ESXi is NOT Linux

ESXi is not built upon the Linux kernel, but uses an own VMware proprietary kernel (the VMkernel) and software, and it misses most of the applications and components that are commonly found in all Linux distributions.

Because ESXi uses "-ix" commands (Unix, Linux, POSIX), it "looks and smells" like Linux, but in fact, these commands are similar to the package CygWin that one can run on a Windows system to get a Linux terminal and command line interpreter. ESXi does not use CygWin, however. They run something called BusyBox.

BusyBox is something used on a lot of small-factor home networking gear. PfSense for example, runs Berkeley Unix (BSD). But many small routers (Ubiquiti EdgeMax comes to mind) use different chipsets, different OS kernels, and then use BusyBox to abstract this kernel away from users by providing a common interface - meaning users don't need to learn a whole slew of new OS commands.

 ESXi has a LOT of things that Linux does NOT have:

1. File systems VMFS6 for example is the newest revision of VMFS.

2. Process Scheduler - and algorithms

3. Kernel hooks that tools like esxtop use (think system activity reporting in Unix and Linux) 

 

This article (the source for this post), discusses some nice facts in comparing ESXi to Linux:

ESXi-is-not-based-on-Linux

I learned some interesting things from this article, such as:

ESXi even uses the same binary format for executables (ELF) than Linux does, so it is really not a big surprise anymore that you can run some Linux binaries in an ESXi shell - provided that they are statically linked or only use libraries that are also available in ESXi! (I exploited this "feature" when describing how to run HP's hpacucli tool in ESXi and when building the ProFTPD package for ESXi).

...You cannot use binary Linux driver modules in ESXi. Lots of Linux device drivers can be adapted to ESXi though by modifying their source code and compiling them specifically for ESXi. That means that the VMkernel of ESXi implements a sub-set of the Linux kernel's driver interfaces, but also extends and adapts them to its own hypervisor-specific needs.

In my opinion this was another very clever move of the VMware ESXi architects and developers, because it makes it relatively easy to port an already existing Linux driver of a hardware device to ESXi. So the partners that produce such devices do not need to develop ESXi drivers from scratch. And it also enables non-commercial community developers to write device drivers for devices that are not supported by ESXi out-of-the-box!

There is a PDF download of the ESXi architecture, which can be downloaded here:

 https://www.vmware.com/techpapers/2007/architecture-of-vmware-esxi-1009.html