Checkpoint and Redo Tuning
Copyright © 2006, Oracle. All rights reserved.
Objectives
After completing this lesson, you should be able to do
the following:
• Diagnose checkpoint and redo issues
• Implement Fast Start MTTR Target
• Monitor the performance impact of Fast Start
MTTR Target
• Tune the redo chain
• Size the redo log file
• Size the redo log buffer
11-2
Copyright © 2006, Oracle. All rights reserved.
Checkpoint and Redo
•
Checkpoint:
– Transfers changed data to disk
– Makes buffer space available for more data blocks
– Controls mean time to recover (MTTR)
•
Redo:
– Recovers committed data not on disk
– Provides uncommitted data for rollback operation
– Provides source data for complete recovery
11-3
Copyright © 2006, Oracle. All rights reserved.
Oracle Database Architecture
Instance
SGA
Fixed
size
nK
Default Keep Recycle
Redo
Sort Global Flash
Large Java Streams buffer buffer buffer buffer Shared ASH
log
pool buffer extent context back
pool
pool pool
cache
caches
cache
cache
buffer
pool
pool
buffer
PMON
SMON
RECO
MMON
MMAN
MMNL
QMNC
LGWR
CTWR
ARCn
S000
RVWR
D000
FMON
Qnnn
Password
file
Control
files
Redo log
files
Temp
SYSTEM
SYSAUX
Data file
Change
tracking
file
Undo
11-4
CKPT
CJQ0
PSP0
Spfile
DBWn
Copyright © 2006, Oracle. All rights reserved.
Flashback
logs
Archive
log files
Checkpoint Architecture
The checkpoint architecture provides:
• Checkpoint position: A starting position in the
redo logs to begin recovery
• Checkpoint target: A calculated position in the
redo logs where the checkpoint position should be
• An estimated mean time to recover
• A high-performance incremental checkpoint
• A full checkpoint when required
11-5
Copyright © 2006, Oracle. All rights reserved.
Database Writer (DBWn) Process
Background Information
SGA
Database
buffer cache
Database
Writer
(DBWn)
Data files
11-6
DBWn writes when one of the
following events occurs:
• Checkpoint
• Dirty buffers’ threshold
• No free buffers
• Timeout
• RAC ping request
• Tablespace OFFLINE
• Tablespace READ ONLY
• Table DROP or TRUNCATE
• Tablespace BEGIN BACKUP
Copyright © 2006, Oracle. All rights reserved.
Checkpoint (CKPT) Process
Responsible for:
• Signaling DBWn at checkpoints
• Updating data file headers with
checkpoint information
• Updating control files with
checkpoint information
Checkpoint
(CKPT)
11-7
Copyright © 2006, Oracle. All rights reserved.
SGA
Database
buffer cache
Database
Writer
(DBWn)
Redo Architecture
Redo is designed for minimum performance impact.
• Server processes write to the redo log buffer:
– Circular buffer
– Memory-to-memory writes
– Small fast writes
•
LGWR writes log buffer blocks to log files:
– Circular files
– Memory-to-disk write
– Full blocks if possible
•
ARCn copies log files to archive log files:
– Disk-to-disk writes
– Multiple archiver processes can be started.
11-8
Copyright © 2006, Oracle. All rights reserved.
The Redo Log Buffer
Redo
log
buffer
Database
buffer
cache
Shared pool
Library cache
Data dictionary cache
User global area
Server
process
LGWR
ARCn
Control files
SQL> UPDATE employees
11-9
2
SET salary=salary*1.1
3
WHERE employee_id=736;
Data files
Redo log files
Copyright © 2006, Oracle. All rights reserved.
Archived
log files
Redo Log Files and LogWriter
SGA
Redo log
buffer
Log
Writer
(LGWR)
Group 1
Group 2
Redo log files:
• Record changes to the
database
• Should be multiplexed to
protect against loss
LogWriter writes:
• At commit
• When one-third full
Group 3 • Every three seconds
• Before DBWn writes
Redo log files
11-10
Copyright © 2006, Oracle. All rights reserved.
Archiver (ARCn)
•
•
•
Is an optional
background process
Automatically archives
online redo log files
when ARCHIVELOG mode
is set for the database
Preserves the record of
all changes made to the
database
SGA
Redo log
buffer
LogWriter
(LGWR)
Archiver
(ARCn)
11-11
Copyright © 2006, Oracle. All rights reserved.
