I have a similar issue.
I have a Supermicro server with a 3ware Inc 9650SE SATA-II RAID PCIe with 8x SATA 1TB Drives setup in RAID6 with 2 x onboard 1Gb interfaces and 2 x dual 1Gb interfaces.
eth0: MGMT
eth1: Internet facing interface

Bond0: 172.16.31.200 (eth2-5 bonded in 802.3ad)
eth2: 192.168.1.1
eth3: 192.168.2.1
eth4: 192.168.3.1
eth5: 192.168.4.1

I then have 2 Supermicro 1U servers with 4 interfaces, they are as follows:
eth0: MGMT
eth1: Internet and Dot1q trunking
eth2 & eth3 are setup in a bond and connectet back to iSCSI on IP address 172.16.31.200

I have a linux and windows VM on each XenServer, the windows system running CrystalDiskMark 3.0 gets the following results:
Read:
Seq: 94.95
512K: 89.05
4K: 16.53
4K QD32: 67.09

Write:
Seq: 75.67
512K: 43.64
4K: 3.130
4K QD32: 3.461

Everything runs fine, though these slowish speeds are a bit of a concern. My issue is every now and then I have a problem where sites on the Linux VM's fail to load for 10-15 seconds then they all load fine. During this time if I have a file open via CLI using vi or nano and I try save it, it also waits for 10-15 seconds and then the file saves and I see my bash shell, at the same time all sites start loading.

I know this is not an internet problem as the hosts are all pingable at the time and I can still press enter and see my bash shell new line, it's just read/write seems to hang for 10-20 seconds from time to time.

I read in this thread that some people use something called mpio (never seen or heard of it) and connect to the multiple interfaces IP's rather than a single Bond0 IP address.

As this is a production system in a datacenter I'm reluctant to change too much incase I break it. Can someone with knowledge about iSCSI with opene & xenserver please make suggestions.

Thanks in advance
Barry Murphy