Other
recovery related to data structure
The
write-Ahead log protocol:
write-ahead logging (WAL) is a
family of techniques for providing atomicity and durability (two of the ACID
properties) in database systems.
In a system using WAL, all modifications
are written to a log before they are applied. Usually both redo and undo
information stored in the log. WAL allows updates of a database to be done
in-place. The main advantage of doing updates in-place is that it reduces the
need to modify flexes and block lists. In summary,
1.
Any
change to an object is first recorded in the log
2.
The
log must be written to stabilize storage before changes to the object are
written to disk
3.
used
to recover the database to the state before a crash
Atomicity: Atomicity: This is the property of transaction processing whereby either
all the operations of a transaction are executed or none of them are executed
(all-or-nothing).
Durability:
durability is
the ACID property which guarantees that transactions that have committed will
survive permanently. For example, if a flight booking reports that a seat has
successfully been booked, then the seat will remain booked even if the plane
crashes.
Log: a transaction log (also
transaction journal, database log, binary log or audit trail) is a history of
actions executed by a database management system to guarantee ACID properties
over crashes or hardware failures. Physically, a log is a file of updates done
to the database, stored in stable storage.
Check
pointing: Check
Pointing is basically consists of storing a snapshot of the current application
state, and later on, use it for restarting the execution in case of failure . A
checkpoint record is written into the log periodically at that point when the
system writes out to the database on disk all DBMS buffers that have been
modified.
Checkpoints are used to reuse of
primary and secondary log files. In the case of crash backup files will be used
to recover the database to that point of crash.
Media
recovery
Media recovery: deals with failures of the storage media holding the permanent database, in particular disk failures. The traditional database approach for media recovery uses archive cut (dumps) of the database as well as archive' Archive copies represent snapshots of the database and are periodically taken.
The archive log contains the log
records for all committed changes which are not yet reflected in the archive
copy. In the event of a media fl the current database can be reconstructed the
latest archive copy and redoing all cha chronological order from the archive
log. A faster recovery from disk failures is sup disk organizations like RAID
(redundant arrays of independent disks) which store data redundantly on several
disks. However, they do not eliminate the need for archive based media recovery
since they cannot completely rule out the possibility of data loss, EG. When
multiple disks fail.