When read-only is not read-only

Asked to investigate why large numbers of access times had been updated on a file system, I confidently ruled out an NFS client on the grounds that the file system had been exported read-only on the AIX server. However when I examined the situation more closely, I noticed that reading a file on a client did in fact result in an update of the atime on the server. Something that doesn't appear to happen when serving NFS file systems using Linux.

According to our best hypothesis:

• the server exports the file system read-only
• the client mounts the file system normally
• a program on the client traverses the file system reading each file
• the read triggers the NFS daemon to access the file on the server. It is this action that changes the atime
• the program on the client cannot use utime() to reset the atime because the file system is read-only

The behaviour is odd, but I'm not sure whether it's contrary to the RFC or whether it's just a detail of the AIX implementation of NFS...