Starting an ESXi installation that is a part of a vSAN cluster from a flash device imposes certain restrictions.
When you boot a vSAN host from a USB/SD device, you must use a high-quality USB or SD flash drive of 4 GB or larger.
When you boot a vSAN host from a SATADOM device, you must use single-level cell (SLC) device. The size of the boot device must be at least 16 GB.
During installation, the ESXi installer creates a coredump partition on the boot device. The default size of the coredump partition satisfies most installation requirements.
If the memory of the ESXi host has 512 GB of memory or less, you can boot the host from a USB, SD, or SATADOM device. If the memory of the ESXi host has more than 512 GB, you must boot the host from a SATADOM or disk device.
vSAN 6.5 and later enables you to resize an existing coredump partition on an ESXi host in a vSAN cluster, and enables you to boot from USB/SD devices. For more information, see the VMware knowledge base article at http://kb.vmware.com/kb/2147881.
Hosts that boot from a disk have a local VMFS. If you have a disk with VMFS that runs VMs, you must separate the disk for an ESXi boot that is not for vSAN. In this case you need separate controllers.
Log Information and Boot Devices in vSAN
When you boot ESXi from a USB or SD device, log information and stack traces are lost on host reboot. They are lost because the scratch partition is on a RAM drive. Use persistent storage for logs, stack traces, and memory dumps.
Do not store log information on the vSAN datastore. This configuration is not supported because a failure in the vSAN cluster could impact the accessibility of log information.
Consider the following options for persistent log storage:
Use a storage device that is not used for vSAN 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.