This issue is now resolved.
Several nodes of our Isilon cluster that support files.brown.edu, nfs.brown.edu and nfsint.brown.edu are experiencing an issue that has caused the nodes to have no free space. This may cause performance issues when connecting to one of the affected nodes. This issue may be intermittent and some users may not experience any issues at all, depending on the node they are connected to. We are working with the vendor to fix the issue.