Axllxa
Contributor
Contributor

RDM-LUN mapping

Hi there,

in creating RDM disks I'm asked to store the RDM-LUN mapping files either with the Virtual Machine or in a specific DataStore.

My understanding is that such files are sparse files, meaning they don't eat any space in the filesystem (if the filesystem supports sparse files)

But who designed our system, specifically instruct to store such files locally not on a datastore.

What are you experience with that? I would think to store such files on the NAS, VMFS.

Any comments/suggestions/documents are more than welcomed.

Alex

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4 Replies

Exact, they don't use any real space (you can see they have size, but they don really consume space). I would leave it with the VM - leaving it on local datastore will avoid you to vmotion/migrate the VM if needed.

Marcelo Soares
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Axllxa
Contributor
Contributor

Eventually I was told that RDM files can act as buffer in case the LUN is not available and hence they can be used to afford connections problems.

Of course it is not the solution for connectivity problems but this way if I have to restart the iSCSI service on the NAS VM machines won't be affected.

So far I haven't found any documentation on that...

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a_p_
Leadership
Leadership

Don't search for such a documentation any longer, you won't find any as this is definitely not the case. There's nothing like a buffer for RDMs not virtual disks at all. If the LUNs are not available you are in trouble. Smiley Wink

André

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Axllxa
Contributor
Contributor

Hi there a.p.

well at least I know the true reality thanks Smiley Happy.

Still I haven't found a way to prove that RDM files are sparse files and how to investigate them on ESXi.

Normally I would that as explained here

Creating sparse files on Linux hosts with dd

but ESXi uses busybox and hence you don't have all the full options.

Is there any other way to check that indeed such file are sparse files? And how to access its metadata?

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