Inode has 13 block addresses. The first 10 are direct block addresses of the first
10 data blocks in the file. The 11th address points to a one-level index block.
The 12th address points to a two-level (double in-direction) index block. The 13th
address points to a three-level(triple in-direction)index block. This provides a
very large maximum file size with efficient access to large files, but also small
files are accessed directly in one disk read.