1. A method for concurrent checkpointing and modifications in a transactional clustered file system (CFS) using a processor device, the method comprising: providing a shadow data segment, whose contents are identical to an original data segment currently being written by a checkpoint operation, for users that require access for modification to data segments concurrently being written within a checkpoint operation; including a transaction journal in each node for storing committed transactions generated by users on that node; scanning a list to identify the location of the latest contents of a data segment prior to the transaction using a rollback procedure; for each modified data segment, identifying the location of the latest contents prior to the transaction using the rollback procedure by: if the data segment was marked as modified in the cache at the time it was inserted into the list, then the latest contents of this data segment appears only in the journal, otherwise, if the data segment was not marked as modified in the cache at the time the data segment was inserted into the list, then the latest contents of this data segment appears in its final location in the shared storage; and recording a type of each data segment in the list during insertion of the data segment into the list, wherein: all data segments in the list whose latest contents appear in a final location are discarded from the cache, for all the other data segments in the list, latest contents for the all the other data segments are restored from the journal into the cache by scanning the local transaction journal from ending to beginning, and first occurrences of these data segments in the local transaction journals are considered, and a modification indication of the data segments is set as true, and exclusive permissions on all the data segments involved in a cancellation transaction are released.