Gallery

WLANs: liberators or liabilities?

Posted on 16 Jun 2009 at 00:00

A visit to his accountant leads Steve Cassidy to spout the pros and cons of wireless technology.

The other type of fragmentation we have to cope with nowadays is virtual machine fragmentation, which is well known to be a theoretical barrier to performance of virtual servers. However, it takes a lot of time and accidents before any rules of thumb emerge that help in dealing with this problem, as I discovered when one of my virtual server clients rang up with an error message to the effect that their virtual machine wouldn't be starting today on account of excessive fragmentation of the underlying physical disk volume on the host. Now there are plenty of utilities out there - and the odd warning dialog in VMware Console - to tell you that your guest disks are becoming fragmented. The normal operation of a guest machine will scatter files and structures around just like a regular PC, but there's a secondary layer at work too, because all that normal PC activity is happening inside the collection of files which, once the guest OS is shut down, are just themselves files on the host machine's physical disk - you can end up with a fragmented guest file stored inside a fragmented host file. It isn't reasonable to expect anybody's smart disk format or storage daemon to cope with that level of abstraction: you need to schedule some downtime for your virtual hosts to unscramble those virtual drives and get all your free space back into one place.

Don't get the idea that this is some highly technical issue that doesn't affect you - I just ran iDefrag on the Mac Pro that hosts my main Windows XP install as a virtual machine, and its main VM disk store file showed 1,807 fragments in the middle of a 750GB disk. Even running in the high-throughput environment of a Mac Pro, defragging it took nearly three hours, so scale that up to a server installation and - even when you allow for the performance advantages of faster disk controllers and architectures - defragmentation starts to take on a whole new significance.

Download a year of Steve Cassidy's Networks columns by heading to our Free Downloads site