Information Technology Reference
In-Depth Information
it means physical compatibility mode RDM. You might also see this referred to as a pass-
through disk.
Virtual Mode (vRDM)
In this mode, there is still a mapping i le, but it enables more (not
all) features that are supported with normal VMDKs. Generally, when VMware says “RDM”
without specifying further, it means a virtual mode RDM.
Contrary to common misconception, both modes support almost all vSphere advanced func-
tions such as vSphere HA and vMotion, but there is one important difference: virtual mode
RDMs can be included in a vSphere snapshot, while physical mode RDMs cannot. This inability
to take a native vSphere snapshot of a pRDM also means that features that depend on snapshots
don't work with pRDMs. In addition, a virtual mode RDM can go from virtual mode RDM to a
virtual disk via Storage vMotion, but a physical mode RDM cannot.
Physical or Virtual? Be Sure to Ask!
When a feature specifi es RDM as an option, make sure to check the type: physical compatibility
mode or virtual mode.
The most common use case for RDMs are VMs coni gured as Microsoft Windows clusters. In
Windows Server 2008, this is called Windows Failover Clusters (WFC), and in Windows Server
2003, this is called Microsoft Cluster Services (MSCS). In Chapter 7, the section “Introducing
Windows Server Failover Clustering” provides full details on how to use RDMs with Windows
Server-based clusters.
Another important use case of pRDMs is that they can be presented from a VM to a physical
host interchangeably. This gives pRDMs a l exibility that isn't found with virtual mode RDMs or
virtual disks. This l exibility is especially useful in cases where an independent software vendor
(ISV) hasn't yet embraced virtualization and indicates that virtual coni gurations are not sup-
ported. In this instance, the RDMs can easily be moved to a physical host to reproduce the issue
on a physical machine. For example, this is useful in Oracle on vSphere use cases.
In a small set of use cases, storage vendor features and functions depend on the guest
directly accessing the LUN and therefore need pRDMs. For example, certain arrays, such as
EMC Symmetrix, use in-band communication for management to isolate management from the
IP network. This means the management trafi c is communicated via the block protocol (most
commonly Fibre Channel). In these cases, EMC gatekeeper LUNs are used for host-array com-
munication and, if they are used in a VM (commonly where EMC Solutions Enabler is used),
require pRDMs.
Finally, another example of storage features that are associated with RDMs are those related
to storage array features such as application-integrated snapshot tools. These are applications
that integrate with Microsoft Exchange, SQL Server, SharePoint, Oracle, and other applications
to handle recovery modes and actions. Examples include EMC's Replication Manager, NetApp's
SnapManager family, and Dell/EqualLogic's Auto Volume Replicator tools. Previous generations
of these tools required the use of RDMs, but most of the vendors now can manage these without
the use of RDMs and integrate with vCenter Server APIs. Check with your array vendor for the
latest details.
In Chapter 7, we show you how to create an RDM, and we briel y discuss RDMs in Chapter 9.
We're now ready to shift away from block-based storage in a vSphere environment and move
into a discussion of working with NAS/NFS datastores.
