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)

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)

Is there a reason to give a VM a round base-2 amount (2048MB, 4096MB, etc) of memory?

The title pretty much says it all, is there any advantage to giving a VM 2048MB of memory instead of rounding to base-10 and doing 2000MB?

Source: (StackOverflow)

When deploying a single server on new hardware, do you virtualize it or not?

There are a few questions that I've found on ServerFault that hint around this topic, and while it may be somewhat opinion-based, I think it can fall into that "good subjective" category based on the below:

Constructive subjective questions:

* tend to have long, not short, answers
* have a constructive, fair, and impartial tone
* invite sharing experiences over opinions
* insist that opinion be backed up with facts and references
* are more than just mindless social fun

So that out of the way.

I'm helping out a fellow sysadmin that is replacing an older physical server running Windows 2003 and he's looking to not only replace the hardware but "upgrade" to 2012 R2 in the process.

In our discussions about his replacement hardware, we discussed the possibility of him installing ESXi and then making the 2012 "server" a VM and migrating the old apps/files/roles from the 2003 server to the VM instead of to a non-VM install on the new hardware.

He doesn't perceive any time in the next few years the need to move anything else to a VM or create additional VMs, so in the end this will either be new hardware running a normal install or new hardware running a single VM on ESXi.

My own experience would lean towards a VM still, there isn't a truly compelling reason to do so other than possibilities that may arise to create additional VMs. But there is the additional overhead and management aspect of the hypervisor now, albeit I have experienced better management capabilities and reporting capabilities with a VM.

So with the premise of hoping this can stay in the "good subjective" category to help others in the future, what experiences/facts/references/constructive answers do you have to help support either outcome (virtualizing or not a single "server")?

Source: (StackOverflow)

How do I know if I'm working on a Virtual Machine or not?

Is there a way to know if the Windows machine I'm working on is virtual or physical? (I'm connecting with RDP to the machine. If it's a virtual machine it is working and handled by VMWare).

Source: (StackOverflow)

VMware Linux Server -- how can you tell if you are a vm or real hardware?

An interesting question. I have logged into a Linux (most likely SuSE) host. Is there some way that I can tell programmatically that I am a VM host or not?

Also assume that the vmtools are not installed.

Source: (StackOverflow)

Are VMware ESXi 5 patches cumulative?

This seems basic, but I'm confused about the patching strategy involved with manually updating standalone VMware ESXi hosts. The VMware vSphere blog attempts to explain this, but the actual process is still not clear to me.

From the blog:
Say Patch01 includes updates for the following VIBs: "esxi-base", "driver10" and "driver 44". And then later Patch02 comes out with updates to "esxi-base", "driver20" and "driver 44". P2 is cumulative in that the "esxi-base" and "driver44" VIBs will include the updates in Patch01. However, it's important to note that Patch02 not include the "driver 10" VIB as that module was not updated.

This VMware Communities post gives a different answer. This one contradicts the other.

Many of the ESXi installations I encounter are standalone and do not utilize Update Manager. It is possible to update an individual host using the patches make available through the VMWare patch download portal. The process is quite simple, so that part makes sense.

The bigger issue is determining what exactly to actually download and install. In my case, I have a good number of HP-specific ESXi builds that incorporate sensors and management for HP ProLiant hardware.

  • Let's say that those servers start with an ESXi build #474610 from 9/2011.
  • Looking at the patch portal screenshot below, there is a patch for ESXi update01, build #623860. There are also patches for builds #653509 and #702118.
  • Coming an old version of ESXi (e.g. vendor-specific build), what is the proper approach to bring the system fully up-to-date? Which patches are cumulative and which need to be applied sequentially? Is installing the newest build the right approach, or do I need to step back and patch incrementally?
  • Another consideration is the large size of the patch downloads. At sites with limited bandwidth, downloading of multiple ~300mb patches is difficult.

enter image description here

Source: (StackOverflow)

Consumer (or prosumer) SSD's vs. fast HDD in a server environment

What are the pro's and con's of consumer SSDs vs. fast 10-15k spinning drives in a server environment? We cannot use enterprise SSDs in our case as they are prohibitively expensive. Here's some notes about our particular use case:

  • Hypervisor with 5-10 VM's max. No individual VM will be crazy i/o intensive.
  • Internal RAID 10, no SAN/NAS...

I know that enterprise SSDs:

  1. are rated for longer lifespans
  2. and perform more consistently over long periods

than consumer SSDs... but does that mean consumer SSDs are completely unsuitable for a server environment, or will they still perform better than fast spinning drives?

Since we're protected via RAID/backup, I'm more concerned about performance over lifespan (as long as lifespan isn't expected to be crazy low).

Source: (StackOverflow)