be no inaccessible vSAN objects
8: There should not be any active resync at the start of the upgrade process.
After starting a vSAN Cluster upgrade :
1: Do not attempt to upgrade a cluster by introducing new versions to the cluster and migrating workloads.
2: If you are adding or replacing disks in the midst of an upgrade, ensure that they are formatted with the appropriate legacy on-disk format version : How to format vSAN Disk Groups with a legacy format version (2146221).
Consider the following recommendations when deploying ESXi hosts for use with vSAN:
1: If ESXi hosts are configured with memory capacity of 512 GB or less, use SATADOM, SD, USB, or hard disk devices as the installation media.
2: If ESXi hosts are configured with memory capacity greater than 512 GB, use a separate magnetic disk or flash device as the installation device. If you are using a separate device, verify that vSAN is not claiming the device.
3: When you boot a vSAN host from a SATADOM device, you must use a single-level cell (SLC) device and the size of the boot device must be at least 16 GB.
4:To ensure your hardware meets the requirements for vSAN, refer to Hardware Requirements for vSAN.
Please check the vSAN upgrade requirements (2145248) : https://kb.vmware.com/s/article/2145248