this is by far one of the strangest things I have seen. I have a win 2008R2 cluster with a CSV. the CSV itself is on an iSCSI storage (hitachi HUS 110) basic config of the two hosts in the cluster is

• Dell R610
• Win 2008 R2 with all patches
• 64GB
• 1 NIC for host access
• 2 NICs for guest access
• 2 NICs for iSCSI

these machine work great and I can load a 2008R2 test guest machine on them in less than 90 seconds

after the above config is running for over a year, I now need to add a new host. now the host is

• Dell R620 (Still intel but different CPU)
• Win 2008 R2 with all patches
• 64GB
• 1 NIC for host access
• 2 NICs for guest access
• 2 NICs for iSCSI

I added this new host to the domain and to the cluster, I gave it access to the CSV and I tried loading the same guest machine that loads in 90 seconds in the other hosts. the machine loads in about 6 minutes. no matter how many times I try this the old hosts load the machine in about 90 seconds and this new host in around 6 minutes

to eliminate any problems with the iSCSI connection, I added a new LUN and directly accessed it from the new host and I was working at around 300MB/s so no problem there. I also tested the connection between the other hosts and the new one and network is working fine there too.

to eliminate problems in HyperV, I copied the machine to the local disk of the new host and it loaded in less than 20 seconds.

now is the point were things get a lot stranger: in my tests I tried installing a fresh windows guest machine to the CSV from the new host. I noticed that while the fresh windows was installing, my test guest was loading in less than 90 seconds even on the new host (I repeated this a few times). If I paused the fresh install guest and tried loading the test guest again it loaded in 6 minutes. and again after I resumed the guest installation the test guest loaded fast.

after the fresh windows was also loaded, I ran tests loading the fresh window and my test machine. each one of them loaded in about 5 minutes when I tried loading them separately. however when I started both of them in the same time they both loaded in around 2.5 minutes

it seems that the iSCSI disk access is only working if it is under some load (although I never got to above 10% utilization according to the task manager)

does anyone have any idea what could be the problem?