Undo Optimization (Contd)

• If pages are not available (e.g media failure)

  • continue with redo recovery of other pages
    • once pages are available again (from archival dump) redos of the relevant pages must be done first, before any undo
  • for physical undos in undo pass
    • we can generate CLRs and apply later; new txns can run on other pages
  • for logical undos in undo pass
    • postpone undos of loser txns if the undo needs to access these pages - ``stopped transaction''
    • undo of other txns can proceed; new txns can start provided appropriate locks are first acquired for loser txns

Notes: