Friday, March 13, 2009

Damn Small Linux as a VM for Capacity Testing a virtual managment system

Here is a little something that I did over a year ago.

My objective was to look at virtual machine management systems (such as SCVMM and Virtual Center) and to consider how many VMs they could manage.

The potential items that you can run into are many.

1) Overhead on the management system just because of the number of VMs.
2) Limits to the number of VMs on a single host.
3) Host storage

And this is without actually powering on a single VM.

Now, begin to consider performing actions on virtual machines and we get into more complexity:
1) How many VMs can a host support (the RAM implications)
2) Can we deploy more VMs than can be booted.
3) How long do the boot operations take?

And I could keep going.

When I was first challenged with this I tried to make it as simple as possible. A single VM with an operating system installed to a virtual disk (I would concern myself with PXE VMs later on). And, I only had three servers (one I needed for the management server, so I couldn't use it as a host).

I began looking at various operating systems (Windows, Linux, etc.). All failed due to the size of the virtual disk. However, I did fine one really good candidate. And there are a couple different ways that you can set it up.

This is where Damn Small Linux comes in.

By default DSL boots from an ISO, detects the hardware and runs.
The only hitch now is installing, and RAM.

In this scenario the DSL VM can be created in two different ways.

1) As many VMs sharing a common boot ISO file. (one boot ISO per host).
2) As many single VMs with unique virtual disks.

In my case I began with option 1 but later switched to option 2.

In the case of option 2, there is a special option to install DSL to the hard disk. I found that here:

And, through trial and error I learned that you can set the RAM on a DSL VM as low as 24Mb of RAM and the VM would still boot. This gave me many possibilities. My requirement was that the VM was able to boot, and that it could run without crashing. The VM ran better at 36Mb of RAM and really good at 128Mb. But for density, I went to as low as I could.

What did this end up giving me?
(other than claims that - I did it!)

This gave me an environment with a virtualization manager server and 1500 virtual machines.
At the time that I did this, just the act of having that many VMs caused a great deal of overhead on the management system.

And I also learned that there was another system that only allowed 355 VMs per host (an undocumented feature) and the host would 'fall silent' as soon as you deployed VM 356 - the host didn't crash, but its internal management daemon did, and then refused to start. I rebuilt a few hosts before I narrowed down that problem.

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