System and method for reallocating blocks in checkpointing bitmap-based file systems

A computerized file system has a first software layer for writing data blocks to persistent storage, and a second software layer for receiving requests from a client, and in response to receiving a request passing a command to the first software layer. A current bit map shows data blocks of persistent storage which are allocated to holding data of the file system. A safe copy of the current bit map is maintained. A third copy of the current bit map is passed, at a selected time, to the first software layer to enable the first software layer to write data blocks containing the bit map current at the selected time to the persistent storage, and simultaneously changing the current bit map as maintained in a computer memory while data blocks containing the bit map current at the selected time are written to the persistent storage.