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enq: FB – contention – Oracle Database Internal Mechanism
Posts about enq: FB – contention written by Nitish Srivastava.
Source: databaseinternalmechanism.com
Date Published: 1/15/2021
View: 5235
Enq: FB contention – No its not Facebook Contention
Mostly, the enq: FB contention wait event was followed by log buffer space. Well, the enq: FB contention is associated with block formatting and …
Source: aprakash.wordpress.com
Date Published: 2/7/2021
View: 1690
enq: FB – contention – 文章整合
enq: FB – contention · 1、Pre allocate the extents to the segment having a high number of inserts. · 2、Look for any IO issue that is causing …
Source: chowdera.com
Date Published: 8/1/2022
View: 8275
enq: FB – contention – CodeAntenna
selectwait_,namefromv$event_name wherename=’enq:FB-contention’;WAIT_CLASS …,CodeAntenna技术文章技术问题代码片段及聚合.
Source: codeantenna.com
Date Published: 12/26/2022
View: 1741
enq: FB – contention – 豆奶特
Troubleshooting ‘enq: FB – contention’ Format Block Enqueues. (Doc ID 1379986.1)In this DocumentPurposeDetails If this wait appears in the Top 5 waits, …
Source: www.dounaite.com
Date Published: 12/12/2022
View: 2785
Oracle enqueue wait tips – Burleson Consulting
FB enqueue – This is the Format Block enqueue, used only when data blocks are … HV enqueue – The HV enqueue (a.k.a. enq: HV – contention) is similar to …
Source: www.dba-oracle.com
Date Published: 6/20/2022
View: 6223
Enq: fB-contention – Alibaba Cloud Topic Center
Enq: fB-contention … “This is the format block enqueue, used only when data blocks are using assm (automatic segment space management or …
Source: topic.alibabacloud.com
Date Published: 7/12/2022
View: 1469
Flashback Logging – Oracle Scratchpad – WordPress.com
One of the waits that is specific to ASSM (automatic segment space management) is the “enq: FB – contention” wait. Despite the title, the …
Source: jonathanlewis.wordpress.com
Date Published: 4/13/2022
View: 5200
주제와 관련된 이미지 enq fb contention
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주제에 대한 기사 평가 enq fb contention
- Author: Hoàng Anh Official
- Views: 조회수 5,213회
- Likes: 좋아요 52개
- Date Published: 2021. 6. 6.
- Video Url link: https://www.youtube.com/watch?v=IlYdpx9KPwQ
Oracle Database Internal Mechanism
If you performed lot of DMLs (specially Deletes) on a table, you may have lots of empty leaf blocks in the associated indexes on that table. These empty leaf blocks are left in the b-tree structure AND attached to the freelist. On subsequent inserts, the process might need to split a block so it takes…
Enq: FB contention – No its not Facebook Contention
I am not a facebook fan neither an addict of it, but i still got to see “enq: FB contention”…thats bad :). Well this was for the first time i saw this wait event and it was on 11.2.0.2 version.Mostly, the enq: FB contention wait event was followed by log buffer space.
Well, the enq: FB contention is associated with block formatting and it ensures that only one process format the blocks in an ASSM tablespace.As i can think of, we had multiple sessions doing insert and could be that each session tried to format the block which caused the contention.The value on P1 is name|mode , P2 tells the tablespace number, and P3 gives the dba
SYS@test1 > select session_id,SQL_ID,EVENT,P1,P1text,P2,P3,WAIT_TIME,SESSION_STATE,TIME_WAITED from v$active_session_history where event like ‘%FB%’; SESSION_ID SQL_ID EVENT P1 P1TEXT P2 P3 WAIT_TIME SESSION TIME_WAITED ———- ————- ————————- ———- ————- ———- ———- ———- ——- ———– 775 bg65mx8g077g5 enq: FB – contention 1178730502 name|mode 9 285422249 0 WAITING 189987 …… …… SYS@test1 > select name from v$tablespace where ts#=9; NAME —————————————- ABC_TBS_01 SYS@test1 > SELECT dbms_utility.data_block_address_block(285422249) “BLOCK”, 2 dbms_utility.data_block_address_file(285422249) “FILE” 3 from dual; BLOCK FILE ———- ———- 209577 68
contention
Troubleshooting ‘enq: FB – contention’ Format Block Enqueues. (Doc ID 1379986.1)
In this Document
Purpose
Details
If this wait appears in the Top 5 waits, one can look to implement the following:
References
APPLIES TO:
Oracle Database – Enterprise Edition
Information in this document applies to any platform.
