Shared virtual storage area for multiple Virtual Machines using VMWare

I'm using a provider of hosted services who provide a VMWare environment (ESX3.5 and vSphere 4.0) to manage multiple VMs. Datastores are provided via iSCSI. My guest OS is Ubuntu Server 9.10. I wish to have two VMs access the same storage area on the SAN, so that a database can be hosted on the SAN and can be accessed simultaneously from multiple VMs.

Is anyone aware of any pitfalls with this configuration in the VMWare environment? Also, what is the best way to enable this configuration? From one VM, I can go to "Edit VM settings/Hardware/Add/Hard Disk" and select "Create a new virtual disk" (the option for "Raw Device Mappings" is grayed out and unavailable). On a second VM I can select this new disk with "Use an existing disk". From the OS of either VM I can now create a partition on the new disk.

Is this approach likely to cause any issues?
tnglwoodAsked:
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vmwarun - ArunCommented:
You can use RDMs only in case of MSCS (Microsoft Clustering Services) when you need 2 or more Server Nodes to access the disk directly.
You can use RDMs for a single Ubuntu VM by formatting it as a ext3 filesystem but you cannot make 2 Ubuntu VMs share the same RDM disk.
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Paul SolovyovskySenior IT AdvisorCommented:
You will need to add the iscsi lun via storage.  To do this you will need to setup the VMware iscsi initiator and add the the initiator to the target SAN  then discover the LUNs.  Once the LUNs are discovered you should see them in storage and have the capability to add as an RDM.  Make sure the two VMs that are sharing the RDM are in cluster mode.
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tnglwoodAuthor Commented:
Thanks for the responses!

Since I'm using a hosted platform, I don't have any access to how VMWare or its associated storage is configured - I only have freedom to configure the VM hosts. And from the hosts all I can do is "add a new virtual disk"/"use an existing virtual disk". From your responses, I take it that sharing the same virtual disk in this manner is not a good idea?
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vmwarun - ArunCommented:
Why do you want to share the RDM between two VMs ? Are you trying to configure Clustering ?
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tnglwoodAuthor Commented:
Yes that's right...
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vmwarun - ArunCommented:
For a detailed understanding of RDMs, please read from Page 121 to 129 of this PDF
http://www.vmware.com/pdf/vsphere4/r40_u1/vsp_40_u1_esx_server_config.pdf 
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tnglwoodAuthor Commented:
Thanks for the link, that helps a lot. Looks like I do need to use RDM, which the hosting provider hasn't given permissions for. I'll see if they can help.
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