1. Buffer Cache Waits

2. #.2 In This Section 1.latch: cache buffers chains 2.latch: cache buffers lru chain 3.latch: cache buffer handles 4.Free Buffer Wait 5.Buffer Busy Wait 6.Write Complete Wait 7.Buffer Exterminate

3. #.3 Buffer Cache Redo Lib Cache Buffer Cache IO Locks Network

4. #.4 REDO Log Files Data Files DBWR LGWR User2 User1 User3 Log Buffer Buffer Cache Log Buffer Buffer Cache SGA Library Cache Oracle Memory Structures

5. #.5 Buffer Cache Access  Buffer Cache Management  Locating Free blocks  Finding data blocks  Managing LRU lists  Cleaning Dirty Blocks  Buffer Cache management can cause contention  Different from IO ( reading blocks of disk )

6. #.6 Query 0. Parse statement 1.Find object information in data dictionary 2.Calculate execution plan 3.If full table scan  Look at all blocks of table 4.If index find root of index and follow to key 5.Data Dictionary will have info about table or index block  File #  Block # 6.Once you know the block DBA (file# + block#) … Select ename from emp where empno = 12;

7. #.7 Is Block in cache? Now you have a file# and block# How do you know if a block is cached? ShadowProcess ? Do you search all the blocks? Could be 1000s of blocks to search. Buffer caches are in the multi Gig

8. #.8 Buffer Cache Find a block by: 1) Hash of Data file # Block# 2) Result = Bucket # 3) Search linked list for that bucket # What is a hash value What are Buckets What is the linked list?

9. #.9 Concepts To understand contention on the buffer cache, need to understand : 1.Linked Lists 2.Hashing 3.Buckets

10. #.10 Double Linked Lists 03C39000 03C39478 03C38F60 03C39000 03C38F60 03C39478 Address Next Previous

11. #.11 Hashing Function  Simple hash could be a Mod function  1 mod 4 = 1  2 mod 4 = 2  3 mod 4 = 3  4 mod 4 = 0  5 mod 4 = 1  6 mod 4 = 2  7 mod 4 = 3  8 mod 4 = 0  Using “mod 4” as a hash funtion creates 4 “buckets” to store things

12. #.12 Hash Bucket Fill Data Block 1 2 0 3 Hash Block’s file# block #’s Result in a bucket# Put Block in bucket ? ? ? ? Hash Block’s 1 file# 437 block #’s (1+437) mod 4 = 2 After a while the buckets become populated with blocks

13. #.13 Latches Protect Bucket Contents Buffer Headers Data Blocks Hash bucket latches Buffer Headers contents described by X$BH

14. #.14 X$bh  Describes Contents of Buffer Headers SQL> desc x$bh Name Type ------------ -------- ADDR RAW(4) DBARFIL NUMBER DBABLK NUMBER OBJ NUMBER HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4) … much more ADDR DBARFIL DBABLK OBJ HLADDR NXT_HASH PRV_HASH … A each buffer header contains Information about the data block It points to and the previous and next Buffer header in a linked list

15. #.15 Cache 03C39000 03C39478 03C38F60 03C39000 03C38F60 03C39478 ADDR NXT_HASH PRV_HASH

16. #.16 X$BH describes Headers Buffer Headers Data Blocks Hash bucket latches HLADDR NXT_HASH PRV_HASH ADDR DBARFIL DBABLK OBJ x$bh ADDR DBARFIL DBABLK OBJ HLADDR NXT_HASH PRV_HASH

17. #.17 To Find a Block 1.Hash the block address 2.Get Bucket latch 3.Look for header 4.Found, read block in cache 5.Not Found Read block off disk ShadowProcess Buffer Headers Data Blocks Hash bucket 235 4 latches 1 2 3 4 5

18. #.18 Cache Buffers Chains Hash Buckets s5 s4 s3 s2 s1 Sessions Contention if too many accesses on a bucket latches Block Headers Cache Buffer Chain Data Blocks

19. #.19 Examples S1 S2 S3 S4 1.Look up Table 2.Nested Loops Select t1.val, t2.val from t1, t2 where t1.c1 = {value} and t2.id = t1.id; t1 Index_t2 t2

20. #.20 CBC Solutions Find SQL ( Why is application hitting the block so hard? )  Nested loops, possibly  Hash Partition  Uses Hash Join  Hash clusters  Look up tables (“ select language from lang_table where...”)  Change application  Use plsql function  Spread data out to reduce contention  Select from dual  Possibly use x$dual How do you find the SQL?

