1. A method of processing a user request to perform an operation on a user file, comprising: receiving one or more user requests to perform a specified operation on a specified user file; selecting, from among a group of rules, a rule applicable to the specified user file, each rule in the group of rules including at least one parameter specifying, for files to which the rule is applicable, how to distribute storage of each such file over the file servers in a group of file servers; performing the specified operation in accordance with the selected rule, including, when the specified operation changes the size of the specified user file, sending commands to a plurality of the file servers so as to continue to distribute storage of the specified user file in accordance with the selected rule; wherein the rules in the group of rules are ordered and wherein each rule in the group of rules has an associated range of files to which the rule is applicable; and wherein the range of files for a particular rule in the group of rules is specified by files not falling within the range of any higher order rule and having one or more characteristics.