By default, the delta file lives in the same directory as the database itself. The file name is also the same, but with .delta appended. This is usually not a problem, but sometimes it is desirable or even necessary to change the location, e.g. when the database is stored on a raw device. nbackup itself has no provision for setting the location; this must be done through SQL.
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
You can also specify a custom delta location while creating a new database:
create database 'path-and-dbname' difference file 'path-and-deltaname'
Notes: