The dynamic virtual hard drive is one of the best things about virtualization. It is like Dr Who\u2019s Tardis. The virtualized OS thinks it has plenty of space, while on the host machine your 128GB virtual drive might occupy just 4 or 5GB \u2013 this is typical of the test VMs I set up, running say Server 2008 and a server application or two.<\/p>\n
Trouble is, there\u2019s a performance penalty. I first came across this with a hilariously slow Ubuntu install, where the problem is made worse by the lack of integration services, the utilities and drivers that install into the guest to enable smooth interaction with the host.<\/p>\n
As an experiment, I created a second Ubuntu VM using a 30GB fixed-size drive. Better? Yes, much better. Here are the figures on my admittedly slow low-end HP Xeon server:<\/p>\n
Copy an 891MB file:<\/p>\n
As a further test, I copied the same file in Server 2008:<\/p>\n
My immediate thoughts: you would be crazy to use VMs in production with dynamic drives. Always use fixed drives. You can still expand them manually if necessary. Note that Hyper-V defaults to dynamic drives.<\/p>\n
Still, these tests are not extensive or rigorous; I\u2019d be interested in other results. I\u2019ll also be creating my next Server 2008 VM on a fixed drive and will repeat the test there.<\/p>\n
I\u2019ve posted some further hyper-v tips and gotchas here<\/a>.<\/p>\n The dynamic virtual hard drive is one of the best things about virtualization. It is like Dr Who\u2019s Tardis. The virtualized OS thinks it has plenty of space, while on the host machine your 128GB virtual drive might occupy just 4 or 5GB \u2013 this is typical of the test VMs I set up, running … Continue reading Hyper-V disk I\/O: performance of dynamic vs fixed virtual hard disks<\/span>