Incremental Checkpointing
t2
Incremental checkpoint
b1
Checkpoint
queue
b2
b3
b4
Redo stream
c2
c1
Checkpoint
position
c3
c4
Target RBA
t1
FAST_START_MTTR_TARGET=T
t1 + t2 < T
11-12
Copyright © 2006, Oracle. All rights reserved.
c5
Tail of
redo thread
Incremental Checkpoint and Log File Size
•
•
•
The maximum checkpoint lag is:
90%*(SUM(log_sizei) – MAX(log_sizei))
Checkpoint lag is designed to prevent log switch
from blocking.
A few small log files can result in excess
checkpoint writes.
Current
tail
9,000
blocks
Log file#2:
10,000 blocks
Target checkpoint
11-14
Log file#1:
10,000 blocks
Copyright © 2006, Oracle. All rights reserved.
Adjusting the Checkpoint Rate
The checkpoint rate is determined by the most
aggressive of:
• FAST_START_MTTR_TARGET parameter (only on
Enterprise Edition)
• Size of the smallest redo log file
• LOG_CHECKPOINT_TIMEOUT parameter (overrides
FAST_START_MTTR_TARGET if set)
• LOG_CHECKPOINT_INTERVAL parameter (overrides
FAST_START_MTTR_TARGET if set)
11-15
Copyright © 2006, Oracle. All rights reserved.
Redo Logfile Size Advisor
•
This advisor
determines the
optimal size of your
online redo logs:
– No additional
checkpoint writes
beyond those
caused by FAST_START_MTTR_TARGET.
•
11-16
FAST_START_MTTR_TARGET must be set.
View name
V$INSTANCE_RECOVERY
Column name
OPTIMAL_LOGFILE_SIZE
Description
This column shows the redo log file size (in megabytes) that is
considered as minimal.
Copyright © 2006, Oracle. All rights reserved.
Impact of the Checkpoint Rate
From the V$ views:
SELECT c.value-nc.value
FROM V$SYSSTAT c, V$SYSSTAT nc
WHERE c.name = 'physical writes' AND
nc.name = 'physical writes non checkpoint';
From the Statspack report:
Statistic
Total
-------------------------- -----------------physical writes
47,308
physical writes non checkpoint
44,674
11-17
Copyright © 2006, Oracle. All rights reserved.
Automatic Checkpoint Tuning
•
•
•
•
11-19
There is no longer a continuous manual tuning
effort.
Automatic checkpoint tuning is the best-effort
checkpointing, without much overhead.
It reduces average recovery time by making use of
unused bandwidth.
Automatic checkpoint tuning is enabled when
FAST_START_MTTR_TARGET is not explicitly set to
zero.
Copyright © 2006, Oracle. All rights reserved.
ADDM Report: Checkpoints
11-20
Copyright © 2006, Oracle. All rights reserved.
ADDM Report: Redo Logs
11-21
Copyright © 2006, Oracle. All rights reserved.
Statspack and AWR Reports
Checkpoint and redo show certain symptoms:
• Alert log shows log switch not complete
• I/O symptoms caused by excessive checkpoints
• Log switches per hour > 4
• log file and latch:redo in Top Timed Events
Top 5 Timed Events
Avg %Total
~~~~~~~~~~~~~~~~~~
wait
Call
Event
Waits
Time (s)
(ms)
Time
----------------------- ------- ----------- ------ -----CPU time
551
56.8
log file parallel write
3,899
201
52
20.7
log file sync
823
58
71
6.0
latch: redo copy
635
44
70
4.6
latch: redo allocation
1,109
42
38
4.4
11-22
Copyright © 2006, Oracle. All rights reserved.
Check Parameters
Review checkpoint parameters for reasonable values:
Parameter Name
Begin value
---------------------------- ----------fast_start_mttr_target
25
Use V$MTTR_TARGET_ADVICE for optimum value:
SQL> SELECT mttr_target_for_estimate,
2> estd_total_ios, estd_total_io_factor
3> FROM V$MTTR_TARGET_ADVICE ORDER BY 1
MTTR_TARGET_FOR_ESTIMATE ESTD_TOTAL_IOS ESTD_TOTAL_IO_FACTOR
------------------------ -------------- -------------------20
2436690
1.0739
22
2330674
1.0272
25
2268973
1
37
2204817
.9717
62
2181841
.9616
11-24
Copyright © 2006, Oracle. All rights reserved.
