What are Dynamic Disks?

Dynamic Disks provide a subset of functionality of LVM (Logical Volume Management) disks and offer the user the ability to eliminate some of the limitations of classical basic disk partitioning. 

With Basic Disks, the user is limited to a single physical disk volume that may contain no more than four primary partitions.  

With Dynamic Disks, multiple physical disks can be combined in various ways to extend total disk capacity. This can  provide redundancy via software mirroring and RAID striping, as well as allowing for more than four partitions or ‘Volumes’ to be carved out of the disk space.

Note: Microsoft Windows Dynamic Disks were a legacy mechanism originally provided to help address the limitations of physical storage at a time when data demand was outreaching physical disk media capacity. Except in the specific case of providing a mechanism for software mirroring of the operating system boot volume, Microsoft considers Dynamic Disks to be deprecated.

That said, many partners may still find that they have client systems that are leveraging Dynamic Disks that they need to protect and be able to recover in the event of a disaster.

Why does it matter?

When performing a Bare Metal Restore, the objective is to recover the full disk image(s) of a protected system.  This is accomplished by generating a set of disk images from the backup data and cloning the virtual disks back to the recovery system physical media.

The tools used for disk cloning provide very good support for basic disks and simple disk partitions. Cloning operations can leverage ‘smart’ copy techniques that only transfer ‘used’ data blocks and skip over ‘empty’ space to save time. 

To do this,  the copy utility has to have a deep understanding of the logical disk structure and underlying filesystem properties.

Dynamic disks and LVM volumes are not supported by this ‘Smart’ cloning technique and must be copied blindly.  In this case, since the tools don’t understand the underlying file system structure, we cannot simply skip over empty space and must copy every block of the disk, which can take much longer to complete.

So, what do we do about it?

If you’re recovering back to a physical system using multiple disks that are configured in a spanned or RAID configuration to expand the volume capacity, then there isn’t much choice but to perform the recovery using Smart Recovery to restore the original Dynamic Disk structure, since the data just won’t fit onto the physical media any other way.

However, if you are recovering to new hardware, or (better yet) to a virtual environment, you have some options.

Smart Recovery vs Legacy Recovery mode

When you select to use ‘Smart Recovery’ mode, the system will leverage metadata captured from the original protected system to rebuild a set of virtual disks that exactly match the original protected system volume, including disk UUID identifiers, partition types and layout,   -- and even Dynamic Disk structures.  

This is generally the preferred method of recovery in most cases, since it retains the original disk identity information. Volumes will be recognized by Windows on boot, so that original disk drive letter assignments will be maintained, etc.

However, in the case of disaster recovery via Bare Metal Restore where large Dynamic Disks are present, you may wish to use Legacy recovery mode instead.

When you opt not to use Smart Recovery, the system will return a simple Basic disk for each volume present on the original system.  

So, for example, if you had (5) 900GB physical volumes configured in a RAID-5 Dynamic Disk array, with a 500GB C: volume and a 4TB D: volume, Legacy recovery would return 2 virtual disks; a 500GB C disk and a 4TB D Disk.  With Legacy disk creation, the system will automatically return MBR style disk partitioning for disks less than 2TB and will return GPT style partitioning for larger disks.

Recovering Dynamic Disks as Basic Disks

If you are recovering a protected system that is using Dynamic Disks to a virtual machine rather than physical hardware, you have the option to change the disk structure of the protected system. 

In this case. it is generally beneficial to choose Legacy recovery mode and allow the system to convert the Dynamic Disks back to Basic disks.  Using Basic disks has the advantage of leveraging smart cloning during Bare Metal Restore to speed up the recovery time, as well as simplifies the disk structure of the machine being recovered.

But what if I need my operating system disk to be mirrored for resiliency?

No problem!  Windows allows converting a Basic disk to a Dynamic disk.  You can perform the Bare Metal Restore using Legacy recovery mode, which will return Basic disks for the operating system volume.  Once the system is recovered, boot into Windows, convert the operating system disk to Dynamic Disk mode, then add another volume for mirroring.

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