1. Kyle Hailey http://perfvision.com/ftp/emea Kyle.hailey@hotmail.com
Redo Waits
Kyle Hailey

2. In This Section: Log Buffer Space Log File Sync
Log File Switch Completion
Log File Switch (checkpoint incomplete)
Log File Switch (private strand flush incomplete)
Log File Switch (archiving needed)
Switch Log File Command
Copyright 2006 Kyle Hailey

3. Log File Waits Log Buffer Buffer Cache SGA Library Cache Buffer Cache
DBWR
LGWR
User1
User2
User3
REDO Log Files
Data Files
Copyright 2006 Kyle Hailey

4. Log File Operations Redo is written to disk when
User commits
Log Buffer 1/3 full (_log_io_size)
Log Buffer fills 1M
Every 3 seconds
DBWR asks LGWR to flush redo
Sessions Commiting wait for LGWR
Copyright 2006 Kyle Hailey

5. log buffer space Wait for space in the redo log buffer in SGA Solution
Increase log_buffer parameter in init.ora
Above 3M log_buffer little affect, if still a problem then backup is at disk level
Improve disk IO for redo
Faster disk
Raw file
Direct IO
Dedicated disk
p1, p2, p3 – no values
Copyright 2006 Kyle Hailey

6. Log Buffer Space Log Buffer Buffer Cache SGA Library Cache
LGWR
User1
Log Buffer too small
LGWR too slow
Slow disk
User2
Users who are creating redo information typically from
Insert
update
delete
Statements have to write the information into the redo log buffer. If the buffer fills up before the LGWR can write out the information then users have to wait.
User3
Insert
Update
delete
REDO Log Files
Copyright 2006 Kyle Hailey

7. log file sync Wait for redo flush upon: Arguments Commit Rollback
P1 = buffer# in log buffer that needs to be flushed
P2 = not used
P3 = not used
select parameter1, parameter2, parameter3
from v$event_name
where name=‘log file sync';
PARAMETER1 PARAMETER2 PARAMETER3
buffer#
Copyright 2006 Kyle Hailey

8. Log File Sync Log Buffer Buffer Cache SGA Library Cache DBWR LGWR
User1
User2
User3
Commit;
wait for LGWR
Data Files
REDO Log Files
Copyright 2006 Kyle Hailey

9. Log File Sync: Solutions
Commit less
Often possible in loops that commit every loop
Commit every 50 or 100 instead
Possibly 10gR2
ALTER SYSTEM SET COMMIT_WRITE = BATCH, NOWAIT
Commit could be lost if machine crash or IO error
Improve IO
Use Raw Device or Direct IO
Consider Ram Disks
Can stripe if redo writes are comparable to stripe size
Striping shouldn’t hurt
Striping can help
Ex: imp – can have large redo writes – can improve by 10-30%
Alternate disks for redo and archiving of redo
Copyright 2006 Kyle Hailey

10. Log File Sync depends on:
log file parallel write
Time it takes for LGWR to write out changes
If log file sync =~ log file parallel write
And the time is slow ( > 3ms) look into IO issues
If log file sync >> log file parallel write
Look at CPU starvation issues
Copyright 2006 Kyle Hailey

11. Separating LGWR and ARCH
Redo 1
Redo 3
Redo 2
Redo 4
Copyright 2006 Kyle Hailey

12. Log file switch completion
No p1,p2,p3
Wait for lgwr to switch log files when generating redo
Solution:
Increase redo log file size
Reduce frequency of switches
Copyright 2006 Kyle Hailey

13. log file switch completion
Log Buffer
Buffer Cache
SGA
Library Cache
Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx
Sessions writing redo wait while switch happens
Buffer Cache
Log Buffer
DBWR
LGWR
User3
User2
User1
Redo log 1 fills up.
LGWR switches to log Redo log 2 which requires
Get next log file from control file
Get Redo Copy and Redo Allocation latch
Flush redo
Close File
Update Controlfile
Set new file to Current
Set old file to Active
If in Archivelog mode add file to archive list
Open all members of new logfile group
Write the SCN to the headers
Enable redo log generation
At the same time
DBWR makes a list of all blocks in the buffer cache that are dirty
And have redo in log 1. This list of blocks has to be written out
To disk before LGWR can reuse log 1
Commit;
Wait for LGWR 1 2 3
REDO Log Files
Data Files
Wait : log file switch completion
Copyright 2006 Kyle Hailey

14. log file switch (checkpoint incomplete)
No p1,p2,p3 args
Wait for checkpoint to complete because all log files are full
Solutions
Increase size of log files
Copyright 2006 Kyle Hailey

15. log file switch (checkpoint incomplete) : Diag
Sessions writing redo wait while switch AND checkpoint happen
Log Buffer
Buffer Cache
SGA
Library Cache
Checkpoint 1
Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx 1
Checkpoint 2
Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx 2
Buffer Cache
Checkpoint 3
Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx 3
Log Buffer
DBWR
LGWR
User3
User2
User1
In this case none of the DBWR checkpoints 1, 2 or 3 finish before
LGWR filled up all 3 redo logs.
IN this case users must wait until DBWR finishes the checkpoints of writing all the dirty blocks out.
In older versions of Oracle the checkpoints were merged together, so all 3 checkpoints had to finish before Redo log 1 could be reusued. In later versions ( I think starting in 9) the checkpoints were kept separate thus once checkpoint 1 had finished, then log 1 could be reused.
Commit;
Wait for LGWR 3
REDO Log Files
Data Files
Copyright 2006 Kyle Hailey

16. log file switch (archiving needed)
No p1,p2,p3
Database “hangs” for transactions
archive log stop;
-- make room in log_archive_dest
archive log start;
Copyright 2006 Kyle Hailey

