code: private void backup(final FileObject _backup,
                          final int _number)
          throws FileSystemException
      {
          if (_number < this.numberBackup) {
              final FileObject backFile = this.manager.resolveFile(this.manager.getBaseFile(),
                      this.storeFileName + VFSStoreResource.EXTENSION_BACKUP + _number);
              if (backFile.exists()) {
                  backup(backFile, _number + 1);
              }
              _backup.moveTo(backFile);
          } else {
              _backup.delete();
          }
      }
docs: Method that deletes the oldest backup and moves the others one up.
  
  @param _backup   file to backup
  @param _number         number of backup
  @throws FileSystemException on error