21. #.21 CBC: Statspack 9i Top 5 Timed Events ~~~~~~~~~~~~~~~~~~ % Total Event Waits Time (s) Ela Time ---------------------------- ------------ ----------- -------- latch free 21,428 1,914 81.37 CPU time 360 15.29 PL/SQL lock timer 16 48 2.04 SQL*Net message from dblink 4,690 14.58 db file sequential read 1,427 5.19 Top 5 Timed Events ~~~~~~~~~~~~~~~~~~ % Total Event Waits Time (s) Ela Time ---------------------------- ------------ ----------- -------- latch free 21,428 1,914 81.37 CPU time 360 15.29 PL/SQL lock timer 16 48 2.04 SQL*Net message from dblink 4,690 14.58 db file sequential read 1,427 5.19 Top 5 Timed Events ~~~~~~~~~~~~~~~~~~ % Total Event Waits Time (s) Ela Time ---------------------------- ------------ ----------- -------- latch free 21,428 1,914 81.37 CPU time 360 15.29 PL/SQL lock timer 16 48 2.04 SQL*Net message from dblink 4,690 14.58 db file sequential read 1,427 5.19 Latch Sleep breakdown for DB: CDB Instance: cdb Snaps: 1 -2 -> ordered by misses desc Latch Name Requests Misses Sleeps Sleeps 1->4 -------------------- ---------- ------- ------ ------------ cache buffers chains 12,123,500 608,415 15,759 0/0/0/0/0 library cache pin 12,027,599 173,446 2,862 172694/743/8/1/0 library cache 12,072,503 98,065 2,373 97739/279/47/0/0 simulator lru latch 606 436 434 6/426/4/0/0 Latch Sleep breakdown for DB: CDB Instance: cdb Snaps: 1 -2 -> ordered by misses desc Latch Name Requests Misses Sleeps Sleeps 1->4 -------------------- ---------- ------- ------ ------------ cache buffers chains 12,123,500 608,415 15,759 0/0/0/0/0 library cache pin 12,027,599 173,446 2,862 172694/743/8/1/0 library cache 12,072,503 98,065 2,373 97739/279/47/0/0 simulator lru latch 606 436 434 6/426/4/0/0 Fails to find SQL

22. #.22 CBC: Statspack 10g Top 5 Timed Events Avg %Total ~~~~~~~~~~~~~~~~~~ wait Call Event Waits Time (s) (ms) Time ---------------------------------- ----------- ------ ------ CPU time 35 54.3 latch: cache buffers chains 46 11 243 17.6 latch: library cache pin 35 8 229 12.6 latch: library cache 27 6 231 9.8 log file sequential read 15 1 60 1.4 Top 5 Timed Events Avg %Total ~~~~~~~~~~~~~~~~~~ wait Call Event Waits Time (s) (ms) Time ---------------------------------- ----------- ------ ------ CPU time 35 54.3 latch: cache buffers chains 46 11 243 17.6 latch: library cache pin 35 8 229 12.6 latch: library cache 27 6 231 9.8 log file sequential read 15 1 60 1.4 Top 5 Timed Events Avg %Total ~~~~~~~~~~~~~~~~~~ wait Call Event Waits Time (s) (ms) Time ---------------------------------- ----------- ------ ------ CPU time 35 54.3 latch: cache buffers chains 46 11 243 17.6 latch: library cache pin 35 8 229 12.6 latch: library cache 27 6 231 9.8 log file sequential read 15 1 60 1.4 Fails to find SQL

23. #.23 CBC: ASH select count(*), sql_id, nvl(o.object_name,ash.current_obj#) objn, substr(o.object_type,0,10) otype, CURRENT_FILE# fn, CURRENT_BLOCK# blockn from v$active_session_history ash, all_objects o where event like 'latch: cache buffers chains' and o.object_id (+)= ash.CURRENT_OBJ# group by sql_id, current_obj#, current_file#, current_block#, o.object_name,o.object_type order by count(*) / CNT SQL_ID OBJN OTYPE FN BLOCKN ---- ------------- -------- ------ --- ------ 84 a09r4dwjpv01q MYDUAL TABLE 1 93170 SQL Statement: Success Extra: Hot block

24. #.24 CBC: OEM

25. #.25 CBC: ADDM Problem SQL Statement Solution?

