 Indirect block pointer

An address in a traditional inode-based file system that points to a block of further disk block numbers, rather than directly to user contents. If a block can store $q$ addresses, one singly indirect block can locate up to $q$ file data blocks, and a doubly indirect arrangement can locate up to $q^2$. The address blocks themselves consume allocated storage and must be reclaimed when their file no longer needs them. This lets small files use a short direct layout while supporting larger files.