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 ?


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 bluetoothThe device doesn't allow step 2 if step 1 didn't s…

How to solve "user locked out due to failed logins" in vSphere vMA

In vSphere 6, if the vi-admin account get locked because of too many failed logins, and you don't have the root password of the appliance, you can reset the account(s) using these steps:

reboot the vMAfrom GRUB, "e"dit the entry"a"ppend init=/bin/bash"b"oot# pam_tally2 --user=vi-admin --reset# passwd vi-admin # Optional. Only if you want to change the password for vi-admin.# exitreset the vMAlog in with vi-admin These steps can be repeated for root or any other account that gets locked out.

If you do have root or vi-admin access, "sudo pam_tally2 --user=mylockeduser --reset" would do it, no reboot required.

Multiple VLANs on a Synology NAS

Synology, like other SOHO/SMB NAS vendors, touts VLAN functionality with their current DSM 4.1 software. However, the web interface just lets you specify one VLAN tag to use over each eth interface (or bond interface).
Manual approachIn the busybox environment that you can ssh into as root (after enabling ssh through the webinterface), there's all the tools you need to use multiple VLANs over one link (eth or bond), however:
First you insert the 802.1q module into the Linux kernel:
 /sbin/lsmod | /bin/grep -q 8021q || /sbin/insmod /lib/modules/8021q.koThen you add each VLAN you need to every interface (bond0 in this example)
 /sbin/vconfig add bond0 4And finally you can configure IP addresses on every interface.vlan combination (bond0.4 in this example)
 /sbin/ifconfig bond0.4 broadcast netmask same type of script would work on a QNAP NAS too, by the way. They offer 8021q.ko and vconfig in their commandline environment as well.
Packets from…