virtualization interview questions

Top 15 virtualization interview questions

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vSphere education - What are the downsides of configuring VMs with *too* much RAM?

VMware memory management seems to be a tricky balancing act. With cluster RAM, Resource Pools, VMware's management techniques (TPS, ballooning, host swapping), in-guest RAM utilization, swapping, reservations, shares and limits, there are a lot of variables.

I'm in a situation where clients are using dedicated vSphere cluster resources. However, they are configuring the virtual machines as though they were on physical hardware. In turn, this means a standard VM build may have 4 vCPUs and 16GB or more of RAM. I come from the school of starting small (1 vCPU, minimal RAM), checking real-world use and adjusting up as necessary. Unfortunately, many vendor requirements and people unfamiliar with virtualization request more resources than necessary... I'm interested in quantifying the impact of this decision.

Some examples from a "problem" cluster.

Resource pool summary - Looks almost 4:1 overcommitted. Note the high amount of ballooned RAM. enter image description here

Resource allocation - The Worst Case Allocation column shows that these VMs would have access to less than 50% of their configured RAM under constrained conditions. enter image description here

The real-time memory utilization graph of the top VM in the listing above. 4 vCPU and 64GB RAM allocated. It averages under 9GB use. enter image description here

Summary of the same VM enter image description here

  • What are the downsides of overcommitting and overconfiguring resources (specifically RAM) in vSphere environments?

  • Assuming that the VMs can run in less RAM, is it fair to say that there's overhead to configuring virtual machines with more RAM than they actually need?

  • What is the counter-argument to: "if a VM has 16GB of RAM allocated, but only uses 4GB, what's the problem??"? E.g. do customers need to be educated that VMs are not the same as physical hardware?

  • What specific metric(s) should be used to meter RAM usage. Tracking the peaks of "Active" versus time? Watching "Consumed"?

Update: I used vCenter Operations Manager to profile this environment and get some detail on the cluster stats listed above. While things are definitely overcommitted, the VMs are actually so overconfigured with unnecessary RAM that the real (tiny) memory footprint shows no memory contention at the cluster/host level...

My takeaway is that VMs should really be right-sized with a little bit of buffer for OS-level caching. Overcommitting out of ignorance or vendor "requirements" leads to the situation presented here. Memory ballooning seems to be bad in every case, as there is a performance impact, so right-sizing can help prevent this.

Update 2: Some of these VMs are beginning to crash with:

kernel:BUG: soft lockup - CPU#1 stuck for 71s! 

VMware describes this as a symptom of heavy memory overcommitment. So I guess that answers the question.

enter image description here

vCops "Oversized Virtual Machines" report... enter image description here

vCops "Reclaimable Waste" graph...

enter image description here

Source: (StackOverflow)

How do you increase a KVM guest's disk space?

I setup an Ubuntu guest on a CentOS KVM host with initially 6GB of disk space. How do I go about increasing the Ubuntu guest's disk space from the command line?

EDIT #1: I'm using a disk image file (qemu).

Source: (StackOverflow)

Difference between Xen PV, Xen KVM and HVM?

I know that Xen is usually better than OpenVZ as the provider cannot oversell in Xen. However, what is the difference between Xen PV, Xen KVM and HVM (I was going through this provider's specs? Which one is better for what purposes and why?


For an end-user who will just be hosting websites, which is better? From efficiency or other point of view, is there any advantage of one over the other?

Source: (StackOverflow)

Is virtual machine slower than the underlying physical machine?

This question is quite general, but most specifically I'm interested in knowing if virtual machine running Ubuntu Enterprise Cloud will be any slower than the same physical machine without any virtualization. How much (1%, 5%, 10%)?

Did anyone measure performance difference of web server or db server (virtual VS physical)?

If it depends on configuration, let's imagine two quad core processors, 12 GB of memory and a bunch of SSD disks, running 64-bit ubuntu enterprise server. On top of that, just 1 virtual machine allowed to use all resources available.

Source: (StackOverflow)

Can windows domain controller be virtualized?

Just the question on the title. There is any problem? Any experience on this?

Source: (StackOverflow)

How to run VMWare ESX or ESXi in a virtual machine?

Can VMWare ESX or ESXi be installed and used inside a virtual machine?

It can be installed inside VMWare Workstation or Server, but then it doesn't work; the main symptoms are:

  • It runs REALLY slowly.
  • It lets you create VMs, but when powering up them it gives an error stating "You may not power on a virtual machine in a virtual machine".

Source: (StackOverflow)

Top - what does Virtual memory size mean? ...linux/ubuntu

I am running top to monitor my server performance and 2 of my java processes show virtual memory of up to 800MB-1GB. Is that a bad thing?

What does virtual memory mean?

And oh btw, I have swap of 1GB and it shows 0% used. So I am confused.

Java process = 1 Tomcat server + my own java daemon Server = Ubuntu 9.10 (karmic)

Source: (StackOverflow)

Why do systems generally disable virtualization by default in BIOS settings?

I have yet to see a system whose default configuration enables MMU and directed I/O virtualization. Often this necessitates rebooting and going into the BIOS to enable it if you want, e.g., 64-bit support on your VMs.