26. #.26 CBC – Further Investigation select * from v$event_name where name = 'latch: cache buffers chains' EVENT# NAME ---------- ---------------------------- 58 latch: cache buffers chains PARAMETER1 PARAMETER2 PARAMETER3 ---------- ---------- ---------- address number tries NOTE: _db_block_hash_buckets = # of hash buckets _db_blocks_per_hash_latch = # of hash latches

27. #.27 CBC: what’s the hot block  Can get it from ASH  Current_file#  Current_block#  Where event=‘latch: cache buffers chains”  Sometimes file and block = 0  Seems to happen for Nested Loops  Get the hot block real time  Use Hash Latch Address  Ash.p2 = x$bh.hladdr

28. #.28 Hot Block: X$BH.TCH  Updated when block read  Updated by no more than 1 every 3 seconds  Can be used to find “hot” blocks  Note: set back to zero when block cycles through the buffer cache

29. #.29 CBC – Real Time select count(*), lpad(replace(to_char(p1,'XXXXXXXXX'),' ','0'),16,0) laddr from v$active_session_history where event= 'latch: cache buffers chains' group by p1; select o.name, bh.dbarfil, bh.dbablk, bh.tch from x$bh bh, obj$ o where tch > 100 and hladdr='00000004D8108330' and o.obj#=bh.obj order by tch COUNT(*) LADDR ---------- ---------------- 4933 00000004D8108330 NAME DBARFIL DBABLK TCH ----------- ------- ------ ---- EMP_CLUSTER 4 394 120

30. #.30 Putting into one Query select name, file#, dbablk, obj, tch, hladdr from x$bh bh, obj$ o where o.obj#(+)=bh.obj and hladdr in ( select ltrim(to_char(p1,'XXXXXXXXXX') ) from v$active_session_history where event like 'latch: cache%' group by p1 having count(*) > 5 ) and tch > 5 order by tch NAME FILE# DBABLK OBJ TCH HLADDR ------------- ----- ------ ------ --- -------- BBW_INDEX 1 110997 66051 17 6BD91180 IDL_UB1$ 1 54837 73 18 6BDB8A80 VIEW$ 1 6885 63 20 6BD91180 VIEW$ 1 6886 63 24 6BDB8A80 DUAL 1 2082 258 32 6BDB8A80 DUAL 1 2081 258 32 6BD91180 MGMT_EMD_PING 3 26479 50312 272 6BDB8A80 This can be misleading, as TCH gets set to 0 ever rap around the LRU and it only gets updated once every 3 seconds, so in this case DUAL was my problem table not MGMT_EMD_PING

31. #.31 Consistent Read Blocks Current Block (XCUR) s1 s2 Update Select Consistent Read (CR) Clone & Undo Both have same file# and block# and hash to same bucket

32. #.32 latches CBC: Consistent Read Blocks Cache Buffer Chain Contention: Too Many Buffers in Bucket s5 s4 s3 s2 s1 Hash Buckets Block Headers Max length : _db_block_max_cr_dba 10g = 6

33. #.33 Consistent Read Copies select count(*), name, file#, dbablk, hladdr from x$bh bh, obj$ o where o.obj#(+)=bh.obj and hladdr in ( select ltrim(to_char(p1,'XXXXXXXXXX') ) from v$active_session_history where event like 'latch: cache%' group by p1 ) group by name,file#, dbablk, hladdr having count(*) > 1 order by count(*); CNT NAME FILE# DBABLK HLADDR --- ---------- ------ ------- -------- 14 MYDUAL 1 93170 2C9F4B20

34. #.34 CBC : Solution  Fine the SQL causing the problem  Change Application Logic  Eliminate hot spots  Look up tables  Uses pl/sql functions  Minimize data per block  Possibly using x$dual instead of dual  Index Nested loops  Hash join  Hash partition index  Hah Cluster  Updates, inserts, select for update on blocks while reading those blocks  Cause multiple copies select ash.sql_id, count(*), sql_text from v$active_session_history ash, v$sqlstats sql where event='latch: cache buffers chains' and sql.sql_id(+)=ash.sql_id group by ash.sql_id, sql_text;

