Redo Waits Kyle Hailey [email protected]. #.2 Copyright 2006 Kyle Hailey Redo REDO Lib Cache Buffer Cache Locks Network I/O

Redo Waits

Kyle Hailey

[email protected]

Copyright 2006 Kyle Hailey

#.2

Redo

REDOREDO Lib Lib CacheCache

Buffer Buffer CacheCache

LocksLocks

NetworkNetwork

I/OI/O


#.3

In This Section:

1. Log Buffer Space

2. Log File Sync

3. Log File Switch Completion

4. Log File Switch (checkpoint incomplete)

5. Log File Switch (private strand flush incomplete)

6. Log File Switch (archiving needed)

7. Switch Log File Command


#.4

Log File Waits

REDO Log FilesREDO Log Files Data FilesData Files

DBWRDBWRLGWRLGWR

User2User2

User1User1

User3User3

Log BufferLog Buffer

Buffer CacheBuffer CacheLog Log BufferBuffer


SGASGALibrary Library CacheCache


#.5

Log File Operations

Redo is written to disk whenUser commitsLog Buffer 1/3 full (_log_io_size)Log Buffer fills 1MEvery 3 secondsDBWR asks LGWR to flush redo

Sessions Commiting wait for LGWR


#.6

log buffer space

Wait for space in the redo log buffer in SGA Solution

1. Increase log_buffer parameter in init.ora Above 3M log_buffer little affect, if still a problem then

backup is at disk level

2. Improve disk IO for redo Faster disk Raw file Direct IO Dedicated disk

p1, p2, p3 – no values


#.7Log Buffer Space

REDO Log FilesREDO Log Files


LGWRLGWR

Buffer CacheBuffer Cache

User2User2

User1User1

User3User3

Insert Insert UpdateUpdatedeletedelete

Log Log BufferBuffer



1.1. Log Buffer too smallLog Buffer too small

2.2. LGWR too slowLGWR too slow

• Disk problemDisk problem


#.8

log file sync Wait for redo flush upon:

CommitRollback

ArgumentsP1 = buffer# in log buffer that needs to be flushedP2 = not usedP3 = not used

select parameter1, parameter2, parameter3 select parameter1, parameter2, parameter3 from v$event_namefrom v$event_namewhere name=‘log file sync';where name=‘log file sync';

PARAMETER1 PARAMETER2 PARAMETER3PARAMETER1 PARAMETER2 PARAMETER3

---------- ---------- -------------------- ---------- ----------

buffer#buffer#


#.9Log File Sync


DBWRDBWRLGWRLGWR

User2User2

User1User1

User3User3

Commit;Commit;

wait for LGWRwait for LGWR





#.10

Log File Sync: Solutions Commit less

Often possible in PL/SQL loops that commit every loop Commit every 50 or 100 instead

Put redo on dedicated disk Use Raw Device or Direct IO More Radical

Consider Ram Disks Can stripe if redo writes are comparable to stripe size

Striping shouldn’t heard Striping can help

Ex: imp – can have large redo writes – can improve by 10-30%

Alternate disks for redo and archiving of redo Possibly 10gR2

ALTER SYSTEM SET COMMIT_WRITE = BATCH, NOWAIT Commit could be lost if machine crash Or IO error


#.11

Separating LGWR and ARCH

Redo 1Redo 1

Redo 3Redo 3

Redo 2Redo 2

Redo 4Redo 4

LGWRLGWR ARCHARCH


#.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


#.13

Log File Switch completion

Log File Switch stepsA lot of activity:

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

DBWR makes a list of DBWR makes a list of blocks that need to be blocks that need to be written out in order to over written out in order to over write the Redo log file a write the Redo log file a list of blocks that need to list of blocks that need to be written out in order to be written out in order to over write the Redo log over write the Redo log

filefile


#.14

log file switch completion



DBWRDBWRLGWRLGWR


Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxxBlock xxxx

Wait : log file switch completion

User2User2User1User1

User3User3


User3User3

Commit;Commit;

