VMware ESXi Core Processes

There are not much information on VMware ESXi architecture out there. There is an old whitepaper “The Architecture of VMware ESXi” which dates back to 2007. In fact, from the management perspective there are only two OS processes critical two ESXi management. These are: hostd (vmware-hostd in ESXi 4) and vpxa (vmware-vpxa in ESXi 4) which are called “Management Agents”.

hostd process is a communication layer between vmkernel and the outside world. It invokes all management operations on VMs, storage, network, etc by directly talking to the OS kernel. If hostd dies you won’t be able to connect to the host neither from vCenter nor VI client. But you will still have an option to connect directly to the host console via SSH.

vpxa is an intermediate layer between vCenter and hostd and is called “vCenter Server Agent”. If you have communication failure between vCenter and ESXi host, it’s the first thing to blame.

Say you have a storage LUN failure and hostd can’t release the datastore. You can restart hostd or both of the processes using these scripts:

# vmware-hostd restart
# vmware-vpxa restart

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Migrating physical Linux host to the VMware ESXi

Well, perhaps the easiest way to accomplish that is using VMware Converter from the start. I believe there is a Linux version. However, I took another route. I already had an Acronis backup image. So my solution was to use this image as a source, which I fed into Windows version of VMware Converter, which in its turn converted it to VMware format and created VMware virtual machine on ESXi server automatically.

Using this simple procedure you can get a working system. Not in my case. Original OS used a software RAID of two hard drives. So I had to boot from a live CD. Then I changed fstab and GRUB’s menu.lst and set /dev/sda1 (root volume) instead of /dev/md0 and /dev/sda2 (swap) in place of /dev/md1. Additionally, I had to reinject GRUB’s boot files:

grub-install –root-directory=/media/sda1/boot /dev/sda

Then, if it’s SUSE you will have to change “resume” switch in GRUB’s boot menu line to /dev/null. Then after you boot into the system, recreate swap partition and point to it in “resume” switch. If you won’t do that, you will end up with the following error during boot process:

Kernel panic – not syncing: I/O error reading memory image

One tricky issue I had in all this story was related to kernel. As I’ve already mentioned original operating system worked on top of software RAID. And its initrd image won’t detect ordinary virtual SCSI hard drive during boot. So I had to boot from the SUSE installation CD and install standard kernel on top of original system. It solved the issue. Additionally I had to choose Russian language as primary during kernel installation, otherwise I ended up with unreadable symbols inside the system. But it’s not necessary for majority of cases.

I hope my experience will be helpful for other sysadmins.