Snapshots of VMs with multiple disks on different LUNs - Question

[pegasus20111014]

I have a few questions on how to handle taking snapshots of VMs that have multiple disks on different LUNs.  First off, the LUNS are VMFS.  We are just beginning to add some VMs with dedicated LUNs for data storage formatted as VMFS.  I'm thinking that additional space should be calculated in for snapshots, so that if a 100GB LUN is needed for data storage, then say, add about 25 to 50% to that amount?  Set the vmdk file for 100GB but have the extra 25 to 50GB of disk space for snapshots if needed (instead of just getting a 100GB lun needed for data and dedicating the entire 100GB to the vmdk file).  Also, if a VM has multiple vmdk files on different LUNs, will that pose a problem taking a snapshot, and is it possible to just do a snapshot of one disk of a VM (like the OS volume)?

Thanks for any assistance.

[a_p_ · Accepted solution]

Snapshot files are created in the VM's base folder by default for all virtual disks, no matter on which datastore they are. You need to make sure you have enough free disk space on this datastore for the snapshots.

If you want to exclude a virtual disks from snapshotting, you can set it to "Independent" - "Persistent" in the VM's settings for the HDD.

André

[pegasus20111014]

Ok, thanks a.p.  The  VMs base folder will be on our shared datastore and that is where we place the vmdks for the OS as well.  The shared datastore we will keep enough space for snapshots, so if its all stored there in the base folder, we are ok.

[a_p_]

Just one more point to remember. If you are planning to create virtual disks larger than 256GB  (i.e. use a datastore with a block size of 2, 4 or 8 MB) make sure to setup the datatstore for the base folder with a least the same block size. Otherwise you will not be able to create snapshots for this VM! Also take a look at the table at the bottom of http://kb.vmware.com/kb/1012384 for the max. virtual disk sizes allowed for virtual disks with snapshots.

André

[pegasus20111014]

Is there a reason why you shouldn't just always set the block size for the datastore to 8MB?

[DSTAVERT]

I would always use the 8MB block size. There is no reason to do otherwise.

[a_p_]

Not really, there shouldn't be any isues with the larger block size. However, I like to keep the block size the smallest one needed.

André

[DSTAVERT]

http://www.yellow-bricks.com/2009/03/24/an-8mb-vmfs-blocksize-doesnt-increase-performance/ There are several good links in there if it helps.