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Storage Foundation vs VMWare and RDM's

Created: 12 Dec 2013 | 3 comments
Marius Gordon's picture

Hi

We need to share a disk between a physical server and a Virtual server.

My Proposed solution - use Storage Foundation to share the RDM disk between the physical and virtual server.

However our VMware team are completely against using RDM's. They say it’s not best practice and can brake VMware.

Is that true. Maybe there is a better way sharing a disk between physical and virtual servers?

Regards

Marius Gordon

Operating Systems:

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Gaurav Sangamnerkar's picture

Hi Marius,

What is the plan to use this disk as ?  as a boot disk or data disk ?

I was browsing for some documentation & found few links ..

From below link (though its a cluster guide but worth to read) page 263

http://sort-cdn.symantec.com/public/documents/sf/5...

VMware provides the ability to create virtual machines that uses a shared raw device for it s boot image. The VMware infrastructure leverages its raw device mapping (RDM) feature to provide this functionality. VCS supports this use of raw devices and raw device mapping in certain high -availability configurations with some limitations. Although VMotion is supported in this configuration, the possibility of data corruption can exist. This can occur when two virtual machines try to access the same shared raw device at the same time on two different physical nodes.

mapping it to VMWare article

https://www.vmware.com/support/ws55/doc/ws_disk_ra...

Note: You should not use a physical disk to share files between host and guest operating systems. It is not safe to make the same partition visible to both host and guest. You can cause data corruption if you do this. To share files between host and guest operating systems, use shared folders

Above gives clue to using shared folders

https://www.vmware.com/support/ws55/doc/ws_running...

I am still looking more around the Storage Foundation sharing, will let you know if I get any success

G

 

 

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Marius Gordon's picture

Thanks for the feedback. 

This will be a data disk only. The basic requirement is to move 3TB data on a daily basis from the physical server to the virtual server. Copying via the network takes too long. Note this is not an active active setup. When the disk is imported on the physical it will be in a deported state on the virtual server. 

mikebounds's picture

http://pubs.vmware.com/vsphere-4-esxi-installable-vcenter/index.jsp?topic=/com.vmware.vsphere.esxi_server_config.doc_41/esx_server_config/raw_device_mapping/c_benefits_of_raw_device_mapping.html  says

An RDM provides a number of benefits, but it should not be used in every situation. In general, virtual disk files are preferable to RDMs for manageability. However, when you need raw devices, you must use the RDM.

 

So as your VMWare team say, virtual disk files are preferable to RDMs, but also as this link says when you need raw devices, you must use the RDM and a phyiscal server can't use a virtual disk file, hence the need for RDM.

The only other solutions I can see would be to use NAS, so present a network share which can be moved between the 2 servers (or share the data from one of the nodes to the other) or use VVR so you have 2 copies of the data that you replicate between your nodes.

VVR is host-base replication over IP so can replicate from VMDK on virtual server to phyical disk on physical server (see https://www-secure.symantec.com/connect/forums/sfhadr-vvr-running-physical-and-virtual-server)

Mike

UK Symantec Consultant in VCS, GCO, SF, VVR, VxAT on Solaris, AIX, HP-ux, Linux & Windows

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