A request to access to a logical location in a file stored in a content addressable storage (CAS) system can be handled by retrieving first tree data from a first node in a hash tree that represents the file, the first tree data including a first hash tree depth, a first CAS signature, a block size and a file size. Based on the tree data, a second node is selected from a higher level in the hash tree. Second tree data from the second node of the hash tree that represents the file is retrieved, including a second CAS signature. The second CAS signature is determined to match a reserved CAS signature, and in response, an indication that the requested logical location is unallocated within the file is provided.