So, you either just setup iSCSI or are having performance issues with your current iSCSI device. Here are some pointers to ensure "networking" is not the limiting factor:
1. Are my packets even making it to the iSCSI target?
Always check in XenCenter that your NICS responsible for storage are pointing to the correct target IPS. If they are, ensure you can ping these targets from within XenServer's command line:
If you cannot ping the target, that may be the issue.
Use the 'route' command to show if XenServer has a device and target to hit on the iSCSI target's subnet. If route shows nothing related to your iSCSI target IPs or takes a long time to show the target's IP/Route information, revisit your network configuration: working from the iSCSI device config, switch ports, and all the way up to the storage interface defined for your XenServer(s).
Odds are the packets are trying to route out via another interface or there is a cable mismatch/VLAN tag mismatch. Or, at worse, the network cable is bad!
2. Is your network really setup for Jumbo Frames?
If you can ping our iSCSI targets, but Re having performance issues with Jumbo Frames (9000 or 4500 Mtu size, based on vendor) ensure your storage interface on XenServer is configured to leverage this Mtu size.
One can also execute a ping command to see if there is fragmentation or support enabled for the larger MTUs:
ping x.x.x.x -M do -s 8972
This tells XenServer to ping, without fragmenting frames, your iSCSI target with an Mtu of 9000 (the rest comes from the ping and other overhead, so use 8972).
If this return fragments or other errors, check the cabling from XenServer along with the switch settings AND iSCSI setup. Sometimes these attributes can be powered after firmware updates to the iSCSI enabled, managed storage devicd
3. Always make sure your network firmware and drivers are up to date!
And these are but three simple ways to isolate issues with iSCSI connectivity/performance. The rest, well, more to come...