35. #.35 Latch: cache buffer handles  Buffers can be pinned  Possibly increase  _db_handles_cached 5  Unsupported  Used when pinning block headers for expected reuse

36. #.36 Free Buffer Wait  Data Block Cache lack free buffers  Tune by  Increase data blocks  Try to tune DBWR  Improving Inefficient SQL  requesting large # of blocks

37. #.37 Free Buffer Wait Finding a Free Block  If the data block isn’t in cache  Get a free block and header in the buffer cache  Read it off disk  Update the free header  Read the block into the buffer cache Need Free Block to Read in New Data Block

38. #.38 Finding a Free Block ShadowProcess When a session reads a block Into the bufffer cache how does it find a FREE spot?

39. #.39 Finding a Free Block Buffer Headers Data Blocks Hash bucket latches 1.Arrange the Buffer Headers into an LRU List 2.Scan LRU for a free block

40. #.40 Cache Buffers LRU = entry in x$bh

41. #.41 X$bh  Describes Buffer Headers SQL> desc x$bh Name Type ---------------------- --------- ADDR RAW(4) DBARFIL NUMBER DBABLK NUMBER OBJ NUMBER HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4) NXT_REPL RAW(4) PRV_REPL RAW(4) NXT_REPL RAW(4) PRV_REPL RAW(4) HLADDR RAW(4) NXT_HASH RAW(4) PRV_HASH RAW(4) Cache buffer chains LRU

42. #.42 LRU Chain 03C39000 03C38F60 03C39000 03C38F60 03C39478 ADDR NXT_HASH PRV_HASH 03C39478 03C38514 03C38638 03C38620 03C385F4 03C38554 NXT_REPL PRV_REPL

43. #.43 Cache Buffers LRU list

44. #.44 Cache Buffers LRU list LRU Chain of Buffer Headers Buffer Cache

45. #.45 Cache Buffers LRU Latch MRU LRU Buffer Headers “Cold” LRU = Least Recently Used MRU = Most Recently Used One LRU Latch protects the linked list during changes to the list “Hot” LRU latch

46. #.46 Session Searching for Free Blocks MRU LRU Buffer Headers Session Shadow 1.Go to the LRU end of data blocks 2.Look for first non-dirty block 3.If search too many post DBWR to make free 4.Free Buffer wait

47. #.47 Free Buffer Wait Solutions  Tune by  Increase data blocks  Try to tune DBWR  ASYNC  If no ASYNC use I/O Slaves (dbwr_io_slaves)  Multiple DBWR (db_writer_processes)  Direct I/O  Tune Inefficient SQL  requesting large # of blocks

48. #.48 Session Finding a Free BlockMRU LRU Hot End Mid-Point Insertion Get LRU Latch Find Free Block Insert Header Release LRU Latch session LRU Latch

49. #.49 DBWR taking Dirty Blocks offMRU LRU Buffer Headers LRU DBWR Dirty List of Buffer Headers LRUW latch LRU latch also covers DBWR list of dirty blocs

50. #.50 Cache Buffers LRU Latch MRU LRU Mid-Point Insertion Oracle Tracks the touch count of blocks. As the block is pushed to the LRU end, if it’s touch count is 3 or more, it’s promoted to the MRU end

51. #.51 Multiple Sets Solution: Multiple Sets _db_block_lru_latches = 8 10gR2 with cpu_count = 2 X$KCBWDS – set descriptor Set 1 Set 2 LRU Latch 1 LRU Latch 2

52. #.52 Working Sets select ds.set_id, ds.blk_size, bp.BUFFERS, nvl(bp.name.’unused’) from x$kcbwds ds, v$buffer_pool bp where ds.set_id >= bp.lo_setid (+) and ds.set_id <= bp.hi_setid (+) / SET_ID BLK_SIZE BUFFERS NAME ---------- ---------- -------- ------- 16 32768 15 32768 14 16384 13 16384 12 8192 11 8192 10 4096 9 4096 8 2048 7 2048 6 8192 4972 DEFAULT 5 8192 4972 DEFAULT 4 8192 3 8192 2 8192 1 8192

