Dynamic Disks

Why we need to use Dynamic Disks with Windows Server Failover Clustering:

Are Dynamic Disks supported on Failover Clusters?

Yes, they are, however support is not provided natively in-box in Windows for Failover Clusters. It requires an add-on product from Symantec called Storage Foundation for Windows to enable support of Dynamic Disks on Windows Server Failover Clusters. This is also true for the recently released Windows Server 2008 R2. You can learn more about the Storage Foundation for Windows product here:

This KB article also discusses support for Dynamic Disks on Windows Server Failover Clusters: http://support.microsoft.com/kb/237853

Now let me ask you…why do you want to use Dynamic Disks?

Dynamic Disks do provide a number of different features, so we like to understand why you use them.

commonly hear two answers when I ask this question:

· “I need to have large partitions”

· “I need to be able to dynamically grow partitions”

Well, did you know that you actually don’t need Dynamic Disks to accomplish those?

Large Partitions

While Basic disks that use MBR partition table only support 2 TB partitions, GUID partition table (GPT) disks enable partitions that are greater than 2 TB and are fully supported on Failover Clusters, using Windows Server 2008 and 2008 R2. If you happen to still be using Windows Server 2003, you can add support for GPT based disks with a post Service Pack 2 hotfix, available at: http://support.microsoft.com/kb/919117.

If you want to learn more about the advantages of GPT disks, here is a good FAQ: http://www.microsoft.com/whdc/device/storage/GPT_FAQ.mspx .

So, you can create large volumes with Basic disks, and there is no need for Dynamic Disks.

Dynamically Growing Partitions

With Windows Server 2003, 2008 and 2008 R2 you can dynamically increase the size of a partition on a Failover Cluster. In Windows Server 2003 this needed to be done via the command line with DiskPart, as described in this KB article: http://support.microsoft.com/kb/304736.

In Windows Server 2008 and 2008 R2, there is a simple right-click option in the Disk Management (DiskMgmt.msc) snap-in to “Extend Volume”. Another new option in Windows Server 2008 R2 is that you can now not only extend a volume, but you can also “Shrink Volume”.

So, you can dynamically grow or shrink volumes with Basic disks, no need for Dynamic Disks.

So, is there really a need for Dynamic Disks?

One benefit of dynamic disks in a cluster out of the box still exists though: You'd be able to migrate disks from one storage array to another transparently to the application and without interruption of the service.

Right now you need third party tools for that.

There are fewer reasons why you might need Dynamic Disks these days, since much of this functionality is now possible with Basic disks. What are the reasons why you might actually need them? That is fair to discuss as well and I commonly hear two answers:

· “I want to use Software RAID”

· “I need to be able to span a single volume over multiple LUNs”

Software RAID & Spanning Volumes

With Failover Clustering it requires external storage (Fibre Channel, iSCSI or SAS), so most customers choose to go with the Hardware RAID they already have in the storage array instead of using Software RAID. With spanning volumes, that really is a matter of how you do your SAN management when increasing capacity. Most storage arrays these days support dynamically expanding the size of a LUN. As I said earlier, with Basic disks you can dynamically increase the size of that volume to match the new larger LUNs. However some people prefer to concatenate LUNs and span a single volume over multiple LUNs, then they just create a new LUN and span the volume over that new LUN when they want to add capacity. For IT departments that are segmented, I sometimes hear this is ‘easier’ for SAN admins to just create a new LUN, opposed to tracking down the right LUN and expanding it. I have no right or wrong answer for you here, it’s a matter of how you manage your SAN’s.

I hope this helps in understanding that Dynamic Disks are supported with Windows Server Failover Clustering with the add-on product from Symantec Storage Foundations for Windows. Microsoft has continually strived to build functionality into Windows that provides ease of use and convenience for server administrators. This is the case with the ability to dynamically expand and shrink volumes and create large volumes using GPT formatted disks. As mentioned, using Dynamic Disks do provide a great add-on product that extends the functionality of Failover Clustering and I hope the above information allows you to make informed choices on what is right for you and your customers.

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