Manual:$wgReadOnlyFile

Details
If this file is found and its contents are more than 0 bytes long, the database is locked for the reason given in the contents of the file.

The file must be writable by the web server for lockdb>Special:MyLanguage/Manual:Lock the database|locking the database when using the special pages "Special:LockDB" (locking) and "Special:UnlockDB" (unlocking).