53. #.53 Test Case  8 Sessions  reading separate tables  Tables were too big to hold in cache  cache option set on each table  Result : lots of buffer cache churn  Expected to get “latch: cache buffer chains LRU”

54. #.54 simulator lru latch

55. #.55 CBC – Further Investigation select p2, count(*) from v$active_session_history where event= 'latch free' group by p2 select * from v$latchname where latch#=127 P2 COUNT(*) ---------- 127 3556 LATCH# NAME ---------- -------------- 127 simulator lru latch select * from v$event_name where name = 'latch free' PARAMETER1 PARAMETER2 PARAMETER3 ---------- ---------- ---------- address number tries

56. #.56 db_cache_advice Alter system set db_cache_advice=off; Group “other” is very small compared to I/O wait time – not a problem

57. #.57 Cache Buffers LRU Latch : Solution Other  Increase Size of Buffer Cache  Using multiple cache buffers  Keep, recycle  Possibly increase _db_block_lru_latches  Not supported

58. #.58 Buffer Busy Waits  User 1 tries to change a buffer header  User 2 has buffer header “locked” (pinned) 1 2 0 3 User1 User2

59. #.59 BBW Solution Paths 1.Find Block type  Resolve if possible 2.Tune SQL  Find SQL  How often is it called  By how many Users 3.Eliminate Hot Block  Find Object  Find Block Type Block Types:  Undo Header  use AUM (or add more RBS)  Undo Block – hot spot in UNDO  Data  index – hot spot, partition  table – free lists, ASSM, partition  Segment header – free lists  table datablock -> freelists  Freelist blocks – free lists groups  File Header Block – look at extent allocation There is a hot block, eliminate the hot block

60. #.60 BBW: Statspack Top 5 Timed Events Avg %Total ~~~~~~~~~~~~~~~~~~ wait Call Event Waits Time(s) (ms) Time ------------------------ ----- ------- ----- ------ buffer busy waits 5,832 263 45 28.2 log file parallel write 248 125 505 13.4 read by other session 902 103 114 11.1 db file parallel write 2,166 94 43 10.1 db file sequential read 653 81 125 8.7 Top 5 Timed Events Avg %Total ~~~~~~~~~~~~~~~~~~ wait Call Event Waits Time(s) (ms) Time ------------------------ ----- ------- ----- ------ buffer busy waits 5,832 263 45 28.2 log file parallel write 248 125 505 13.4 read by other session 902 103 114 11.1 db file parallel write 2,166 94 43 10.1 db file sequential read 653 81 125 8.7 Class Waits Wait Time (s) Avg Time (ms) ------------------ ----- ------------- ------------- file header block 264 203 769 data block 6,070 162 27 undo header 355 0 1 segment header 44 0 1 Class Waits Wait Time (s) Avg Time (ms) ------------------ ----- ------------- ------------- file header block 264 203 769 data block 6,070 162 27 undo header 355 0 1 segment header 44 0 1 fails to find Object

61. #.61 BBW: ASH Finds  Object  Block Type  SQL Statement CNT OBJ OTYPE SQL_ID BLOCK_TYPE TBS --- --------------- ----- ------------- ----------------- -------- 2 BBW_INDEX_VAL_I INDEX 635xhydd6fzgg segment header SYSTEM 2 0 635xhydd6fzgg usn 5 header UNDOTBS1 3 0 1hsb81ypyrfs5 file header block UNDOTBS1 32 BBW_INDEX_VAL_I INDEX 1hsb81ypyrfs5 data block SYSTEM 33 BBW_INDEX_VAL_I INDEX 6avm49ys4k7t6 data block SYSTEM 34 BBW_INDEX_VAL_I INDEX 5wqps1quuxqr4 data block SYSTEM

