1. Database Recovery Recovery Buffer Management Recovery Facilities
Recover Techniques
Chapter 22.3(20.3)

2. Recovery(1)
Recovery:Restoring the database to a correct state in the event of a failure
Storage device: memory, Disk, Tape (or other offline storage)
Failure
system crashes: memory loss
media failures: parts of disk loss
application software errors:one or more transactions fail
disasters and carelessness
Sabotage (惡意破壞）

3. Recovery(2) Objective:guarantee atomicity and durability
Write: Write to Buffer then flush to disk
Transaction committed: redo(rollforward)
Transaction uncomitted:undo(rollback)

4. Buffer Management Write back policy
steal policy:allows write back before commit
force policy:all page updated write back immediately when commit
no-steal policy need very large buffer
force policy has lower efficiency
most DBMS employ steal,no-force policy

5. Recovery Facilities(1)
backup mechanism
full or incremental
store backup on offline storage
logging facilities
DBMS maintains a special file called a log or journal contains info about all updates
log contains:Transaction and checkpoint records Fig. 22(20).25,may contain other info for management
maybe 2 or 3 copies,store on separated disk
log file may be bottleneck of DBMS

6. Recovery Facilities(2)
checkpoint facility
how far the log file need to be traced back when failure occurs
use checkpointing to limit the amount of searching and processing on log file.
checkpoint: the point of synchronization between DB and log. All buffers are flush to Disk. once per min.
redo Transactions after CP, undo Transactions when failure occure
recovery manager

7. Recover Techniques
extensively damaged:restore backup data and redo transactions on log file
inconsistent:use undo and redo
deferred update:write to log file until commited then write into DB by using log records
only redo is needed, can redo any times
immediate update:write-ahead log protocol
need redo and undo(reverse order)
Shadow paging:use current and shadow page, not log file system

8. Example PostgreSQL MySQL
MySQL
prevention.html