Check the Redo Log Size
Review the current size of the redo log files:
Check alert log for log switch rate:
11-25
Copyright © 2006, Oracle. All rights reserved.
Redo Log Chain Tuning
Redo tuning starts with the slowest part.
• Reduce the amount of redo generated.
• Check archive logging (waits for archiving
needed).
• Check the redo log file size and log switch rate.
• Check the checkpoint parameters.
• Look for log space requests.
• The Redo Buffer Allocation Retries value should
be near 0 and should be less than 1% of redo
entries.
11-26
Copyright © 2006, Oracle. All rights reserved.
Reducing Redo Operations
Ways to avoid logging bulk operations in the redo log:
• Direct Path loading without archiving does not
generate redo.
• Direct Path loading with archiving can use
NOLOGGING mode.
• Direct Load INSERT can use NOLOGGING mode.
• Some SQL statements can use NOLOGGING mode.
11-28
Copyright © 2006, Oracle. All rights reserved.
Increasing the Performance of Archiving
•
Share the archiving work during a temporary
increase in workload:
ALTER SYSTEM ARCHIVE LOG ALL
TO <log_archive_dest>
•
Increase the number of archiver processes with
LOG_ARCHIVE_MAX_PROCESSES.
•
Multiplex the redo log files, and add more
members.
Change the number of archive destinations:
– LOG_ARCHIVE_DEST_n
•
11-30
Copyright © 2006, Oracle. All rights reserved.
Diagnostic Tools
V$ARCHIVE_DEST
V$ARCHIVED_LOG
V$ARCHIVE_PROCESSES
LOG_ARCHIVE_DEST_n
LOG_ARCHIVE_DEST_STATE_n
11-32
Copyright © 2006, Oracle. All rights reserved.
Archived
logs
Redo Log Groups and Members
LGWR
Group 1
Group 2
Group 3
Disk 1
Member
Member
Member
Disk 2
Member
11-33
Member
Copyright © 2006, Oracle. All rights reserved.
Member
Online Redo Log File Configuration
•
•
•
•
11-34
Size redo log files to minimize contention.
Provide enough groups to prevent waiting.
Store redo log files on separate, fast devices.
Monitor the redo log file configuration with:
– V$LOGFILE
– V$LOG
– V$LOG_HISTORY
Copyright © 2006, Oracle. All rights reserved.
Monitoring Online Redo Log File I/O
11-35
Copyright © 2006, Oracle. All rights reserved.
Sizing the Redo Log Buffer
The size of the redo log buffer is determined by:
• LOG_BUFFER parameter
• Remaining space in the fixed area granule
Default value: Either 2 MB or 128 KB  the value of
CPU_COUNT, whichever is greater
11-36
Copyright © 2006, Oracle. All rights reserved.
Diagnosing Redo Log Buffer Inefficiency
SQL> UPDATE employees
2
SET salary=salary*1.1
3
WHERE employee_id=736;
Server
process
Server
process
LGRW
SQL> DELETE FROM employees
2
WHERE employee_id=7400;
ARCH
Redo log files
11-37
Copyright © 2006, Oracle. All rights reserved.
Archived
log files
Diagnosing Log Buffer Problems
V$SESSION_WAIT
Redo log buffer
Log Buffer Space event
V$SYSSTAT
Redo Buffer Allocation Retries
Redo Entries
11-38
Copyright © 2006, Oracle. All rights reserved.
Log Space Request Waits:
Further Investigation
Possible reasons for log space request waits:
• There is disk I/O contention on redo log files.
• LGWR is waiting on DBWn to complete the
checkpointing of the required redo log file.
• LGWR is waiting on ARCn to complete archiving
of the required redo log file.
• Log buffer is too small.
11-40
Copyright © 2006, Oracle. All rights reserved.
Practice Overview:
Diagnose Checkpoints and Redo
This practice covers the following topics:
• Diagnose checkpoint and redo issues
• Resize log files
• Adjust the checkpoint parameters
11-42
Copyright © 2006, Oracle. All rights reserved.
Summary
In this lesson, you should have learned how to:
• Diagnose checkpoint and redo issues
• Implement Fast Start MTTR Target
• Monitor the performance impact of Fast Start
MTTR Target
• Implement multiple database writers
• Tune the redo chain
• Size the redo log file
• Size the redo log buffer
11-43
Copyright © 2006, Oracle. All rights reserved.