ESXi 5.0, the system creates at least five partitions whose size and layout the user cannot control.
Bootpartition (4 MB):
This partition is needed for booting
Bootbank (250 MB):
The compressed boot image is saved on this FAT partition.
It will be extracted during the boot process and loaded into the system memory.
At the time of vSphere 4 this file had a size of about 70 MB, with vSphere 5 it is now grown to 250 MB.
AltBootbank (250 MB):
This partition is empty after a fresh install. Once you perform an update of ESXi, the current image is copied from the bootbank partition here.
This makes it possible to return to the last known good configuration by typing “Shift + R” while booting if there occures an error during the update of an ESXi host.
Dump/crash partition (110 MB):
In the case of a total crash of the host a dump file is written on this partition.
Store (285 MB):
On this partition the different ISO files for the VMWare Tools are available for all supported operating systems.
Scratch partition (4 GB):
This partition is only created if the installation media has at least 5 GB of space. It is used for the log files of the VMKernel.
If this partition is missing, the logs of the host are lost after a reboot or shutdown.
This partition is only created if the installation medium is not a flash memory.
It extends over the total available space of the medium and is formatted with VMFS 5.