After what would appear to be months of dragging its feet with the XenSource code, Citrix Systems today marched out the final version of XenServer 4.1.

The last time upgrade to the virtualization package was in August (http://www.theregister.co.uk/2007/08/14/xensource_v4/) - around the time Citrix bought Xensource for $500m.

The upgrade has some 50 features added to the software — although perhaps its most striking change how Citrix calculates the bill. With the release of 4.1, the company is moving away from counting sockets to charging on a per-physical server basis.

This should let customers have an unlimited number of virtual machines or guest operating systems running on a box for the same price, the company said. The new model applies to all editions of XenServer: Standard, Enterprise, (and the upcoming Platinum).

According to Citrix CTO Simon Crosby, the change lightens a burden of having to track the total number of deployed CPU sockets across a data center. And the new pricing structure will generally be easier on the wallet, he says

VMWare, Xen's biggest rival, still charges on a per-processor basis. When Microsoft enters the virtualization scene in August, Windows Server 2008 Data Center edition with Hyper-V is charged per-processor as well. With blades and multi-processor servers becoming regular company in the data center, Citrix's pricing sounds pretty agreeable and may well motivate change for the competition.

Another change in 4.1 comes from a partnership with NetApp, which allows XenServer to use some advanced features of the Data OnTap operating system. The jointly developed solution lets XenSource tap into tools such as data snapshots, cloning, back-up, replication and thin provisioning via the XenCenter management console.

The update aincludes new capabilities such NIC driver updates, support for 10Gb/s networking, and 64-bit enterprise Linux guests.

We dwell on the 4.1 updates a bit more over here (http://www.theregister.co.uk/2008/02/12/citrix_xensource_fourdotone/).

XenServer starts at $600 per server for an annual license, and $900 per server for a perpetual license. Customers using the old pricing structure will need to wait until their current licenses run out before making the change. ®

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