1. A method of managing concurrent access among computers to a bitmap stored on disk storage, comprising: receiving a command from a requesting computer of the computers, the command indicating that one or more bits in the bitmap are to be cleared; updating the bitmap as stored on the disk storage responsive to the command to clear the one or more bits in the bitmap; and storing synchronization data associated with the bitmap, the synchronization data configured to distinguish between a current generation and a previous generation of the bitmap, and to specify an action to be performed in response to receiving a subsequent command for accessing the bitmap if the subsequent command identifies the previous generation.