Jingyi;

I believe you can use a snapshot of a RDM LUN.  However, only one VM can be on that LUN.

The Replication Manager Admin Guide, section on  Replicating an RDM or iSCSI initiator LUNs on Clariion or VNX section on page 39, discusses that RM will allow for the application to be restarted on a snapshot.  An excerpt from the section:

When you create a replica that uses CLARiiON snapshots in a VMware environment, Replication Manager creates a snapshot session. The snapshot session is a point-in-time copy of the source LUN.

Note: The EMC Knowledgebase article emc184439 offers detailed information on how to perform these steps in a VMware environment.

There is a good discussion on RDM vs VMDK as it relates to virtualizing Oracle in the "Everything Oracle at EMC > Discussion area.  Look for  What are the most common storage options for virtualized Oracle?

Darryl breaks down  'There are many pros and cons for the RDM vs. VMDK comparison.'

RDMs will outperform VMDK, slightly.

RDMs allow for array based replication. (snapshots, clones, proxy backup and RDF/replication manager)

VMDKs have better SRM support

VMDKs allow for storage vMotion.

VMDKs allow for DRS with Oracle RAC.

Have you looked into Replication Manager?  The Replication Manager Admin Guide, section on  Replicating an RDM or iSCSI initiator LUNs on Clariion or VNX section on page 39, discusses that RM will allow for the application to be restarted on a snapshot.  An excerpt from the section:

When you create a replica that uses CLARiiON snapshots in a VMware environment, Replication Manager creates a snapshot session. The snapshot session is a point-in-time copy of the source LUN.

Note: The EMC Knowledgebase article emc184439 offers detailed information on how to perform these steps in a VMware environment.

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VMware's Storage (vSphere 5.0) Guide has a section on snapshotting and replication including RDM LUNS :

Layered Applications

SAN administrators customarily use specialized array-based software for backup, disaster recovery, data

mining, forensics, and configuration testing.

Storage providers typically supply two types of advanced services for their LUNs: snapshotting and replication.

• Snapshotting creates space with efficient copies of LUNs that share common blocks of data. In general,

snapshotting is used locally on the same storage systems as the primary LUN for quick backups,

application testing, forensics, or data mining.

• Replication creates full copies of LUNs. Replicas are usually made to separate storage systems, possibly

separate sites to protect against major outages that incapacitate or destroy an entire array or site.

When you use an ESXi system in conjunction with a SAN, you must decide whether array-based or host-based

tools are more suitable for your particular situation.

Array-Based (Third-Party) Solution

When you use an ESXi system in conjunction with a SAN, you must decide whether array-based tools are more

suitable for your particular situation.

When you consider an array-based solution, keep in mind the following points:

• Array-based solutions usually result in more comprehensive statistics. With RDMs, data always takes the

same path, which results in easier performance management.

• Security is more transparent to the storage administrator when you use an RDM and an array-based

solution because with RDMs, virtual machines more closely resemble physical machines.

• If you use an array-based solution, physical compatibility RDMs are often used for the storage of virtual
• machines. If you do not intend to use RDMs, check the storage vendor documentation to see if operations

on LUNs with VMFS volumes are supported. If you use array operations on VMFS LUNs, carefully read

the section on resignaturing.

File-Based (VMFS) Solution

When you use an ESXi system in conjunction with a SAN, you must decide whether file-based tools are more

suitable for your particular situation.

When you consider a file-based solution that uses VMware tools and VMFS instead of the array tools, be aware

of the following points:

• Using VMware tools and VMFS is better for provisioning. One large LUN is allocated and multiple .vmdk files can be placed on that LUN. With an RDM, a new LUN is required for each virtual machine.   
• Snapshotting is included with your ESXi host at no extra cost.
• Using VMFS is easier for ESXi administrators.
• ESXi administrators who use the file-based solution are more independent from the SAN administrator.

Nick