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pass though lun vs vodka file

sara2000 asked
i am trying to find out the advantages of using pass-through lun for a vm instead of using vmdk file.
This is what i noticed in my client who has three esxi hosts which are attached to netapp and a san device.
The vm are using its own iscsi initiator to connect to lun in the storages. That is , the vm server has either ms iscsi initiator or sanpdrive and connecting the lun.
 I want to know what the advantage of using this instead of using a vmdk file which support more than 2TB?
The esxi are ver 5.1.
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Paul SolovyovskySenior IT Advisor
Top Expert 2008
There are several reasons why you would use a MS iSCSI initiator or RDM versus a VMDK files, especially on a Netapp.

For instance if a customer is using SnapManager for Exchange it needs to see the physical properties on the Netapp LUN and therefore needs an direct access to the LUN whether it be iscsi via MS initiator or RDM. In 5.1 you still the the 2TB-512b limit on RDMs and VMDK volumes.  

With Snapmamager for sql you can use VMDK or ms iscsi/rdm as well but using vmdk volumes requires some configuration decisions to be made (one vmdk per volume due to Netapp snapshots)
Virtualization Expert, Sr B&R, Storage Specialist

Paulsolov describe it well, especially with Netapp storage, I may add some general rules.
RDM or direct iSCSI to the vm will be always little faster than vmdk files when used with high random iops traffic because it lack of one more filesystem layer (VMFS), it also allow vm to see and do almost all scsi commands on the lun which is importand and sometimes required with some low level apps like Clustering and storage snapshot management/sync.
Additional advantage is ease of moving one vm on rdm/iscsi to another environment only by redirecting a lun but not moving a data to other datastore (if storage is the same of course) or just cloning/backing up vm on a storage itself without vmware interactions.
Last advantage I know is better security management of single vm on rdm/iscsi lun than on vmdk datastore (it's just safer), for example vmware administrator that have root/administrator privilige may delete single vm or them vmdk files for accident but he will not do it so easly with rdm and especially iscsi lun (sure it's still possible but happens much more rare than deleting vmdk files), also there is a possibility to delete (from esxi/vsphere console) a lot of vmdk files at just one click ;((, but it's not possible to do that with RDM luns and especially with propertly secured iSCSI luns.

Kind regards


Thank you both for the good explanation .
Dawid FusekVirtualization Expert, Sr B&R, Storage Specialist

Glad we can help mate.