***Checked for relevance on 02-Sep-2016***
Database performance may get affected, inserts/DML might become slow.
PURPOSE
To understand enq: FB Contention, impact of this on Database Performance and how to resolve this contention.
DETAILS
enq: FB – contention is associated with block formatting. This enqueue is acquired to ensure only one process can format the blocks.
**If this wait appears in the Top 5 waits, one can look to implement the following:
1、Pre allocate the extents to the segment having a high number of inserts.
2、Look for any IO issue that is causing slowdown in formatting the blocks. One can see the average wait time for other IO waits like db file sequential read,log file parallel write etc. to see any generic IO issues.
3、Some time high IO operation like large index block split may require more extent allocation and may cause the enq: FB contention.
4、Avoid doing Multiple sessions doing the insert. Each session may try to format the block and can trigger the contention.
5、Check if any Parallel DML is happening; parallel slaves may try to format the blocks simultaneously.**
Bugs related to enq: FB contention:
Bug 6468758: STBH: USER SESSION BLOCKED BY ENQ: FB – CONTENTION
Bug 5557421: SELF-DEADLOCK OCCURS WHEN IMP SESSION IS KILLED
Bug 9239623: STBH: MMON IN CONTROL FILE SEQUENTIAL READ BLOCKING OTHER SESSIONS
BUG 9864080 – INSERT STATEMENT HAS SLOW PERFORMANCE WITH GC CR MULTIBLOCK REQUESTS; BB BUG 8735005
BUG 8735005 – SLOW INSERT – POSSIBLY DUE TO INDEX BLOCK SPLIT
Random recommendation
Oracle enqueue wait tips
Oracle enqueue wait tips Oracle Database Tips by Donald BurlesonNovember 26, 2015 There are many types of Oracle enqueues, here is a sample of the most common types: – CF enqueue – The CF enqueue is a Control File enqueue (a.k.a. enq: CF – contention) and happens during parallel access 6to the control files. The CF enqueue can be seen during any action that requires reading the control file, such as redo log archiving, redo log switches and begin backup commands. – CI Enqueue – The CI enqueue is the Cross Instance enqueue (a.k.a. enq: US – contention) and happens when a session executes a cross instance call such as a query over a database link.
– FB enqueue – This is the Format Block enqueue, used only when data blocks are using ASSM (Automatic Segment Space Management or bitmapped freelists). As we might expect, common FB enqueue relate to buffer busy conditions, especially since ASSM tends to cause performance problems under heavily DML loads.
– HV enqueue – The HV enqueue (a.k.a. enq: HV – contention) is similar to the HW enqueue but for parallel direct path INSERTs.
– HW enqueue – The HW High Water enqueue (a.k.a. enq: HW – contention) occurs when competing processing are inserting into the same table and are trying to increase the high water mark of a table simultaneously. The HW enqueue can sometimes be removed by adding freelists or moving the segment to ASSM.
– KO enqueue – The KO enqueue (a.k.a. enq: KO – fast object checkpoint) is seem in Oracle STAR transformations and high enqueue waits can indicate a sub-optimal DBWR background process.
– PE enqueue – The PE enqueue (a.k.a. enq: PE – contention) is the Parameter Enqueue, which happens after alter system or alter session statements.
– PS enqueue – The PS enqueue is the Parallel Slave synchronization enqueue (a.k.a enq: PS – contention), which is only seen with Oracle parallel query. The PS enqueue happens when pre-processing problems occur when allocating the factotum (slave) processes for OPQ.
– RO Enqueue – The RO enqueue is the Reuse Object enqueue and is a cross-instance enqueue related to truncate table and drop table DDL operations.
– SQ enqueue – The SQ enqueue is the Sequence Cache enqueue (a.k.a. enq: SQ – contention) is used to serialize access to Oracle sequences.
– SS enqueue – The SS enqueue is the Sort Segment enqueue (a.k.a. enq:SS – contention) and these are related to the sorting of large result sets.
– SS enqueue – The SS enqueues are Sort Segment enqueues (a.k.a. enq: SS – contention), and occur when a process is performing a large sort operation.
– ST enqueue – The ST enqueue can be seen in a partitioned environment when a large number of partitions are created simultaneously.
