What is the best iSCSI approach for additional disk in the guest ?

[oban]

Hi all,
Taking the following configuration : 1 host connected to a NAS. Proxmox 1.8 running on the host with iSCSI storage (LVM) from the NAS configured. Now, as example, I want to create a guest with 2 disks, that is one system disk and one data disk.

The system disk will be create through the normal procedure by using Proxmox web interface (VM configuration->hardware).
then :

case a) I can create the data disk the same way I have created the system disk.
case b) I can use the guest OS (windows 2008 server in my case) and use the iSCSI initiator tool to add a iSCSI target disk.

In both cases, I have finally 2 local disks, one system disk and one data disk.

Now comes my question: is any valuable difference between a) and b) regarding R/W performance ?

Thanks for your comments.
oban

 

[tom]

never did such a benchmark but I would prefer case a for many reasons.

 

[oban]

To give a first answer to my question, I did some read benchmark on my environment. I created 2 identical Ubuntu 10.10 VMs :

VM1 = 2 disks, first disk where Ubuntu is installed and 2nd disk for bechmark, both created on the same iSCSI storage build from Proxmox web interface
VM2 = 2 disks, first disk identical to VM1 and 2nd disk added after having installed the iSCSI Initiafor on Ubuntu and connected to the iSCSI target.

Next, I choose the "disk utility" tool and its benchmark feature available in Ubuntu 10.10 to test the different cases. I don't want to list all the values because there are relative to the NAS, network and host performance, and will not express anything valuable. But the test demonstrates the following :

The 2nd disk of VM1 gave me almost 3 times the performance compared to the 2nd disk of VM2. Have a look on the attached benchmark pictures.

.

The way I did this benchmark is probably not the best way to do it, I mean, the test should be in R/W, with many applications as usual when doing benchmarks, but for me it shows the essential, that is the ratio between one config and the other.

Best regards
oban