Sectors

Disk interface presents linear array of sectors
Generally 512 bytes, written atomically
Disk maps logical sector #s to physical sectors
Zoning--puts more sectors on longer tracks
Track skewing--sector 0 pos. varies for sequential I/O
Sparing--flawed sectors remapped elsewhere
OS doesn't know logical to physical sector mapping
Larger logical sector # difference means larger seek
Highly non-linear relationship (and depends on zone)
OS has no info on rotational positions
Can empirically build table to estimate times