1 Gigabyte+ boot device: Installing or upgrading to ESXi 6.0 requires a minimum of a 1 GB boot device.
Note: Although a 1 GB USB or SD device suffices for a minimal installation, you should use a 4 GB or larger device. The extra space is used for an expanded coredump partition on the USB/SD device.
4 GB extra for scratch partition: When booting from a local disk, a SAN or an iSCSI LUN, a 5.2 GB disk is required to allow for the creation of the VMFS volume and a 4 GB scratch partition on the boot device.
If a smaller disk or LUN is used, the installer attempts to allocate a scratch region on another available local disk. If no local disk can be found to serve as a scratch partition, /scratch is located on the ESXi host ramdisk, linked to /tmp/scratch. You can later reconfigure /scratch to use a separate disk or LUN.
Due to the I/O sensitivity of USB and SD devices the installer does not create a scratch partition on these devices. Again, the host attempts to configure the /scratch on an available local disk, if no local disk is available it is placed on the ramdisk.
For environments that boot from a SAN or use Auto Deploy, it is not necessary to allocate a separate LUN for each ESXi host. You can co-locate the scratch regions for many ESXi hosts onto a single LUN. The number of hosts assigned to any single LUN should be weighed against the LUN size and the I/O behavior of the virtual machines.
For best performance and memory optimization, do not leave the /scratch partition configured to use the ramdisk. To reconfigure /scratch, see the Set the Scratch Partition from the vSphere Web Client section in the vSphere Installation and Setup Guide.