G Locality Master scheduling policy Asks gfS for locations of replicas of input file blocks Map tasks typically split into 64MB(=gFS block size Map tasks scheduled so gFS input block replica are on same machine or same rac Effect Thousands of machines read input at local disk speed Without this, rack switches limit read rateLocality 23 • Master scheduling policy – Asks GFS for locations of replicas of input file blocks – Map tasks typically split into 64MB (== GFS block size) – Map tasks scheduled so GFS input block replica are on same machine or same rack • Effect – Thousands of machines read input at local disk speed – Without this, rack switches limit read rate