Open File Backup (OFB) is a utility that helps GBM capture files that are open, even if they are changing during the backup, without locking users out of the applications or forcing them to log off the network

Open File Backup monitors the file system for read requests coming from GBM. When the backup operation is initiated, OFB determines a point in time when there are no partial transactions pending on the system. Once it discovers this state, it begins maintaining a dynamically allocated pre-write cache for each open file on the system; this is called Preview Data

From this point on, any file modification goes directly to the intended file - after Open File Backup has placed a copy of the data to be overwritten into the Preview Data. When GBM gets to a part of a file that has been changed during the backup, OFB substitutes the original, pre-write Preview Data to fulfill the backup request

The file on the backup destination will then be identical to what was on the disk when the backup application first began backing up the system. Meanwhile, users of the data have normal read and write access to their files

Open File Backup does not add any noticeable systems overhead. The speed of the backup will not be affected by using OFB. If backup time increases it is because more files are being copied that were previously skipped, which is the main reason for using OFB.