A backup is performed by a client at a time when a backup server is unable to process the backup. The client maintains a cache including a root tag vector and hash entries. The client begins a backup by writing the root tag vector to a journal file and breaking files into pieces. For each piece, the client performs a hash and compares the resulting hash to entries in the cache. If the hash does not match any entries, the client records a request in the journal file to add the corresponding piece of data to an archive. After completing the backup, the journal file can be sent to the server. Before processing the journal file, the server validates the root tag vector. If the root tag vector is valid, the server processes each of the requests to add data. Otherwise, the server discards the journal file.