62. #.62 BBW: OEM

63. #.63 Solutions

64. #.64 BBW Block Types select rownum n,ws.class from v$waitstat; NAME P1 P2 P3 ----------------- ----- ------ ----- buffer busy waits file# block# class# NAME P1 P2 P3 ----------------- ----- ------ ----- buffer busy waits file# block# class# select * from v$event_name where name = 'buffer busy waits' N CLASS --- ------------------ 1 data block 2 sort block 3 save undo block 4 segment header 5 save undo header 6 free list 7 extent map 8 1st level bmb 9 2nd level bmb 10 3rd level bmb 11 bitmap block 12 bitmap index block 13 file header block 14 unused 15 system undo header 16 system undo block 17 undo header 18 undo block Note: Before 10g, P3 was BBW type If P3 in 100,110,120,130 then read Now “read by other session” Else Write, P3 in 200,210,220,230, 231

65. #.65 Joining ASH with v$waitstat select o.object_name obj, o.object_type otype, ash.SQL_ID, w.class from v$active_session_history ash, ( select rownum class#, class from v$waitstat ) w, all_objects o where event='buffer busy waits' and w.class#(+)=ash.p3 and o.object_id (+)= ash.CURRENT_OBJ# Order by sample_time; OBJ OTYPE SQL_ID CLASS ------ ------ ------------- ------------------ TOTO1 TABLE 8gz51m9hg5yuf data block TOTO1 TABLE 8gz51m9hg5yuf segment header TOTO1 TABLE 8gz51m9hg5yuf data block

66. #.66 Alternative to ASH: AWR select to_char(BEGIN_INTERVAL_TIME,'DD-MON HH:MI'), o.name, s.BUFFER_BUSY_WAITS_DELTA from dba_hist_seg_stat s, dba_hist_snapshot sn, obj$ o where BUFFER_BUSY_WAITS_DELTA > 100 and sn.snap_id = s.snap_id and o.obj# = s.obj#; TO_CHAR(BEGI NAME BUFFER_BUSY_WAITS_DELTA ------------ ----- ---------------------- 11-JAN 10:21 TOTO1 58447

67. #.67 Example: BBW with Insert  Concurrent inserts will insert into the same block  Each session has to wait for the previous session to finish it’s write  Usually pretty fast  Contention builds on highly concurrent applications  Lack of Free Lists  Not Using ASSM (Automatic Segment Space Management)

68. #.68 Example: Lack of Free List S1 S2 S3 S4 4 Sessions running Insert into toto values (null, ‘a’); Commit; OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE ----------- ------ ------ ------ ------------- ------------ 54962 TOTO1 TABLE 16 45012 8gz51m9hg5yuf data block 54962 TOTO1 TABLE 16 161 8gz51m9hg5yuf segment header

69. #.69 Solution1: Free Lists S1 S2 S3 S4 4 Sessions running Insert into toto values (null, ‘a’); Commit;

70. #.70 Solution 2: ASSM  Multiple Bitmap Blocks Track Free Space  Unformatted  Up to 25% Free  Up to 50% Free  Up to 75% Free  Full  Free block chosen by Process ID  Possibly instance # for RAC

71. #.71 Solution 2: ASSM Header Level 2 Level 1 DataBlocks BitmapBlocks

72. #.72 Tablespace Types : ASSM select tablespace_name, extent_management LOCAL, allocation_type EXTENTS, segment_space_management ASSM, initial_extent from dba_tablespaces TABLESPACE_NAME LOCAL EXTENTS ASSM --------------- ---------- --------- ------ SYSTEM LOCAL SYSTEM MANUAL UNDOTBS1 LOCAL SYSTEM MANUAL SYSAUX LOCAL SYSTEM AUTO TEMP LOCAL UNIFORM MANUAL USERS LOCAL SYSTEM AUTO EXAMPLE LOCAL SYSTEM AUTO DATA LOCAL SYSTEM MANUAL create tablespace data2 datafile '/d3/kyle/data2_01.dbf' size 200M segment space management auto;

73. #.73 BBW: ASSM  Consider using Freelists instead of ASSM  Normally waits on ASSM blocks should be too small to warrant using Freelists  ASSM is easier, automatically managed 1st level bmb 2nd level bmb 3rd level bmb

74. #.74 BBW on Index Index Session 1 Session 2 Session 3 Increasing index key creates a hot spot on the leading index leaf OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE --------- ----- ----- ------- ------------- ------------ BBW_INDEX_INDEX 1 113599 97dgthz60u28d data block 1  Use Reverse Key indexes  Breaks Index scans  Hash Partition Index  More IOs per index access

