StartESX
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Contributor

Raw device maping in virtual mode and snapshot

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Hi,

I have some dificulty to hundersand how works Snapshot with RDM. For exemple, I make a RDM with a Lun mapped to 1TB, what happend if I do a snapshot ? ESX create a file in my Lun where is my mapping file ? what is the size of this new file ?

Please, help me to understand this point.

ps : Escuse me for my poor english... Tell me if you don't understand what I've said.

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oschistad
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Enthusiast

You specify this under VM > Edit Settings > Hard Disk N

In the lower right hand pane, under "Compatibility mode", there's a tickbox for Independent mode and persistent or nonpersistent snapshots.

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oschistad
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Enthusiast

When creating an RDM, you get two options - physical and virtual compatibility mode.

If you choose physical compatibility you lose the ability to snapshot that virtual machine (amongst other things), however this is a requirement for clustering between boxes.

In Virtual Compatibility mode you can snapshot, and in the default configuration a snapshot would result in a copy being made of the entire LUN which the RDM points to as well. Since RDMs tend to be used for large disks, this may not necessarily be desirable or even possible, and so VMware have also included the ability to specify that the RDM should be independent from the parent VMs snapshots.

So to answer your question, if you choose virtual compatibility mode and set it to independent snapshots, you only snapshot the VM itself and not its data disk when you trigger a snap. If you do not set it to independent snapshots, your ESX would try (and probably fail) to create a new VMDK of 1TB inside the same folder as the RDM.

Hope this clears things up a bit.

StartESX
Contributor
Contributor

thanks for your answer. Whare I can specified "independent snapshot" ?

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StartESX

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oschistad
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Enthusiast

You specify this under VM > Edit Settings > Hard Disk N

In the lower right hand pane, under "Compatibility mode", there's a tickbox for Independent mode and persistent or nonpersistent snapshots.

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