Is a Single VM on a Single Host viable?

I had a question from a company that had lots of small regional sites, asking “Is a single VM on a single host viable?”, in response to my query about why they were buying one or two servers for these sites. There was an assumption that it was only worthwhile to virtualise when there was a requirement for more than three operating system installations at a site. When I recommended that the new servers were deployed with ESXi, there was a questionning gaze from the Infrastructure manager…

Is a single VM on a single host viable?

There are of course benefits and drawbacks, however I believe that the benefits cannot be ignored. I am going to focus on a scenario where a remote/small site has a single Windows server that is a domain controller, DHCP, DNS, NTP, print server and fileserver, with branch cache and proxy for anti-virus update distribution and WSUS/SCCM package cache.

Benefits

• Hardware abstraction. It no longer matters what hardware the Windows server is on – it’s virtual now.
• No more hardware drivers to update.
• The ability to add more VMs in the future, without buying more hardware…
• Portability – once you have a VM, it can be moved to new hardware without a rebuild (see the section below on P2V)
• Any server class system you can buy now will be over-specified for the needs and mostly idle, by making the physical server a virtual host, it opens up opportunities to make the most of the investment.
• Remote administration is easier – up to 40 concurrent remote consoles.
• Higher security. You can’t have a user plug in a USB drive or insert a CD and then run something or copy data off. You no longer need a keyboard and screen on-site.
• If at any stage there is a plan to move the server back to a centralised site, this can be done with the VM.
• Can leverage abilities such as vSphere Replication and VDP for backup, including features like CBT (changed block tracking).
• Core virtualisation benefits, like clones, snapshots, virtual networking (my customer wanted 5 NICs), virtual CPU and memory and disk allocation and expansion, etc. etc.

Missed opportunities

Not quite drawbacks, but things you can’t really take advantage of with a single host

• No HA – you can’t have high availability by just ticking a box, because there is no other host to fail over to
• No vMotion – without shared storage (and a second host), you can’t move a VM from host to host.
• No advanced features from the higher editions of ESXi – unless you want to pay to get them
• Remote out of band management is reduced – things like iLO and iDRAC are considerably less effective, but you can still do tasks like BIOS level management and DCUI access

Drawbacks

It’s true, there are problems.

• Lose hardware management of RAID. You can’t add a new disk and expand it on the fly, like you can with the major hardware vendors (HP, Dell etc). You need to do this through the BIOS level tools – or you can add storage through NFS, iSCSI etc. However, you can still replace failed disks without the RAID management tools.
• Nothing much can be done on the console onsite – a user can’t put in a CD to install software or plug in a USB drive to pull a file off (see benefits above!) This also means that if there are network (WAN) problems, you can’t talk someone through the resolution unless they already have the vSphere client installed.
• You can’t use the vSphere Web Client – you have to use the full/fat vSphere client (applies up to version 5.x)
• If you have the free version of ESXi, you can only administer one host at a time. If you have vSphere Essentials or Essentials Plus, you can only have up to 3 hosts managed by the vCenter Essentials license. If you have vCenter Standard, you can administer remote ESXi Essentials hosts.

It’s important to understand that all versions of ESXi, including the free edition, can be installed on an SD card or USB key, taking up no space on the internal disks. If you purchase the Essentials or Essentials Plus license, it starts out at around $300 per host.

Transition benefits

Back to the original question, asking if a single VM on a Single host is viable, one of the key points that I raised with my customer that really raised their interest, was the transition. They had originally planned to have downtime whilst they moved all their configuration and data from one Windows server to new hardware. I pointed out that they can deploy their new host, and then perform a P2V of the old server onto the new host. This would allow them to test the transition had worked, and still be able to switch back to the old server (even remotely). They also had a problem where there was an over-provisioned logical partition that they did not want to keep – and the disk layout can be changed during the P2V conversion process. This was the clincher for them; no rebuild, less risk, more flexibility.