Microsoft has been quite clear about this. Dynamic and Differential Disks are not supported in production. There is a serious performance hit when you move away from Pass through or Fixed disks. This means that things like snapshots are not supported in production. Snapshots use differential disks. There’s another catch: if a serious amount of time has passed since the snapshot then the differential disk will grow substantially and a merge might take a very long time to process.
We have one lightweight box that I P2V’d a while back using VMM 2008. When I did the conversion the target host I used was my lab server. It has a minimal amount of storage so I used dynamic VHD’s. I’ve since moved the VM onto our production cluster and upgraded the VM OS from W2003 to W2008. It does some very lightweight management work but once a month the amount of data on it bursts. I decided to leave the disks as VHD for a while – mainly because I’ve been snowed under with higher priority technical work for clients and supporting our sales/marketing efforts.
A few weeks ago we had a series of disk performance alerts from OpsMgr for the VM in question. It was performing below par. That was before the OS upgrade. Last night, it had a series of alerts again. Is it a disk subsystem fault? Nope – It’s an EVA SAN with fibre channel. The other VM’s on the host are being actively pushed while the alerting VM is barely hitting host resources at all.
The issue is pretty simple: the dynamic disk is having to expand when new data comes along and this slows down the process. This is clear supporting evidence for Microsoft’s recommendation.
DO NOT USE DYNAMIC DISKS IN PRODUCTION!
I know, it can save you on disk space. But is that really worth it when you have production systems and performance is important? It’s like RAM oversubscription on other virtualisation platforms. In theory it’s great. When managed properly it’s a money saver and performance isn’t hit. But when it’s not managed correctly then you take a huge hit and performance/availability issues arise, e.g. I heard of one hosting company in Ireland that likes to “burst” memory/RAM and they’re having major issues with customer VM uptime/performance issues.
Performance = vendor support = better availability = happy customers/users = your job is a little safer.
That’s enough blogging for now. I’ve some dynamic disks to convert.
Does this restriction also apply to 2008 R2 ?
I know there are improvements in the qay Dynamic discs work
Windows Server 2008 R2 totally changes the scene. Dynamic disks have pretty much the same performance as Fixed VHD’s. I’ve switched to using dyanamic disk in production now. We’re seeing a drop in storage requirements in most situations now without a drop in performance.
Now your decision is: what type of underlying physical storage and SAN do I use.