Tester's Notebook: Ready, Get Set, Run

We evaluated XenSource XenEnterprise 3.2, including its storage capabilities. Its lean code base and Windows and Linux paravirtualization tools make XenEnterprise a bargain compared with VMware, but some shops will want to hold out for 4.0.

Joe Hernick, IT Director

July 13, 2007

3 Min Read

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.

  1. Create an igroup

  2. Add nodes to the igroup

  3. Create LUNs (logic unit numbers)

  4. Map the LUNs to the appropriate igroup

  5. Create a unique iSCSI Qualified Name IGN (Microsoft's iSCSI initiator creates one by default, Xen requires you to key one in)

  6. Using the included helper scripts, configure and use the mapped LUN as a storage repository for the XenServer host

  7. Generate and customize XenVMs using the admin console or CLI

  8. With all VMs powered off, discover remote LUNs, create storage repositories and clone/populate LUN with storage repositories. LUN can then be attached to a second XenServer for migration.

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.

Return to the story:
XenSource's XenEnterprise Is A Virtualization Bargain View gallery:
A Virtualization Bargain View the full benchmark chart:
XenSource Performance Numbers

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About the Author(s)

Joe Hernick

IT Director

Joe Hernick is in his seventh year as director of academic technology at Suffield Academy, where he teaches, sits on the Academic Committee, provides faculty training and is a general proponent of information literacy. He was formerly the director of IT and computer studies chair at the Loomis Chaffee School in Windsor, CT, and spent 10 years in the insurance industry as a director and program manager at CIGNA.

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