Category Archives: Theory

Theories, designs, sketch-ups. Whatever comes to my mind and plans how to make it running.

How much memory should be free for VMkernel?

Recently I have made a small research to see how much free RAM does VMkernel need to work without any hiccups due to:

  • Memory Reclamation Techniques
  • Memory Reservation for the VMkernel itself

I have gathered this data from live environment. However one very important metric is not included in the below measures and graphs, and that is the Virtual Machine overhead that is individual for each environment and is dependant on the VMs’ Memory and vCPU amount.

A quick explanation:

  • RAM [GB]: How many Gigabytes of RAM are installed in the Server.
  • VMKernel [MB]: How many MB are reserved for the VMkernel itself (you can find this value in Configuration -> System Resources Tab).
  • Reclamation [MB]: Calculated with a Memory Reclamation Formula (900 MB + 1% of memory above 24GB).
  • Total [MB]: Sum of VMKernel & Reclamation values. This should be the governing baseline value.
  • Free [%]: How much % of the total server’s memory should be free.
RAM [GB] VMKernel [MB] Reclamation [MB] Total [MB] Free [%]
8 1393 900.0 2293.0 28.0
16 1749 900.0 2649.0 16.2
24 2378 900.0 3278.0 13.3
48 2682 1145.8 3827.8 7.8
64 2745 1309.6 4054.6 6.2
96 3514.5 1637.3 5151.8 5.2
128 3612 1965.0 5577.0 4.3
192 5218.5 2620.3 7838.8 4.0
384 8220 4586.4 12806.4 3.3
512 9985 5897.1 15882.1 3.0

And a graph is below:

Memory Reservation Graph

A Graph representing the GB Installed vs. MB reserved memory.

I hope this table comes in useful when deciding how much RAM there is in your environment for the hosts to use.

My VCP Exam Experience

Hi there! Today I’d like to share my VCP Exam experience with you. I hope this will encourage the people who are preparing to do it at some point in time (be it tomorrow or in a few months) and to share some heart-felt moments with you that have also successfully passed this examination. So stay awhile, and listen 🙂


I got my free exam voucher from Karel Novak by winning a drawing at his vmware-veeam blog. This voucher was time-limited to the end of September, so I wanted to schedule it just for that time so I could soak up the most hands-on experience from my newly acquired position as a last level specialist. Since hands-on experience is the king, I have relied primarily on that resource in order to “prepare” myself.

Since I like to soak up as much information possible on tech stuff I get my hands on, I have purchased a Mastering VMware vSphere 5.5 on which I have written a review in one of my earlier posts (spoiler: it’s helped me a LOT to pass the exam!). Since trip to my job takes around an hour, I was reading it on the bus for about a month before I actually got my exam scheduled.

The Actual Exam.

I arrived at the Pearson-certified testing center and everything went full swing. I have signed a few papers regarding NDA on question reproduction and order in the exam room. Then I got my photo taken by a webcam (you have to sit still for about 10 seconds which was nearly impossible for me), provide an electronic signature and off I went for my exam.

The rumours you might have heard about the draconian exam room rules were all true – you aren’t allowed to drink, chew gum, make any loud noises and do anything else that would disturb other people in the room. You are under surveillance the whole time and the only things you are allowed to carry with you to the exam room is a pen, an erasable board (you also give it back with all your scribblings) and a pen you are given by the attendant. If you want to leave, you have to ring for the attendant to pick you up and provide her with your ID. It sounds horrid but trust me, it’s not bad at all when the exam timer starts ticking – then you’ll have the time to worry about.

My initial (and foolish) thoughts were: Pfff, I’ll be done in a few minutes, take my certificate and go home. No need for all 135 minutes I got for the exam. Oh, I couldn’t have been more wrong – I consumed every single second of that exam and let that timeout state my fate. Halfway through the exam I had an impending doom come down, telling myself “there is no way in hell I’m gonna make this”.

Some of the questions concerned technologies we don’t use in our environment, some (all right, most of them) had pretty tricky wording. I had to read them several times over to make sure the authors don’t catch me off guard this required a tremendous amount of concentration. Overall this exam tests your vSphere knowledge of vSphere thoroughly – configuration, storage, networking, resource management – such a happy concoction. Fortunately, some questions correlated to each other so if I contradicted myself on some of those, I stopped for a minute (and watch  the timer tick away) to re-think my approach to the question. Again, hands-on experience is invaluable in this exam.

In the end, I managed, and a huge boulder fell of my heart.

What’s next?

Passing this certification gave me such a huge boost that I decided to finally start a blog I’d just been rambling about for quite some time, become active in VMware Communities participation, and established another personal goal to try VCAP-DCD or DCA in the next year. It is a gret feeling of accomplishment that you stood the trial by fire, and I hope that all of you who will be undertaking this exam in the future will eventually feel the same way.

What is Virtualization?

Since I’d also like to have this blog more accessible for people of different skillsets, I’d like to make a short introduction to the Virtualization technology that is very progressive the last few years.

