Block Allocation

Goal

Optimize for sequential access

• If available, use rotationally close block in same cylinder
• Otherwise, use block in same cylinder group
• If CG full, find another CG with quadratic hashing
  • If CG n is full, try $n+1^2,n+2^2,n+3^2,...$
• Else use any CG

Problem

Don’t want large files filling up whole CG

• Otherwise other inode will not have room for their data

Solution

Break large files over many CGs

• Still need to have large extents in each CG
  • So sequential access is still quick