– TC enqueue – The TC enqueue is related to the DBWR background process and occur when -alter tablespace- commands are issued. You will also see the TC enqueue when doing parallel full-table scans where rows are accessed directly, without being loaded into the data buffer cache.
– TM enqueue – The TM enqueue related to Transaction Management (a.k.a. enq: TM – contention) and can be seen when tables are explicitly locked with reorganization activities that require locking of a table.
– TQ enqueue – The TQ enqueue is the Queue Table enqueue (a.k.a. enq: TQ – DDL contention) and happens during Data ump (export import) operations.
– TS enqueue – The TS enqueue is the temporary segment enqueue (a.k.a. enq: TS – contention) and these enqueues happen during disk sort operations.
– TT enqueue – The TT enqueue (a.k.a. enq: TT – contention) is used to avoid deadlocks in parallel tablespace operations. The TT enqueue can be seen with parallel create tablespace and parallel point in time recovery (PITR)
– TX runqueue – The TX enqueue is the transaction enqueue (a.k.a. enq: TX – contention) and is commonly related to buffer busy waits, in conditions where multiple transaction attempt to update the same data blocks.
enq: TX – row lock contention enq: TX – allocate ITL entry enq: TX – row lock contention – UL enqueue – The UL enqueue is a User Lock enqueue (a.k.a. enq: UL – contention) and happens when a lock is requested in dbms_lock.request. The UL enqueue can be seen in Oracle Data Pump.
– US Enqueue – The US enqueue happens with Oracle automatic UNDO management was undo segments are moved online and offline. Also, see my notes on tuning to reduce index contention.
Enq: fB-contention
Select wait_class, name from V $ event_name where name = ‘enq: fB-contention ‘; wait_class name——————————————————————————————–Other Enq: fB-contention SQL> select name, parameter1, parameter2, parameter3 from V $ event_name where name = ‘enq: fB-contention ‘; Name parameter1 parameter2 parameter3——————————————————————————————————————————–Enq: fB-contention name | mode tablespace # DBA”This is the format block enqueue, used only when data blocks are using assm (automatic segment space management or bitmapped freelists).We might fail CT, common FB enqueue relate to buffer busy conditions, especially since assm tends to cause performance problems under heavily DML loads .”
Http://www.dba-oracle.com/t_enqueue_wait_types_.htm
“Well the format block enqueue (” Enq: FB contention “) is related to grabbing
New blocks in assm tablespaces, during an insert operation for example. It
Might be a good idea at this point to find out what the ‘query’ is that you
Are actually doing and if it is happening in parallel etc. Incidentally are
Both of these queries on the one system, or is one prod and one UAT? ”
Http://www.freelists.org/post/oracle-l/Execution-time-in-SHARED-SERVER-vs-DEDICATED-SERVER-RAC-10gR2,12
Reprinted: http://blog.chinaunix.net/uid-499812-id-3046288.html
Flashback Logging
One of the waits that is specific to ASSM (automatic segment space management) is the “enq: FB – contention” wait. Despite the title, the “FB” in this case doesn’t mean “FlashBack”; instead when you query v$lock_type, and v$event_name you will find that the “FB” enqueue has the following description and wait information :
SQL> execute print_table(‘select * from v$lock_type where type = ”FB”’) TYPE : FB NAME : Format Block ID1_TAG : tablespace # ID2_TAG : dba IS_USER : NO DESCRIPTION : Ensures that only one process can format data blocks in auto segment space managed tablespaces SQL> execute print_table(‘select * from v$event_name where name like ”enq: FB%”’) EVENT# : 806 EVENT_ID : 1238611814 NAME : enq: FB – contention PARAMETER1 : name|mode PARAMETER2 : tablespace # PARAMETER3 : dba WAIT_CLASS_ID : 1893977003 WAIT_CLASS# : 0 WAIT_CLASS : Other
This tells us that a process will acquire the lock when it wants to format a batch of blocks in a segment in a tablespace using ASSM – and prior experience tells us that this is a batch of 16 consecutive blocks in the current extent of the segment. When we see a wait for an FB enqueue we can assume that two sessions have simultaneously tried to format the same new batch of blocks and one of them is waiting for the other to complete the format. In some ways this wait can be viewed (like the “read by other session” wait) in a positive light – if the second session weren’t waiting for the first session to complete the block format it would have to do the formatting itself, which means the second end-user is actually going to get an improved response time thanks to the work done by the first session. On the other hand the set of 16 blocks picked by a session is dependent on its process id so the second session might have picked a different set of 16 blocks to format, which means that in the elapsed time of one format call the segment could have had 32 blocks formatted – this wouldn’t have improved the end-user’s response time, but it would mean that more time would pass before another session had to spend time formatting blocks. Basically, in a highly concurrent system, there’s not a lot you can do about FB waits (unless, of course, you do some clever partitioning of the hot objects).
