In-Guest iSCSI vMotion Support

[ukDC201110141]

Hi, we're using in-guest iSCSI using Microsoft iSCSI Initiator to host our Exchange 2010 volumes. We just noticed that snapshots are not supported, can anyone advise if vMotion is supported with this configuration and does anyone know if snapshots will be supported in ESX5?

[rickardnobel · Accepted solution]

Hi, we're using in-guest iSCSI using Microsoft iSCSI Initiator to host our Exchange 2010 volumes. We just noticed that snapshots are not supported, can anyone advise if vMotion is supported with this configuration and does anyone know if snapshots will be supported in ESX5?

vMotion is supported, but you will have to verify for yourself that the iSCSI target is reachable from all ESXi hosts. That is, with all other virtual disks, both *.vmdk and with Raw Device Mappings the Vmkernel knows it is a disk that the VM needs and will check and verify that it is accessable from the other host.

When you do internal iSCSI inside the guest then this is just network frames from the Vmkernel point of view. It has no knowledge what the VM is sending and why.

This is also the reason why you can not take VMware Snapshots on internal iSCSI disks. The disks are not "visible" at all from the ESXi host and can not use any method of snapshoting those.

[ukDC201110141]

Thanks, that's really useful. Do you have any recommendations between ESX RDM vs in-guest iSCSI initiators? We went with in-guest for simplicity and it seems to perform well enough. 

[rickardnobel]

ukDC201110141 wrote:

Thanks, that's really useful. Do you have any recommendations between ESX RDM vs in-guest iSCSI initiators? We went with in-guest for simplicity and it seems to perform well enough. 

I do prefer to use RDM since it is more in line with the whole vSphere thought, that the VM should not have to care about anything but using seemingly local disks and all remote disk access is taken care of by the Vmkernel. It is also a bit more secure, since no VMs has to directly access the iSCSI network. When using internal VM iSCSI there is a (small) risk that something goes wrong inside a VM and, with or without intention, destroys the iSCSI network.

In vSphere 4.x, which I understand you are using, one reason not to use RDMs was if the VM needed disks larger than 2 TB. In ESXi 5 you could create 2 TB+ LUNs and attach those to a VM as a RDM disk. However, this large disks do not support VMware Snapshots either, if this was a need for you.

[ukDC201110141]

And if you have an app that says it requires iSCSI to be supported (Exchange) then presenting it as an RDM still fulfuls this requirement in the same way that in-guest does?

[rickardnobel]

ukDC201110141 wrote:

And if you have an app that says it requires iSCSI to be supported (Exchange) then presenting it as an RDM still fulfuls this requirement in the same way that in-guest does?

I am not familiar with the specific requirements of Exchange, but does it really says it needs iSCSI to be supported?

When using the RDM it will look like an ordinary local SCSI disk to the VM and all disk IO will be taken care of by the VMkernel, even if it would be through iSCSI or Fibre Channel or even FCoE. If an application "requires" a specific protocol to reach a disk than it might not be supported from the app side, even if it really is iSCSI - but invisible from the VM.

[ukDC201110141]

Sorry, I didn't quite explain that properly. Microsoft say that the data has to be on a certain type of block level storage like iSCSI rather than NFS. The specifically say iSCSI is supported so the question should have been that assuming we went with iSCSI through the host it would still count as an iSCSI drive? Hope that makes more sense. Maybe another way of looking at it - it it just the same as if presenting it in the guest.

[rickardnobel]

ukDC201110141 wrote:

Microsoft say that the data has to be on a certain type of block level storage like iSCSI rather than NFS. The specifically say iSCSI is supported so the question should have been that assuming we went with iSCSI through the host it would still count as an iSCSI drive?

If the Exchange server just needs some kind of block level storage then all VMDKs would be good, since it for the VM looks like a local SCSI block device. You should be able to use both ordinary VMDK files (through iSCSI from the host) or RDMs if that makes sense. If it not really is necessary it could make the environment more "clean" to let the ESX host deliver all disk IO.

I found a pdf from VMware about setting up Exchange 2010 and it says really nothing on any need for just iSCSI. It could be worth a read:

http://www.vmware.com/files/pdf/Exchange_2010_on_VMware_-_Best_Practices_Guide.pdf

[ukDC201110141]

It seems to be down to MS, despite VMWare supporting! There is a post here http://social.technet.microsoft.com/Forums/en-US/exchange2010/thread/1b3e88d3-b4e9-455e-8fef-bbad929...

Anyway you've more than answered the original question, thanks a lot for your help.

[rickardnobel]

The link to the MS forum seems to discussing using VMDKs on top of NFS and this is not supported from VMware either (for Exchange, according to the pdf above). It would be interesting to see a Microsoft KB which states this more clearly.

Here is another article from VMware discussing the specific things we have talked above, so this is only the view from VMware:

http://blogs.vmware.com/apps/2011/11/virtualized-exchange-storage-vmdk-or-rdm-or.html

[vGuy]

I was looking on this topic as well some time ago and if I remember correctly VMware too does not support MSCS VM's on iSCSI Datastores...

and the cluster service did not even initialize when the quorum and the shared disks were on NFS datastores.

[rickardnobel]

Obaid wrote:

I was looking on this topic as well some time ago and if I remember correctly VMware too does not support MSCS VM's on iSCSI Datastores...

From what we have seen in this thread it was just Exchange 2010 and no Microsoft Failover Cluster Services involved. If so, it might be other requirements.

[ukDC201110141]

I've asked in the Exchange section - more details can be found here -  http://communities.vmware.com/message/2066544#2066544

[vGuy]

my bad for overlooking...apologies

i guess multi-tasking is not working for me at the end of the day..