Skip to main content

how many physical hosts do you buy: what MS sales didn't tell you

One of the first steps in a virtualization project is building a list of workloads that will get virtualized, with a measurement or estimate of the resources that they will need. X MHz and Y MB, sum everything up, and let's say you get 30 GHz of CPU power and 20 GB of RAM.

The hardware you'd like to run all those virtual machines on can handle two CPUs (dual-socket), four cores each (quad-core). That means that every physical server will give you between 20 and 25 GHz of CPU power. For memory, you'll buy 12 GB of RAM in each server.

So the plan is to buy two of those servers, right ?

Well, as long as your infrastructure is 100% healthy and running OK, two servers will do the job just fine. You've got enough resources, with a bit of headroom for overhead and future growth. But what happens when one of the physical servers is down ? Think of hardware problems, think of virtualization software upgrades, think of patching the hypervisor.

Then the available resources are down to 20 GHz and 12 GB of RAM. For CPU, 20 GHz means that every application will get 30% less than desired, and will therefore run a bit slow, probably noticable for users. Is that acceptable in these cases ?

And last but not least, memory. Temporarily, you've got just 12 GB of RAM available, and your VMs need 20 GB. Did you know that what happens depends on which hypervisor you've chosen ?

  1. With Hyper-V or Xen, you're in trouble. With 12 GB, you can run 60% of your VMs, and the rest stays down.
  2. With ESX and ESXi, you can start all your VMs, and just as with CPU, there's not enough resources so everything will slow down a bit. But it will run. This trick is called "memory overcommitment".
Conclusion: if you chose Hyper-V or Xen, you'll need three of those servers to continue running your business. With VMware, you have the option of buying three and continuing without speed impact, or you just buy two servers, and live with the temporary loss of performance.

Now let me guess, did your Microsoft sales guy tell you about this difference ?
Labels:

Comments

Post a Comment

Popular posts from this blog

Volkswagen UHV bluetooth touch adapter & its problems

My Volkswagen car has the "universal cellphone preparation" UHV built-in. This is the main part of a car kit, but requires an additional adapter for connecting to a cellphone. At first, I was using an adapter for my good old Nokia 6310, even after I changed to the Nokia E71. Connecting was easy: pair the phone with the "VW UHV" bluetooth entity, and done. This has the phone connected to the car kit at all times, so even non-call-related functions use the car audio system (e.g. voice recognition). But progress will have its way, no matter what happens. So in comes the "bluetooth touch adapter". Instead of a phone-specific adapter, this is a small touchscreen device that slots into the UHV dashboard mount. Connecting a phone is very different now: the Bluetooth Touch Adapter connects to the "VW UHV" device via bluetooth the phone connects to "Touch Adapter" device, also via bluetooth The device doesn't allow step 2 if step 1 didn'...
12 comments
Read more

Which vSphere version is my VM running on?

Several years ago, I created a list of ESXi versions with matching VM BIOS identifiers. The list is now complete up to vSphere 7. Your Linux runs on a VMware VM, but on which ESXi version? Even without access to the host nor vCenter, you can see for yourself: run "dmidecode" and look at lines 10, 11 and 12. The BIOS release date, the address and the runtime size are unique for each ESXi version. Starting from 7.0 U3, no address and size is reported, and the "Phoenix Technologies LTD" name changed to "VMware, Inc.". Look up your result in the following table: ESXi version   BIOS release date   Address   Size ESX 2.5 04/21/2004 0xE8480 97152 bytes ESX 3.0 04/17/2006 0xE7C70 99216 bytes ESX 3.5 01/30/2008 0xE7910 100080 bytes ESX 4 08/15/2008 0xEA6C0 88384 bytes ESX 4U1 09/22/2009 0xEA550 88752 bytes ESX 4.1 10/13/2009 0xEA2E0 89376 bytes ESXi 5 01/07/2011 0xE72C0 101696 bytes ESXi 5.1 06/22/2012 0xEA0C0 89920 bytes ESXi 5.5 07...
7 comments
Read more

Reset lost root password on vSphere ESXi 6.7

VMware's solution to a lost or forgotten root password for ESXi is simple: go to  https://kb.vmware.com/s/article/1317898?lang=en_US  and you'll find that "Reinstalling the ESXi host is the only supported way to reset a password on ESXi". If your host is still connected to vCenter, you may be able to use Host Profiles to reset the root password, or alternatively you can join ESXi in Active Directory via vCenter, and log in with a user in the "ESX Admins" AD group. If your host is no longer connected to vCenter, those options are closed. Can you avoid reinstallation? Fortunately, you can. You will need to reset and reboot your ESXi though. If you're ready for an unsupported deep dive into the bowels of ESXi, follow these steps: Create a bootable Linux USB-drive (or something else you can boot your server with). I used a CentOS 7 installation USB-drive that I could use to boot into rescue mode. Reset your ESXi and boot from the Linux medium. Ident...
Post a Comment
Read more