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Microsoft Windows Dynamic Disks (or Dynamic Volumes) are an abstraction layer (introduced in Windows 2000) over the partitions (similar to the LVM layer on Linux environment) to decoupled the volumes (with their NTFS data) from the disk partitions and have a greater flexibility, like hot-extend a volume by simple add new partitions (but also several other advantages).
But in a virtual environment there are other way to handle disk flexibility, like hot-extend a virtual disk at hypervisor level (for VMware this feature was introduced in VI 3.5U2)… so there is no need to use dynamic disks at guest level. The only requirement is that the guest file system also support a hot-extent operation: this is available from the GUI in recent versions of Windows (from Vista, aka NT 6.0), and in a limited way (not, without use 3rd part tools, for OS disks) from the command line (with the diskpart command) also in some previous versions.
There can still be some reasons to still use dynamic disks also in virtual environment (see also Why dynamic disks are still relevant) but there are several work-around that I think that are better than use dynamic disks. For example to have virtual disk greater that 2 TB you can use a guest iSCSI initiator (or in vSphere 5.0 use physical RDM, but with other limitation due to the physical RDM).
I will give instead some reasons to do avoid this kind of disks:
- the basic to dynamic disk conversion is not reversible (or not in simple way)
- a spanned dynamic disk could be less reliable than a basic disk: if you loose a single “piece” you loose everything
- in a virtual environment you increase the complexity: with basic disks you can keep a simple and clean virtual disk <-> guest volumes approach
- it can mean trouble in backup solutions and plans
For the last point I bring the example of Veeam Backup with dynamic disks (but will probably the same also for other similar products): they are fully supported for backup jobs, but you have several limitation in file level restore.
As written in the Veeam Backup 5.0.2 User Guide (page 97): The File Level Restore wizard provides support for Windows Logical Disk Manager (dynamic disks). Spanned/striped/raid-5 volumes and GPT disks are not supported. And is not the only limitation: as written in the Veeam Backup 5.0.2 Release Notes (page 9) Dynamic disks support is limited. Only first partition on dynamic disk is visible during file level restore. Note that I’ve give the references for the 5.0.2 version, but same limitations are also in the latest 6 version.
The possible workarounds for a file level restore are quite complicated (note that the one suggested does not work, if you choose other OS, the “linux-proxy” machine is not able to mount the filesystems): one could be use an instant restore and power-on on a isolated network or add the vmdk to another VM and then import the dynamic disks.