Hans Vredevoort, a Microsoft Most Valuable Professional with the expertise Clustering wrote an article about fragmentation of a Cluster Shared Volume (CSV).

Cluster Shared Volumes is an enhancement, implemented as a file system filter driver introduced in Windows Server 2008 R2 which allows multiple virtual hard disk (VHD) images to be mounted on a single logical volume, allowing migration of virtual machines from one physical host server to another with minimal downtime, called Live Migration within Hyper-V environments.

 

Hans explains that especially dynamic VHDs are more susceptible to fragmentation because initially only a header and a few control blocks are written to the physical disk. As more data is written to the file that the VHD represents, this data written fragmented on the physical disk. Defragmenting a CSV requires it to be placed in maintenance mode, therefore careful planning is needed.

--> The rest is on the blog : http://virtualization.info/en/news/2011/05/tech-how-to-defrag-a-hyper-v-r2-cluster-shared-volume.html