Redologs Mechanism
How redo logs work
. Redo logs are used in a cyclic fashion
. When a redo log file is full,LGWR will move to the next log group
-this is called a log switch
| alter system switch logfile; <---forcing log switch |
-checkpoint operation also occurs
>FAST_START_MTTR_TARGET 1
>FAST_START_TO_TARGET 2
>LOG_CHECKPOINT_TIMEOUT 3
注意:若使用1,则2,3就要disable;
-information is written to the control file
Adding online redo log groups and redo log members
alter database add logfile group 5
('$ORACLE_HOME/oradata/u01/log5a.log REUSE',
'$ORACLE_HOME/oradata/u02/log5b.log REUSE',
'$ORACLE_HOME/oradata/u03/log5c.log REUSE') size 3m;
alter database add logfile member
'$ORACLE_HOME/oradata/u04/log5d.log' REUSE to group 5
说明:注意比较上面两例中REUSE中的用法
Dropping online redo log groups and redo log members
alter database drop logfile group 5;
alter database drop logfile member
'$ORACLE_HOME/oradata/u04/log5d.log'
说明：
1.当前日志组或日志成员(状态为current)不能删除,如果删除,只有先切换,再删除;
2.这里指的删除是逻辑删除，物理文件还是存在的，可重新使用
Renaming(relocating) online redo log files
alter database rename file '$ORACLE_HOME/oradata/u04/log5d.log'
to '$ORACLE_HOME/oradata/u04/log5E.log'
alter database rename file '
$ORACLE_HOME/oradata/u04/log5d.log'
to '$ORACLE_HOME/oradata/u03/log5d.log'
说明：前提条件是DATABASE是在mount状态，而非open状态
Archived(automatic) redo log files
.setup parameter
log_archive_start=TRUE (initialization) --要编辑pfile
log_archive_format=%t_%s.arc
log_archive_dest_1='$ORACLE_HOME/oradata/u01'
log_archive_dest_2='$ORACLE_HOME/oradata/u02'
|
alter system set log_archive_format=%t_%s.arc;
alter system set log_archive_dest_1='$ORACLE_HOME/oradata/u01' scope=spfile;
alter system set log_archive_dest_2='$ORACLE_HOME/oradata/u02' scope=spfile; |
.archivelog database (in mount)
| alter database archivelog; |
.open database
3. Parameters related to incremental checkpointing
Note: Log file switches will always override checkpoints caused by following paarameters.
Since Oracle 9i FAST_START_MTTR_TARGET parameter is the preferred method
of tuning incremental checkpoint target. FAST_START_MTTR_TARGET enables you
to specify the number of seconds the database takes to perform crash recovery
of a single instance. Based on internal statistics, incremental checkpoint
automatically adjusts the checkpoint target to meet the requirement of
FAST_START_MTTR_TARGET.
V$INSTANCE_RECOVERY.ESTIMATED_MTTR shows the current estimated mean time to
recover (MTTR) in seconds. This value is shown even if FAST_START_MTTR_TARGET
is not specified.
V$INSTANCE_RECOVERY.TARGET_MTTR shows the effective MTTR target in seconds
enforced by the system.
V$MTTR_TARGET_ADVICE shows the number of I/Os resulted by the current workload
under the current MTTR setting and the estimated number of I/Os that would be
resulted by the current workload under other MTTR settings. This view helps
the user to assess the trade-off between runtime performance and setting
FAST_START_MTTR_TARGET to achieve better recovery time.
LOG_CHECKPOINT_INTERVAL parameter specifies the maximum number of redo blocks
the incremental checkpoint target should lag the current log tail.
If FAST_START_MTTR_TARGET is specified, LOG_CHECKPOINT_INTERVAL should not
be set or set to 0.
On most Unix systems the operating system block size is 512 bytes.
This means that setting LOG_CHECKPOINT_INTERVAL to a value of 10,000 would
mean the incremental checkpoint target should not lag the current log tail
by more than 5,120,000 (5M) bytes. . If the size of your redo log is 20M, you are taking 4
checkpoints for each log.
LOG_CHECKPOINT_INTERVAL influences when a checkpoint occurs, which means
careful attention should be given to the setting of this parameter, keeping it
updated as the size of the redo log files is changed. The checkpoint
frequency is one of the factors which impacts the time required for the
database to recover from an unexpected failure. Longer intervals between
checkpoints mean that if the system crashes, more time will be needed for the
database to recover. Shorter checkpoint intervals mean that the database will
recover more quickly, at the expense of increased resource utilization during
the checkpoint operation.
This parameter also impacts the time required to complete a database recovery
operation during the roll forward phase of recovery. The actual recovery time
is dependent upon this time, and other factors, such as the type of failure
(instance or system crash, media failure, etc.), and the number of archived
redo logs which need to be applied.
The LOG_CHECKPOINT_TIMEOUT parameter specifies the maximum number of seconds
the incremental checkpoint target should lag the current log tail.
In another word, it specifies how long a dirty buffer in buffer cache can
remain dirty.
Checkpoint frequency impacts the time required for the
database to recover from an unexpected failure. Longer intervals between
checkpoints mean that more time will be required during database recovery.
Oracle recommends using LOG_CHECKPOINT_INTERVAL to control the checkpoint
interval rather than LOG_CHECKPOINT_TIMEOUT, which will initiate a checkpoint
every "n" seconds, regardless of the transaction frequency. This can cause
unnecessary checkpoints in cases where transaction volumes vary. Unnecessary
checkpoints must be avoided whenever possible for optimal performance.
It is a misconception that setting LOG_CHECKPOINT_TIMEOUT to a given
value will initiate a log switch at that interval, enabling a recovery
window used for a stand-by database configuration. Log switches cause a checkpoint,but a checkpoint does not cause a log switch. The only way to cause a log switch is manually with
ALTER SYSTEM SWITCH LOGFILE or resizing the redo logs to cause
more frequent switches. This is controlled by operating system
blocks, not a timed interval.
Sizing of the online redo logs is critical for performance and recovery.
See additional sections below on redo logs and checkpoints.
LOG_CHECKPOINTS_TO_ALERT lets you log your checkpoints to the alert file.
Doing so is useful for determining whether checkpoints are occurring at
the desired frequency.
Prior to Oracle9i this parameter was STATIC.
Oracle generally advises this be set to TRUE as the overhead is
negligible but the information in the alert log may be useful. |
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