07_maintain_online_redo

Chapter 7

07_maintain_online_redo

explain the purpose of online redo log files

outline the structure

log switch and checkpoints

multiplex and maintain online redo log files

manage online redo log files with OMF

obtain online redo log file information

Using the redo log files:

record all changes made to data

provide a recovery mechanism

can ba organized into groups 

at least two groups required

Redo log files work:

call a log switch

checkpoint operation also occurs

information wirtten to the control file

A checkpoint is the writing of dirty blocks from the buffer cache to disk.

LGWR:

when a transaction commits

every three seconds

when the redo log buffer becomes one-third full

when there is more than a megabyte of changed records in the redo log buffer

before the DBWn writs modified blocks in the database buffer cache to the data files

LS  && LSN

alter sysem switch logfile;

CK:

fast_start_mttr_target

FAST_START_MTTR_TARGET enables you to specify the number of seconds the 

database takes to perform crash recovery of a single instance. When 

specified, FAST_START_MTTR_TARGET is overridden by LOG_CHECKPOINT_INTERVAL.

LOG_CHECKPOINT_INTERVAL specifies the frequency of checkpoints in terms of the 

number of redo log file blocks that can exist between an incremental checkpoint

and the last block written to the redo log. This number refers to physical 

operating system blocks, not database blocks.

Regardless of this value, a checkpoint always occurs when switching from one 

online redo log file to another. Therefore, if the value exceeds the actual redo

log file size, checkpoints occur only when switching logs. Checkpoint frequency

is one of the factors that influence the time required for the database to 

recover from an unexpected failure.

Notes:

Specifying a value of 0 (zero) for LOG_CHECKPOINT_INTERVAL has the same effect

as setting the parameter to infinity and causes the parameter to be ignored.

 Only nonzero values of this parameter are considered meaningful.

Recovery I/O can also be limited by setting the LOG_CHECKPOINT_TIMEOUT 

parameter or by the size specified for the smallest redo log. For information 

on which mechanism is controlling checkpointing behavior, query the 

V$INSTANCE_RECOVERY view.

alter system checkpoint;

v$log

v$logfile

v$log_history

v$instance

v$database     

archive log list

可以查看数据库的归档模式