Server Virtualisation – Good or Bad?
Tags: Support   Technical   Tips   SQL   Team  
5/07/2012 12:00 AM

By Duncan Stodart

My first reaction when told customers are going to install Jim2 Server onto a virtual machine is – don't. Second reaction is – really? Third reaction is – why? Having said that there are some very good reasons for using virtual servers, mainly around resource management, so my reaction can be a little harsh.

Lets look at some things to consider:

Resource Management

If your computing environment contains a large number of servers, it can make sense to consolidate from multiple physical machines onto a smaller number of larger physical machines. You are then sharing the resources of the physical machine amongst a larger number of virtual machines. You can potentially control the amount of CPU, memory, and disk, even network bandwidth available to each of your virtual machines.

Generally this works really well, and the only things you have to watch for are peak resource utilisation and resource overloading. If you want to run a big query (maybe sales register in Jim2) on your virtual machine at the same time big queries are being requested on other virtual machines that are sharing the same physical machine, then the performance of your query may well suffer, ie. the physical machine needs to be able to handle the concurrent peak loads of the virtual machines. You can also over-commit resources on a physical machine.

For example, you can run 4 virtual machines with 4GB of memory on a physical machine with only 12GB of memory. This is ok, as long as the virtual machines don't need all their 4GB of memory at once, and you are happy with the performance cost of restoring virtual memory from disk when and if required.

This resource management is ideal for companies providing hosting services.

Backing Up

A popular mechanism for backing up a virtualised computer is via snapshots. You should be aware that shapshots are effectively like switching your computer off (ie. not shutting it down) and taking a copy of the hard drive. Most of the time you will be ok, but ask any IT person if it is a good thing to simply switch off servers without shutting them down, and you should get one answer – 'no'. We highly recommend using a SQL Server supported backup mechanism (eg. use the schedule backup feature in Jim2 Server, set up using the Jim2 Server Console).

Using virtualisation makes it easy to move a virtual machine from one piece of hardware to another, but if this is the sole purpose of virtualisation you should question whether installing virtualisation software instead of installing a Windows OS, SQL Server and Jim2 is worth the performance cost.

Performance

We have many customers who run Jim2 on virtual machines successfully. We have had a couple of customers run into performance issues running on virtual machines, particularly around disk I/O performance. To highlight the potential performance cost, below are 2 performance graphs done on identical physical machines, with identical data. One machine is Windows 2008 virtualised under ESXi, the other is Windows 2008 running natively on the hardware. On one machine it takes about 15 seconds for the disk I/O for queries to complete. On the other it takes about 30 seconds.

Virtualised



Native

Virtualisation can be a worthwhile exercise – just have a think about what you are trying to achieve and the trade-offs you may have to accept before you decide what to do.


References:
The following references include lots of things to think about, and hopefully highlight some issues to watch for when looking at or using virtualisation technologies.

Best Practices for SQL Server [Virtualising]

Considerations for installing SQL Server on VMWare

Top 10 Keys to Deploying SQL Server on VMware

About Duncan

Duncan is one of the gun team of software developers at Happen Business and is a keen cyclist.

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Duncan is lead developer at Happen Business and likes endurance mountain biking and adventure racing.