Wait for Wait for LGWRLGWR




11 22 33

11

Sessions Sessions writing redo writing redo wait while wait while switch happensswitch happens


#.15

log file switch (checkpoint incomplete)

No p1,p2,p3 args Wait for checkpoint to complete because all

log files are full Solutions

Add more log files Increase size of log files


#.16log file switch (checkpoint incomplete) : Diag



LGWRLGWR



User3User3

Commit;Commit;


Checkpoint 1Checkpoint 1






DBWRDBWR




11

22

33

33

Sessions Sessions writing redo writing redo wait while wait while switch happensswitch happens

And checkpoint And checkpoint happenhappen


#.17

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;


#.18

log file switch (archiving needed) :

Diag

REDO Log FilesREDO Log Files Archive DestArchive Dest

ARCHARCH

LGWRLGWRUserUser

22UserUser11 UserUser

33

Commit;Commit;





Buffer CacheBuffer CacheLog Log BufferBuffer



33


#.19

switch logfile command

Same as log file switch completion but the command is executed by the dbaAlter system switch logfile;


#.20

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

Standby DB: ARCHIVE_LAG_TARGET Seconds, limits lag between primary and standby Increases log file switches


#.21

Incremental Checkpoints (9iR2+) FAST_START_MTTR_TARGET

Seconds to RecoveryEasy and accuracy Is overridden by FAST_START_IO_TARGET Is overridden by LOG_CHECKPOINT_INTERVAL

SQL> select ESTIMATED_MTTR from V$INSTANCE_RECOVERY;

ESTIMATED_MTTR-------------- 21

SQL> select ESTIMATED_MTTR from V$INSTANCE_RECOVERY;

ESTIMATED_MTTR-------------- 21

alter system set fast_start_mttr_target=17 scope=both;alter system set fast_start_mttr_target=17 scope=both;


#.22

Recovery and Checkpoints

REDO Log FilesREDO Log Files

Data FilesData Files

DBWRDBWR

LGWRLGWR




Incremental CheckpointIncremental Checkpoint

Current PositionCurrent Position

Needed for RecoveryNeeded for Recovery11 22 33


#.23

DBWR dirty List and LGWRBuffers

Checkpoint aCheckpoint a

Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxxBlock xxxx

LGWRLGWR

DBWRDBWR

Current PositionCurrent Position

Incremental CheckpointIncremental CheckpointDBWRDBWR

DBWR usually DBWR usually just writes out just writes out dirty blocks at dirty blocks at end of LRU end of LRU until checkpointuntil checkpoint

Now, DBWR Now, DBWR keeps a keeps a checkpoint listcheckpoint list

That it cleans aThat it cleans a


#.24

DBWR dirty List

MRU - HotMRU - Hot LRU - ColdLRU - ColdBuffer Headers

Dirty ListDirty List


DBWRDBWRDBWR also has to track dirty DBWR also has to track dirty blocks at the cold end of the blocks at the cold end of the LRULRU


#.25

DBWR merges Dirty and Checkpoint

MRU - HotMRU - Hot LRU - ColdLRU - ColdBuffer Headers

Dirty ListDirty List


DBWRDBWRCheckpoint aCheckpoint a


Write ListWrite List

Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxx Block xxxxBlock xxxx Data FilesData Files

There are 2 checkpoint listsThere are 2 checkpoint listsOne the DBWR can write toOne the DBWR can write to disk While users can disk While users can concurrently update the otherconcurrently update the other


#.26

log file switch (private strand flush incomplete)

New wait 10g Like a “log file switch Completion”


#.27

Redo Wait Solutions

2 log file syncCommit less, put redo logs on faster disks

11 log buffer spaceIncrease log buffer no more than 32M, then tune LGWR

12 log file switch completionIncrease log file sizes

22 log file switch (checkpoint incomplete)Add log files (or increase log file size)

30 switch logfile commandAvoid switching log files

32 log file switch (archiving needed) ***Archive log running out of space

Documents

Redo Waits Kyle Hailey [email protected]. #.2 Copyright 2006 Kyle Hailey Redo REDO Lib Cache Buffer Cache Locks Network I/O