Sun Microsystems is preparing to file a patent suit against storage vendor Network Appliance (NetApp), chief executive Jonathan Schwartz revealed on his blog.

The suit is a response to a legal complaint that NetApp filed in early September over the alleged infringement of seven NetApp patents by Sun's ZFS file system. The NetApp suit alleges that ZSF is essentially a reimplementation of its WAFL file system.

In addition to monetary damages, Sun plans to ask for a permanent injunction that prevents the NetApp from further selling its so-called filer products, storage systems for high performance, mission critical storage.

Sun plans to donate half of any damages that it will be awarded to free software charities. The executive named the Software Freedom Law Center and the Peer to Patent initiative. He also suggested setting up a venture fund that promotes free software innovation.

Schwartz also said that it would re-examine NetApp's licence for the Network File System, a technology that Sun developed in 1984 and that Netapp has licensed.

A file system determines how data is structured and stored on a hard drive or series of hard drives. Sun has released its ZFS technology under the Common Development and Distribution License (CDDL), an open source licence. It is currently used by Solaris and will be implemented in Apple's upcoming OX 10.5 Leopard operating system.

In his blog posting, Schwartz accuses NetApp of attempting to "stop the competitive impact of ZFS on their business". He claims that the storage vendor is asking Sun to 'unfree' ZFS and require that Sun limits the technology to the field of computing, staying clear of storage devices.

Schwartz turned down both requests and instead decided to file a counterclaim. Sun owns a patent portfolio of about 14,000 patents spanning storage, computing and network technologies.

NetApp did not return a request for comment.

When the firm filed its patent suit back in September, NetApp's founder and executive vice president claimed that Sun started the row by demanding $37m in royalty payments for its patents and published an email conversation (PDF) to back up its claim. The subsequent suit was a mere response to Sun's legal threats, he said.

Schwartz states in his blog posting that Sun only uses its patent portfolio defensively, allowing it to file counterclaims against aggressors. In its email message, a Sun lawyer does ask for royalty payments as part of a cross licensing agreement, but does not threaten with any legal action.

At least one of the disputed patents was allegedly filed by StorageTek, a company that Sun acquired in June 2005. The firm was first notified about NetApp's infringement of the patent NetApp approached the company with a request to purchase the patent. StorageTek declined the offer and instead started licensing negotiations that ultimately led to the current patent suits.