Does the Off-Host Backup methods (like HDS ShadowImage, EMC BCV, etc) work within a VM?

[jaikrit]

HI Experts,

Does the Off-Host Backup methods (like HDS ShadowImage, EMC BCV, etc) work within a Virtual Machine (if the volume to be backed up is created on RDM)?

This means that each such VM that need to have off-host backup using ShadowImage have their dedicated LUN (using RDM) and not on the VMFS volumes.

Any suggestions greatly appreciated.

Jaikrit Negi

(VCP, NCDA, BCFP, SCSA)

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[mcowger]

It does if you are using physicalmode - shadowimage/bcvs whatever dont care, they just copy the LUN.

--Matt

[kjb007]

The thing to remember is that when you use an array-based snapshot, you get a crash-consistent "backup". Since the filesystem is not quiesced, not everything is written to disk. For data drives this is ok, and it should be fine in physical or virtual mode, because the data is RAW on the san.

-KjB

[jaikrit]

Hi Matt and KjB,

It seems that atleast off-host backups work wit RDM's but yet not sure whether phsycial or virtual mode.

Does anyone can point to VMware whitepaper or any such document by Storage Vendor to confirm?

Jaikrit Negi

(VCP, NCDA, BCFP, SCSA)

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[mcowger]

The problem with a virtual mode RDM is if you start a (vmware) snapshot on the RDM, the updates will not get written to the RDM, but to the sidefile.

--Matt

[jaikrit]

Thank you Matt. Do you have any document or URL that I can refer to?

Jaikrit Negi

(VCP, NCDA, BCFP, SCSA)

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[titaniumlegs · Accepted solution]

I'm not sure that's really a "problem". If you're taking a storage-side snapshot (BCV, NetApp snapshot, whatever) you're after a point-in-time backup. The fact that virtual mode RDM changes go do a delta file while the VM snapshot exists doesn't make much difference as far as what's in the storage snapshot - except that it's at least guest FS consistent (app consistent is another discussion). You still have a point in time snapshot, and it's still the data that existed when you triggered the process.

So, with virtual mode, you can use VMware snapshots (with the flush option as long as VMware tools are installed), and get a guest-FS consistent snapshot.

With physical mode RDM, it's up to tools, usually from the storage vendor, in the guest to flush/quiesce. One example of that is NetApp SnapDrive. SnapDrive uses the same win32 calls to flush NTFS. You can put SnapManager for Exchange, SQL, SharePoint on top of that, and now you're app-consistent as well.

You can do storage snapshots of VMFS or NFS datastores too. You would need a script or tool to flush the VMs. There's an example of a very basic script in NetApp's TR3428 (), or if you actually have NetApp, you could use SMVI (). These scripts don't flush apps by themselves, but depending on the app, you can embed pre/post scripts in the VM.

Enjoy!