Using iSCSI to get some big ole disk in a Virtual Machine

First, I have lived in the South too long, because I said “Big ole disk” and couldn’t think of a more appropriate phrase. Now someone rescue me if I start to tell you to “mash” the power button on your server or SAN. I kid.

I am sure everyone out there has used this before but I like to document these things just case someone else needs help.

A coworker and I were installing a vSphere environment last week to support some new software for a customer. The software vendor required approximately 30 x 146GB drives in a Raid 5 to store images. Never would guess the software vendor happens to sell SANs too! I exaggerate it actually called for 3TB of usable space.

So my thought was to get over 2TB limit of VMFS we would need to use the MS iSCSI initiator inside the VM. Then my coworker thought we could enable MPIO using two virtual Nics with vmxnet3. We tied each vmxnet3 nic to a separate port group and assigned one of the 2 physical NICs to each port group. Additionally vmxnet3 lets you enable jumbo frames and the physical nics were already set to mtu 9000 because this was on the software iscsi vswitch. So we were able to get multiple paths from the VM to the network and have jumbo frames all the way through.

Next we presented the iSCSI volume of 3TB to the Windows machines. Of course at first it sees it as a couple of smaller volumes. Convert the disk to GPT and align to 64k, then format with NTFS. Just like that a 3TB disk inside a Virtual Machine.


Now we saw IOMETER push better sequential IO than an RDM that was set up for Round Robin, but not quite as good in the Random IO department as a RDM.

The main gain here is to get a file bigger than 2TB minus 512B. Useful for the scan/image servers that store a tons of files for a long time.

To sum up and make it clear.

1. Use the Microsoft iSCSI initiator and MPIO inside the VM when you need more than 2TB on  a disk.

2. Use 2 port groups and bind them to separate physical nics to let the MPIO actually work over 2 nics.

3. With vSphere use the VMXNET3 driver for network to use jumbo frames, the E1000 driver does not support this.

3 thoughts on “Using iSCSI to get some big ole disk in a Virtual Machine”

  1. Hmm….I feel like I’m missing something…..but why not just use an RDM? Less setup? (i.e. just an initiator inside the VM than iSCSI on each ESX host)

  2. In vSphere the RDM creates a pointer file that “claims” to be the size of the RDM. I don’t think it actually uses that space. Even at the highest block size a file over 2TB is not allowed. Unless there is another trick to RDM’s that I didn’t know. All datastores and RDM’s under 2TB we setup the normal way with the Software iSCSI initiator on the host.

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