One or more secondary files may be specified by the database creator, to be used for database storage once the primary file has reached its specified limit. The database can be distributed across as many secondary files (Firebird 2.1 allows you to create and manage up to 65,000 secondary files) as wished.
Usually Firebird/InterBase® databases grow dynamically, when database objects, program code or data are added. The only limitations are the physical limits of the hard disk or file system on which the database is stored.
Some file systems such as, for example, HP UNIX have additional limitations which do not enable the partition size to go over two Gigabytes. To avoid such a limitation, the Firebird/InterBase® database can be spanned across multiple file systems. Each file can be assigned a maximum size. Due to the automatic administration in Firebird/InterBase®, the primary file is first filled until the maximum page size has been reached. Subsequent information is then packed into the secondary files until their capacity has been reached.
Since InterBase® 6.5/Firebird secondary files are really no longer necessary. In those particular cases, where secondary files may need to be considered, please consult the respective database Release Notes.
There are no performance advantages to be expected by distributing the database across several files, so it is not recommended that secondary files be used, unless the disk storage space and database size absolutely require it.
Please note that when a database is dropped/deleted, all secondary and shadow files are also deleted.