For new installations of ESXi, during the autoconfiguration phase, a 4GB VFAT scratch partition is created if the partition is not present on another disk.
Partitioning for hosts that are upgraded to ESXi 5.x from ESXi versions earlier than version 5.0 differs significantly from partitioning for new installations of ESXi 5.x. See the vSphere Upgrade documentation.
When ESXi boots, the system tries to find a suitable partition on a local disk to create a scratch partition.
The scratch partition is not required. It is used to store vm-support output, which you need when you create a support bundle. If the scratch partition is not present, vm-support output is stored in a ramdisk. In low-memory situations, you might want to create a scratch partition if one is not present.
For the installable version of ESXi, the partition is created during installation and is selected. VMware recommends that you do not modify the partition.
To create the VMFS volume and scratch partition, the ESXi installer requires a minimum of 5.2GB of free space on the installation disk.
For ESXi Embedded, if a partition is not found, but an empty local disk exists, the system formats it and creates a scratch partition. If no scratch partition is created, you can configure one, but a scratch partition is not required. You can also override the default configuration. You might want to create the scratch partition on a remote NFS mounted directory.
The installer can create multiple VFAT partitions. The VFAT designation does not always indicate that the partition is a scratch partition. In some cases, a VFAT partition can simply lie idle.