Is there some kind of substantial processor overhead that occurs if this is switched on and you're not using virtualization? If not, then what's the reason for it being off by default?

Source: (StackOverflow)

Virtualization Pitfalls/Lessons Learned

What are some pitfalls or lessons learned after converting existing hardware to a virtualized environment? Is there anything you tried to virtualize but will never do again?

Source: (StackOverflow)

Creating a virtual machine in VirtualBox from a physical one

Is there any way to create a virtual machine that you can use in VirtualBox from a physical installation that you have? For instance, if I have Windows XP installed on a physical computer and want to have a virtual version of that machine on a different computer. This would save a ton of time by not having to reinstall and reconfigure the whole OS.

I would think there would be issues with Microsoft's licensing. But even if it's not possible with Windows would it be possible to take a physical Linux machine and create a VirtualBox version of that? Does any other desktop virtualization software provide this feature?

Source: (StackOverflow)

Running 100 virtual machines on a single VMWare host server

I've been using VMWare for many years, running dozens of production servers with very few issues. But I never tried hosting more than 20 VMs on a single physical host. Here is the idea:

  1. A stripped down version of Windows XP can live with 512MB of RAM and 4GB disk space.
  2. $5,000 gets me an 8-core server class machine with 64GB of RAM and four SAS mirrors.
  3. Since 100 above mentioned VMs fit into this server, my hardware cost is only $50 per VM which is super nice (cheaper than renting VMs at GoDaddy or any other hosting shops).

I'd like to see if anybody is able to achieve this kind of scalability with VMWare? I've done a few tests and bumped into a weird issue. The VM performance starts degrading dramatically once you start up 20 VMs. At the same time, the host server does not show any resource bottlenecks (the disks are 99% idle, CPU utlization is under 15% and there is plenty of free RAM).

I'll appreciate if you can share your success stories around scaling VMWare or any other virtualization technology!

Source: (StackOverflow)

Will virtualizing a server mean another OS layer to patch and update, more work and greater risk?

I've done a search and have not found anything addressing issues regarding patching and system updates. I've got guidelines that say servers need to have necessary patches. If I have a VM host then is that an extra layer to patch and update - even with bare metal hypervisors? As opposed to having a metal server? (ie more work and testing and documentation as per my guidelines).

How often do type 1/bare-metal hyper-visors get updated? Does that matter? Does the fact that it is an extra software layer introduce more complexity and risk (security & reliability)? (eg 99% bug free software x 99% bug free software = 98% bug free system)?

(My practical experience is with VMWare Workstation and Server, and VirtualBox.)

Source: (StackOverflow)

What is the difference between PV and HVM virtualization types in ec2?

AWS EC2 offers two types of virtualization of Ubuntu Linux EC2 machines - PV and HVM.

PV: enter image description here

HVM: enter image description here

What is the difference between these types?

Source: (StackOverflow)

How to zero fill a virtual disk's free space on windows for better compression?

How to zero fill a virtual disk's free space on windows for better compression?

I would like a simple open source tool (or at least free) for that. It should probably write an as big as possible file full of 0and erase it afterwards. Only one pass (this is not for security reasons but for compression, we are backing up virtual machines).

Should run from inside windows and not from a disk.

On Linux I do it like this (as a user):

mkdir wipe
sudo sfill -f -l -l -z ./wipe/

Edit 1: I decided to use sdelete from the accepted answer. I had a look at the sdelete's help:

C:\WINDOWS\system32>sdelete /?

SDelete - Secure Delete v1.51
Copyright (C) 1999-2005 Mark Russinovich
Sysinternals - www.sysinternals.com

usage: sdelete [-p passes] [-s] [-q] <file or directory>
       sdelete [-p passes] [-z|-c] [drive letter]
   -c         Zero free space (good for virtual disk optimization)
   -p passes  Specifies number of overwrite passes (default is 1)
   -q         Don't print errors (Quiet)
   -s         Recurse subdirectories
   -z         Clean free space

This is an old version. I used the -c switch from the 2nd invocation and this was quite fast (syntax only valid for older versions before V1.6):

c:\>sdelete -c c: (OUTDATED!)

Edit 2: As scottbb pointed out in his answer below, there was a September 2011 change to the tool (version 1.6) The -c and -z options have changed meanings. The correct usage from 1.6 onwards is

c:\>sdelete -z c:

I have the impresion this does what I want. The sdelete tool is easy to use and easy to get.

Source: (StackOverflow)

Vagrant set default share permissions

When running a vagrant instance the project folder is mounted on /vagrant automatically. However is mounted with the following permissions

# ll -d /vagrant
drwx------ 1 vagrant vagrant 612 Jun 13 14:41 /vagrant/

I need it to be mounted with (at least) 0770 but I can't find how. If I run the mount command I see this output

# mount
v-root on /vagrant type vboxsf (uid=1000,gid=100,rw)

I've tried both chmod and chown/chgrp, but they won't work on that mounted folder so my apache user can't access that folder. I read in Vagrant manual that I can change owner and group but it doesn't mention nothing about permission.

How can I do that?

Another option could be switch to NFS but in this way it won't work on Windows platforms and it need to edit local /etc/exports file and it would require root privileges and also it's pretty annoying, so I'd prefer to not make this change.

Source: (StackOverflow)