Tester's Notebook: Ready, Get Set, Run

Virtualization And Storage: The SAN Conundrum
XenSource offers basic support for iSCSI and Fibre Channel storage area networks. XenEnterprise contains the open-iSCSI software initiator, used to open a session to the "target." The initiator creates a virtual device node on the XenEnterprise host and redirects all I/O requests over the TCP connection to the SAN or SCSI drive -- they appear like a local hard drive to the Unix subsystem.

You'll need to be very comfortable with Unix storage concepts, the CLI, and Xen's VM migration tools to get up and running with a SAN. Xen-Source is promising a form-based GUI to assist with iSCSI configuration in 4.0. After our experience in 3.2, we hope it delivers.

Until then, here's how to connect XenEnterprise to an iSCSI resource. Remember: This is not for the faint of heart.

After a couple days spent configuring and troubleshooting XenEnterprise to mount our iSCSI array, create a storage repository and migrate our XenVMs, we reran our tests and generated disk performance scores roughly 12% better for our single XenVM test and 18% better for our "fully loaded" test versus the on-board RAID array in our HP test bed, which was connected directly via the server's standard Gigabit Ethernet port. We can assume that the inherent performance benefits of the Equal-Logic NAS contributed directly to our results; faster drives, more disks in the array, and more robust drive controllers provided the uptick in performance. No wonder: EqualLogic's PS3800XV is an enterprise-class, sixteen-drive array that can be configured as RAID 5, 10 or 50 with up to 2.3 terabytes of raw storage at hand. Our loaner came fully loaded with 15K RPM 146-Gbyte SAS drives.

Yeah, we hated sending it back.

We did not use an iSCSI initiator within our virtualized Windows 2003 guest OS, though it is possible to mount a dedicated storage resource to a particular guest. All iSCSI requests from within that XenVM would need to traverse the virtualized I/O environment starting in the guest OS and progressing down through the hypervisor to reach the NIC providing access to the iSCSI resource. While this is a workaround past the current single repository limitation, our browsing of the Xen support boards indicated the "double I/O translation hit" reduced any performance benefit yielded from a dedicated storage resource for the guest. It is viable if simple storage is required, but not advisable as a high-transaction solution.

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XenSource Performance Numbers