Physical Servers

Servers are machines that, under ideal conditions, operate 24 hours a day, 7 days a week, 365 days in a year. For stability and compatibility purposes, they are often running only one application or service for the highest possible uptime. However we know that maintaining this level of service is an exceptional task that is very hard to reach. In the age where multi-core and multi-threaded processors are prominent, having a quad-core processor just sitting there as a file or a simple application server would be a waste of resources. Here’s a little list.

Server operation cost:

  • Electricity that the servers consume and energy used for cooling the server room.
  • Actual physical space where 1 server = 1 slot in a rack.
  • Time and money if anything decides to break down at any time, requiring a technician’s visit to replace a faulty motherboard or a memory module that either crashed the server completely or caused an avalanche of BSODs (or any other kernel panic colors).
  • Money for space rental (if you don’t have your own data center).


  • In most cases, physical hardware  in loaded small percentages when running.
  • Storage may not be easily expanded.

This is remedied with one very effective solution:


This technology enables multiple virtual machines of various Operation Systems to run on one physical server under a so-called hypervisor. Depending on server’s hardware capabilities and application needs, one physical server can run a few resource-demanding , or many lightweight machines.

The hypervisor is a “barebone OS” (in case of VMware, a POSIX-like system with unix-like commandline thanks to the BusyBox Libraries) installed on a bare metal. Once installed, properly configured and managed, it handles all virtual machines’ requests for CPU, memory and IO operations intelligently and scheduling resources when and where needed. Literally it transforms all BIOS instructions the virtual machines generate into the BIOS of physical hardware. Think of it as a robot who catches multiple requests from various sources and has only one output path where he sorts them.

A Virtual machine is a set of files, stored in a folder on a data store using a special file system called VMFS (Virtual Machine File System). These files contain the VM configuration, BIOS’ NVRAM, local Virtual Machine Disks, Virtual Machine Swap Files, snapshot deltas and more.

Virtualization benefits:

  • One physical hardware runs multiple virtual machines – hardware is being used to its maximum potential if scaled well.
  • Hypervisor (or an ESXi host) can be run from local storage, USB drive, SD card, or directly from the memory (with Enterprise version of VMware).
  • VMs can communicate between themselves and have physical network NICs assigned with use of virtual network switches.
  • VMs on shared storage (FCoE / iSCSI / NFS) with clustered ESXi hosts can use High Availability or Fault Tolerance (VM Mirroring) in case of failures.
  • VMs can be dynamically configured how much memory and/or CPU resources they will use.
  • Devices attached to the computer you are running a management console from (VMware vSphere) can be attached to the VMs.
  • VMs can be transferred from one host to another without any issue involving a hardware change in the OS.

And the eventual outcome? Virtualization helps save resources: power, space, and time – mostly downtime that would be connected to a failure of one physical host where an application would be rendered unusable.

Book Review: Mastering VMware vSphere 5.5

Book Cover

Today I’ll be sharing a short review for the of the book, Mastering VMware vSphere 5.5 – Kindle Edition. I purchased it in order to enhance my knowledge, and in hope that it will help me be better prepared for the VCP510 exam. This book has  greatly fulfilled both of my expectations.

I read it from cover to cover, as I usually read all the technically oriented books in order to soak the most knowledge I can. The book is nicely written, with sidebars providing very useful knowledge from working experience. As can be expected from a book that contains “Mastering” in its name, it focuses on every single aspect of vSphere. Starting from the basics in each of its chapters, smoothly transferring to an in-depth level. There is a step-by-step walk-through for every action that is a subject of the chapter along with screenshots. Technically complex matters are displayed graphically to enhance your imagination, which is always nice.

The paperback version has 840 pages – this translates to roughly 15 hours of reading if you don’t just want to skim through the book or just look for enlightenment in certain chapters. But it is certainly good to read the book bit by bit – I guess reading it all in one sitting would result in an information overload 🙂

I wish I could sum up each element this book covers, but I’d be just typing out all vSphere features and aspects such as networking and storage and their underlying components – that many of you are already familiar with. If you have some vSphere experience – or you grasp the concept of virtualization with some IT background and seek a book that will get you started with vSphere, would like to know how certain things work “under the hood”, or just want to see how the authors tackled some real-world scenarios, this book is for you.

As for myself, I read this book with some ~5 months of last-level operator experience and learned many new things. For example did you know that during vMotion the memory snapshot of the source VM is sent to the destination ESXi host in clear text? Or that while using NFS connection to your datastores, only one of the two uplinks being used for the data transfer, even though LACP is used? And there’s much, much more. While sweating in the exam room answering the VCP questions, I recalled what I read in this book many times. Even though this book is revolving around vSphere 5.5 and its features, it is tremendeously helpful for use with earlier versions of vSphere 5.x.

I heartily recommend this book to everyone who wants to learn more about the vast amount features in vSphere. Be it people recently introduced to virtualization, or seasoned vSphere operators who would like to know more. It will even help you to prepare for VCP if you take your time and indulge in the chapters.