Setting the delta file

By default, the delta file lives in the same directory as the database itself. It also has the same name as the database file, but with .delta appended. There is usually no reason to change this, but it can be done if need be – though not via nbackup itself. Make a connection to the database with any client that allows you to enter your own SQL statements and give the command:

alter database add difference file 'path-and-filename'

The custom delta file specification is persistent in the database; it is stored in the system table RDB$FILES. To revert to the default behaviour, issue the following statement:

alter database drop difference file

Notes

If you specify a bare file name with ADD DIFFERENCE FILE, the delta will likely not be created in the same directory as the database, but in the current directory as seen from the server. On Windows this may e.g. be the system directory. The same logic applies to relative paths.
The entire directory path must already exist. Firebird doesn't attempt to create any missing directories.
If you want to change your custom delta specification, you must first DROP the old one and then ADD the new one.