Starting an ESXi installation that is a part of a Virtual SAN cluster from a flash device imposes certain restrictions.
Use a high-quality USB or SD flash drive of 4 GB or larger.
If the memory size of the ESXi host is greater than 512 GB, boot it from a SATADOM or disk device. When you boot a Virtual SAN host from a SATADOM device, you must use single-level cell (SLC) device and the size of the boot device must be at least 16 GB. In addition, hosts that boot from a disk have a local VMFS. Hence, you must separate the disk for ESXi boot that is not for Virtual SAN.
Log Information and Boot Devices in Virtual SAN
When you boot ESXi from a USB or SD device, log information and stack traces are lost on host reboot because the scratch partition is on a RAM drive. Use persistent storage for logs, stack traces and memory dumps.
You should not store log information on the Virtual SAN datastore. A failure in the Virtual SAN cluster might impact the accessibility of log information.
Consider the following options for persistent log storage:
Use a storage device that is not used for Virtual SAN and is formatted with VMFS or NFS.
Configure the ESXi Dump Collector and vSphere Syslog Collector on the host to send memory dumps and system logs to vCenter Server.
For information about setting up the scratch partition with a persistent location, see the vSphere Installation and Setup documentation.