When a database page is being written to, Firebird reserves 20% of the page for future use. This could be used to extend VARCHAR columns that started off small and then were updated to a longer value, for example.
If you wish to use all the available space in each database page, you may use the -use command to configure the database to do so. If you subsequently wish to return to the default behaviour, the -use command can be used to revert back to leaving 20% free space per page.
Note: Once a page has been filled to 'capacity' (80 or 100%) changing the page usage setting will not change those pages, only subsequently written pages will be affected.
The -use command takes two parameters as follows:
gfix -use USAGE database_name
The USAGE is one of:
The following example configures a database to use all available space in each database page:
linux> gfix -use full my_employee linux> gstat -header my_employee | grep Attributes Attributes no reserve
The following example sets the page usage back to the default:
linux> gfix -use reserve my_employee linux> gstat -header my_employee | grep Attributes Attributes
If you are using full page utilisation then the Attributes show up with no reserve in the text. This doesn't appear for normal 80% utilisation mode.