FSX is a file system exerciser originally written at Apple for 
stress testing file system operations, particularly those related to 
seeking, truncating etc.

A good output looks like:
fsx_init done
truncating to largest ever: 0x1cbf7
truncating to largest ever: 0x27453
truncating to largest ever: 0x2d9bd
truncating to largest ever: 0x36c22
truncating to largest ever: 0x3e9f5
truncating to largest ever: 0x3eff0
truncating to largest ever: 0x3fd22
truncating to largest ever: 0x3fe0b
truncating to largest ever: 0x3fe29
truncating to largest ever: 0x3ff4b
truncating to largest ever: 0x3ff8c
truncating to largest ever: 0x3ffa2
truncating to largest ever: 0x3ffe5
truncating to largest ever: 0x3fff8
skipping zero size write
skipping zero size read
truncating to largest ever: 0x3fffd
truncating to largest ever: 0x3ffff
skipping zero size read
skipping zero size read
skipping zero size write
skipping zero size write
skipping zero size read
skipping zero size read
skipping zero size read
skipping zero size read
skipping zero size read
skipping zero size read