ESXi BootBank partitions

VMware ESXi will create some system partitions on its boot storage device, and it's very useful to understand them with respect to troubleshooting tasks. So let's check them a little:

1. System Boot (FAT16) includes boot loader that has a fixed size: 4MB for older versions (Prior to 6.7) / 100MB for ESXi ver7.0

2. BootBanks 0 & 1: Boot Bank partition has a compressed copy of ESXi boot files and modules. BB0 is used as an active boot partition and BB1 for alternative (AltBootBank) so whenever you upgrade the ESXi version all contents of BB0 will copied into the BB1 for fail-safe purposes. When you upgrade the ESXi host, files of currently installed version are loaded into the AltBootBank (It's empty after new installation) and the system is set to use the updated bank when it reboots normally. In some cases, if the ESXi failed to boot or for any possible reasons the BootBank partition became inaccessible, to recover the latest healthy status of the ESXi host, the system automatically boots from the previously used BootBank and will return to the last good situation (However, you can choose between them manually by pressing "Shift + R" while ESXi are booting).

3. Also, all other system partitions that include non-boot modules like the Scratch partition and the CoreDump that will be placed in the new introduce unified partition in ESXi v7.0, called ESX-OSData. (I think I wrote enough about the importance of CoreDump in my blog, like the last one: Why CoreDump files are useful?) This partition can be used for storing virtual machine files, whenever there is no secondary storage device and the only chosen device must provide all VM's required spaces.

One of the major limitations of ESXi system partitions is their fixed size and to avoid related issues to this matter, VMware decided to make this parameter flexible in v7.0 (You can read more details here) So based on the disk space that we choose as the boot device and its capacity, only the size of the BootBank partitions will be different (not the system boot partition).

At last, if you need to know how to recover a failed ESXi and back it to the normal boot, check the kb59418.