File map compression using run length encoding for active blocks

Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for compressing file maps. In one aspect, a method includes accessing a file maintained by a file system that manages access to a block device. The file includes a plurality of active blocks associated with a respective logical block number and a respective block index. The method also includes assigning a file index to the file, analyzing the file to determine a maximum block index and a minimum block index, and identifying runs of blocks in the plurality of active blocks. Each run of blocks includes a respective start block. For each of the runs of blocks, the method includes identifying a respective length. For each start block, the method includes generating a file map entry for each start block. The method also includes storing the file map entries in a file map.