75. #.75 BBW on Index : ADDM Recs Also consider “reversing” the key

76. #.76 Example: BBW on RBS  IF BBW happen on old style RBS  Class# > 18  Switch to UNDO  Old style RBS, the DBA had to figure out # of RBS Segments  With UNDO, it is automatically managed alter system set undo_management=auto scope=spfile;

77. #.77 BBW and RBS Segs OBJN OTYPE FILEN BLOCKN SQL_ID BLOCK_TYPE ----------- ------ ------ ------ ------------- ------------ 54962 TOTO1 TABLE 16 45012 8gz51m9hg5yuf data block 54962 TOTO1 TABLE 16 161 8gz51m9hg5yuf segment header 0 14 9 8gz51m9hg5yuf 87 Select CURRENT_OBJ#||' '||o.object_name objn, o.object_type otype, CURRENT_FILE# filen, CURRENT_BLOCK# blockn, ash.SQL_ID, w.class ||' '||to_char(ash.p3) block_type from v$active_session_history ash, (select rownum class#, class from v$waitstat ) w, all_objects o where event='buffer busy waits' and w.class#(+)=ash.p3 and o.object_id (+)= ash.CURRENT_OBJ# Order by sample_time;

78. #.78 Further Investigation RBS  Old Style RBS if Class# > 18 P1 P2 P3 SQL_ID COUNT(*) CLASS -- ------ -- ------------- -------- -------------- ---- 14 9 87 72wa5hjpzr0by 1 14 9 87 3gkmtvxzu6p2m 1 6 561325 1 7zx1krfcgn88t 8 data block 14 9 87 8s29zyzr55z2t 1 select segment_name, segment_type from dba_extents where file_id = P1 and P2 between block_id and block_id + blocks – 1; select segment_name, segment_type from dba_extents where file_id = P1 and P2 between block_id and block_id + blocks – 1; SEGMENT_NAME SEGMENT_TYPE -------------- R2 ROLLBACK

79. #.79 ADDM finds old style RBS

80. #.80 BBW: File Header  Querying ASH, make sure  P1=current_file#  P2=current_block#  If not, use p1, p2 and not current_object# Time P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE ----- --- --- ---- ----- -- ------ ----------------- 11:44 202 2 -1 0 0 file header block 11:44 202 2 TOTO TABLE 1 60218 file header block SELECT A.OBJECT_ID FROM ALL_OBJECTS A, ( SELECT * FROM ALL_OBJECTS WHERE ROWNUM < 1000) B ORDER BY A.OBJECT_NAME

81. #.81 BBW : File Header Time P1 P2 OBJN OTYPE FN BLOCKN BLOCK_TYPE ----- --- --- ---- ----- -- ------ ----------------- 11:44 202 2 TOTO TABLE 1 60218 file header block Solution is make initial and next extent larger in Temp Table Space ADDM doesn’t say much

82. #.82 write complete waits  Usually happens in tandem with free buffer  Tune by  Increase data block cache  Happens because shadow wants to access blocks that are currently being written to disk by DBWR  also seen it happen when there is a lot of write to sort the waits are on block 2 of the temp tablespace file

83. #.83 Write Complete Waits LRU DBWR Dirty List of Buffer Headers LRUW Sessio n

84. #.84 Buffer Exterminate  Buffer cache dynamically resized  V$SGA_DYNAMIC_COMPONENTS displays information about the dynamic SGA components. This view summarizes information based on all completed SGA resize operations since instance startup.  V$SGA_CURRENT_RESIZE_OPS displays information about SGA resize operations which are currently in progress. An operation can be a grow or a shrink of a dynamic SGA component.  V$SGA_DYNAMIC_FREE_MEMORY displays information about the amount of SGA memory available for future dynamic SGA resize operations. Alter system set db_cache_size=50M;

85. #.85 Summary Buffer Cache Waits 1.latch: cache buffers chains - find SQL  Eliminate hot spots 2.latch: cache buffers lru chain – increase sets 3.Free Buffer Wait - increase cache size 4.Buffer Busy Wait  Index : alleviate hot spots, partition  Data DML : add free lists or use ASSM  File Segment Header : looked at high extent allocations 5.Write Complete Waits - increase cache size