How to make use of an iSCSI LUN bigger than 2 TB?

[Massimo1980]

I have a ESXi servers (4.1 U1) connected to an iSCSI NAS, where 4 LUNS have been defined and presented to this host; all the LUNs are 3 TB in size. ESXi correctly sees the LUNs and acknowledges their 3 TB size.

I know the maximum size for a VMFS datastore is 2 TB, so creating a single datastore for each LUN would be a waste of space; I also know that creating multiple datastores on a single LUN is not a best practice, and it doesn't seem to work either: if there already is a datastore on a LUN, the vSphere Client just doesn't let me create anything else there, it doesn't even list it in the list of available devices for creating datastores.

I know I can use extents to create a datastore bigger than 2 TB, but this only seems to work across multiple LUNs: when I try to increase the size of a datastore using the same LUN where it resides, ESXi tries to actually increase it, so it won't go above 2 TB in size.

My question is: is there any way to combine two extents in the same LUN, so effectively creating a 3 TB datastore made up by 2 1.5 TB extents?

If this is not possible, is it possible to create two datastores in the same iSCSI LUN? I know it's not best practice, but it should at least be possilbe... but looks like it isn't.

If even this is not possible, then... how to make use of these 3 TB LUNs?

[mcowger · Accepted solution]

You cannot carve up the LUN and then recombine it.

The way to use these 3TB LUNs is to get rid of them and re-present them as < 2TB luns.

[Dave_Mac]

Second the above.

The only way you could feasibly use some of these 3TB LUNs would be to use an OS based iSCSI initiator for a VM's additional drives with LUNs that are specifically set aside for that purpose, i.e. not VMFS.

You'd still need to present the correct sized storage to the hosts for the O/S disks.

Quite a faff if all you're wanting to do is save having to reprovision the storage.

[LeftHandVSA]

So of course there is the 2TB (minus 512bytes) limit. There are many other things to consider when sizing LUN's: number of VM's per LUN, number of active snapshots per LUN, general I/O characteristics of the VM's. One really sure way to mess up a vSphere implementation is to poorly plan the storage (and networking).

[kishoreg15]

Hi Massimo1980,

It is not possible to divide the 3 TB LUN in to two extents. More technical brief can be obtained here.

Best Regards,

Kishore G.

[Rumple]

I pray to all that is holy that they move to SCSI 3 with esx5 and get rid of this limitation.  Obviously its not good to put a 100 VM's on a single vmfs volume, but there are times where a single large vmfs volume (which will esentially hold a single vmdk - aka file server) would be much more beneficial and standard then using an iscsi lun inside a VM...

We can only hope at this point...

[mcowger]

They wont need SCSI3 to do > 2TB extents, they will need to use a the SCSI READ32/WRITE32 commands.

[AureusStone]

Sounds like your in luck.  Provided the leak was accurate.

http://virtualization.info/en/news/2011/04/%20more-details-about-vsphere-5-appear-online.html

[J1mbo]

Some proper support for SSD vSwap is good to see.  And of course the stupid 2TB limit gone.