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SCSI Bus Sharing vs RDM

Posted on 2014-11-22
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Why studying for my VCP 5.5 it is making take a more detailed look at items to ensure I am understanding them on of these items is the difference between SCSI Busing Sharing and RDM. Are they the same thing or different?
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Question by:compdigit44
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ID: 40459878
They are very different.

This guide on Failover Clustering setup can be helpful

Setup for Failover Clustering and Microsoft Cluster Service

SCSI Bus Sharing (virtual) is a function in VMware vSphere which allows two virtual machines to share a single virtual machine disk.

It's similar in the real world to a directly connected storage to two physical machines using a SAS controller.

SCSI Bus Sharing can be used to create a "Cluster in a Box" but if you setup a Cluster in a Box, there are some caveats, both virtual machines must be on the same host, you cannot vMotion or use the VM in DRS.

So you are limited to a Cluster in a Box of machines to a single host.

See Clustering support here

Microsoft Clustering on VMware vSphere: Guidelines for supported configurations (1037959)

RDM or RAW Mappings are when your present a LUN (Fibre Channel or iSCSI LUN) directly to a VM.

see here

http://www.vmadmin.co.uk/resources/35-esxserver/58-rdmvm

RDM are useful and the supported method for Microsoft Failover Clustering.

They were very common, when high performance was required on VMs, but VMFS and VMDKs have caught up today.

RDM versus VMDK performance

Conclusion: VMFS and RDM have similar performance. Don’t choose RDM for performance.
Source:http://www.vfrank.org/2011/03/22/performance-rdm-vs-vmfs/

and to top off this response, here is a real world question on EE, about Best Practice SQL Cluster, which the Asker, (we) eventually find is setup as Cluster in a Box, e.g. SCSI bus sharing...

http://www.experts-exchange.com/Database/MS-SQL-Server/Q_28554874.html#a40446430

and I explain the disadvantages in real life...

and he's now going to convert this to RDM based Cluster. (Supported by Microsoft!)
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by:compdigit44
ID: 40460665
Thank you for the great reply Hancock...

So SCSI Bus sharing is "how" the VM access the storage and RDM is how the storage is presented to the VM correct.

When would you use Bus Sharing in physical mode?
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Andrew Hancock (VMware vExpert / EE MVE) earned 500 total points
ID: 40460726
SCSI Bus Sharing is a policy which defines how a virtual disk (VMDK) is to be used, between virtual machines.

Physical mode, allows access to virtual machine disk (VMDK) between virtual machines on different hosts.

A Raw Device Mapping gives your virtual machine direct access to the SAN. This uses existing SAN commands to manage the storage,
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by:compdigit44
ID: 40462090
Thanks!!!
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