17. log file switch (archiving needed) :
SGA
Log Buffer
Library Cache
Buffer Cache
Buffer Cache
Buffer Cache
Log Buffer
Log Buffer
LGWR
ARCH
IN this case the archiver for some reason, hasn’t been able to archive log 1 and now LGWR needs to reause it. IN this case all transactional activity in the database comes to a halt. To any user with tranactions, the database has effectively hung.
This is almost always caused by the archive destination filling.
Make room on the destination disk.
You can manually stop and start the archiver to make sure it restarts after room is made
archive log stop;
-- make room in log_archive_dest
archive log start; 3
REDO Log Files
Archive Dest
Copyright 2006 Kyle Hailey

18. switch logfile command
Same as log file switch completion but the command is executed by the dba
Alter system switch logfile;
Copyright 2006 Kyle Hailey

19. Concerns – Recovery Time
What happens to recovery time if I change my redo log file sizes
Larger Redo Log size can increase recovery time but
There are init.ora parameters to limit this
Copyright 2006 Kyle Hailey

20. Incremental Checkpoints (9iR2+)
FAST_START_MTTR_TARGET
Seconds to Recovery
Easy and accuracy
Is overridden by FAST_START_IO_TARGET
Is overridden by LOG_CHECKPOINT_INTERVAL
alter system set fast_start_mttr_target=17 scope=both;
select ESTIMATED_MTTR from v$instance_recovery;
From Chris Foot
10G Automatic Checkpoint Tuning If you do not set FAST_START_MTTR_TARGET, or set it to a very large value, Oracle10g will provide automatic checkpoint tuning. The database will write out dirty blocks from the cache as fast as possible without negatively impacting database performance. The DBA is no longer required to set any of the aforementioned checkpoint parameters.
SELECT TARGET_MTTR, ESTIMATED_MTTR,
CKPT_BLOCK_WRITES
FROM V$INSTANCE_RECOVERY
CKPT_BLOCK_WRITES = represents overhead from fast_start_mttr_target
SQL> select ESTIMATED_MTTR from V$INSTANCE_RECOVERY;
ESTIMATED_MTTR 21
Copyright 2006 Kyle Hailey

21. Recovery and Checkpoints
Log Buffer
Buffer Cache
SGA
Library Cache
DBWR
LGWR
Data Files
Current Position
Needed for Recovery 1 2 3
Incremental Checkpoint
REDO Log Files
Copyright 2006 Kyle Hailey

22. DBWR dirty List and LGWR
Buffers
DBWR usually just writes out dirty blocks at end of LRU until checkpoint
LGWR Current Position
Incremental Checkpoint
DBWR
DBWR
Checkpoint a
Block xxxx Block xxxx Block xxxx Block xxxx
Now, DBWR keeps a checkpoint list that it writes out
Copyright 2006 Kyle Hailey

23. DBWR dirty List Buffer Headers MRU - Hot LRU - Cold
DBWR also has to track dirty blocks at the cold end of the LRU
Dirty List
Block xxxx Block xxxx Block xxxx Block xxxx
DBWR
Copyright 2006 Kyle Hailey

24. DBWR merges Dirty and Checkpoint
Buffer Headers
MRU - Hot
LRU - Cold
Checkpoint a
Block xxxx Block xxxx Block xxxx Block xxxx
Dirty List
Block xxxx Block xxxx Block xxxx Block xxxx
DBWR
Write List
Block xxxx Block xxxx Block xxxx Block xxxx
Data Files
Copyright 2006 Kyle Hailey

25. log file switch (private strand flush incomplete)
New wait 10g
Like a “log file switch Completion”
From Metalink:
The message means that we haven't completed writing all the redo information to the log when we are trying to switch. It is similar in nature to a "checkpoint not complete" except that is only involves the redo being written to the log. The log switch can not occur until all of the redo has been written. A "strand" is new terminology for 10g and it deals with latches for redo . Strands are a mechanism to allow multiple allocation latches for processes to write redo more efficiently in the redo buffer and is related to the log_parallelism parameter present in 9i. The concept of a strand is to ensure that the redo generation rate for an instance is optimal and that when there is some kind of redo contention then the number of strands is dynamically adjusted to compensate. The initial allocation for the number of strands depends on the number of CPU's and is started with 2 strands with one strand for active redo generation. For large scale enterprise systems the amount of redo generation is large and hence these strands are *made active* as and when the foregrounds encounter this redo contention (allocated latch related contention) when this concept of dynamic strands comes into play. There is always shared strands and a number of private strands . Oracle 10g has some major changes in the mechanisms for redo (and undo), which seem to be aimed at reducing contention. Instead of redo being recorded in real time, it can be recorded 'privately' and pumped into the redo log buffer on commit. Similary the undo can be generated as 'in memory undo' and applied in bulk.
This affect the memory used for redo management and the possibility to flush it in pieces. The message you get is related to internal Cache Redo File management. You can disregard these messages as normal messages. When you switch logs all private strands have to be flushed to the current log before the switch is allowed to proceed.
Copyright 2006 Kyle Hailey

26. Redo Wait Solutions log file sync log buffer space
Commit less, put redo logs on faster disks
log buffer space
Increase log buffer no more than 32M, then tune LGWR
log file switch completion
Increase log file sizes
log file switch (checkpoint incomplete)
Add log files (or increase log file size)
switch logfile command
Avoid switching log files
log file switch (private strand flush incomplete)
increase log file sizes
log file switch (archiving needed) ***
Archive log running out of space
ARCHIVE_LAG_TARGET
Copyright 2006 Kyle Hailey