There is actually one set of circumstances where you can have some control of how much time is spent on the wait but before I mention it I’d like to point out a couple more details about the event itself. First, parameter3 / id2_tag is a little misleading: you can use it to work out which blocks are being formatted (if you really need to) but the “dba” is NOT a data block address (which you might think if you look at the name and a few values). There is a special case when the FB enqueue is being held while you format blocks in a 64KB extent if you’re using system allocated extents, and there’s probably a special case (which I haven’t bothered to examine) if you create a tablespace with uniform extents that aren’t a multiple of 16 blocks, but in the general case the “dba” seems to consist of two parts – a base “data block address” and a single (hex) digit offset identifying which batch of 16 blocks will be formatted.
For example: a value of 0x01800242 means start at data block address 0x01800240, count forward 2 * 16 blocks then format 16 blocks from that point onwards. Since the last digit can only range from 0x0 to 0xf this means the first 7 (hex) digits of a “dba” can only reference 16 batches of 16 blocks, i.e. 256 blocks. It’s not coincidence (I assume) that a single bitmap space management block can cover a maximum of 256 consecutive blocks in a segment – the FB enqueue is tied very closely to the bitmap block.
Connecting the FB enqueue to Flashback logging.
So now it’s time to ask why this discussion of the FB enqueue appears in an article titled “Flashback Logging”. Enable the 10704 trace at level 10, along with the 10046 trace at level 8 and you’ll see. Remember that Oracle may have to log the old version of a block before modifying it and if it’s a block that’s being reused it may contribute to the statistic “physical reads for flashback new” – here’s a trace of a “format block” event:
*** 2015-03-10 12:50:35.496 ksucti: init session DID from txn DID: ksqgtl: ksqlkdid: 0001-0023-00000014 *** 2015-03-10 12:50:35.496 *** ksudidTrace: ksqgtl ktcmydid(): 0001-0023-00000014 ksusesdi: 0000-0000-00000000 ksusetxn: 0001-0023-00000014 ksqgtl: RETURNS 0 WAIT #140627501114184: nam=’db file sequential read’ ela= 4217 file#=6 block#=736 blocks=1 obj#=192544 tim=1425991835501051 WAIT #140627501114184: nam=’db file sequential read’ ela= 674 file#=6 block#=737 blocks=1 obj#=192544 tim=1425991835501761 WAIT #140627501114184: nam=’db file sequential read’ ela= 486 file#=6 block#=738 blocks=1 obj#=192544 tim=1425991835502278 WAIT #140627501114184: nam=’db file sequential read’ ela= 522 file#=6 block#=739 blocks=1 obj#=192544 tim=1425991835502831 WAIT #140627501114184: nam=’db file sequential read’ ela= 460 file#=6 block#=740 blocks=1 obj#=192544 tim=1425991835503326 WAIT #140627501114184: nam=’db file sequential read’ ela= 1148 file#=6 block#=741 blocks=1 obj#=192544 tim=1425991835504506 WAIT #140627501114184: nam=’db file sequential read’ ela= 443 file#=6 block#=742 blocks=1 obj#=192544 tim=1425991835504990 WAIT #140627501114184: nam=’db file sequential read’ ela= 455 file#=6 block#=743 blocks=1 obj#=192544 tim=1425991835505477 WAIT #140627501114184: nam=’db file sequential read’ ela= 449 file#=6 block#=744 blocks=1 obj#=192544 tim=1425991835505985 WAIT #140627501114184: nam=’db file sequential read’ ela= 591 file#=6 block#=745 blocks=1 obj#=192544 tim=1425991835506615 WAIT #140627501114184: nam=’db file sequential read’ ela= 449 file#=6 block#=746 blocks=1 obj#=192544 tim=1425991835507157 WAIT #140627501114184: nam=’db file sequential read’ ela= 489 file#=6 block#=747 blocks=1 obj#=192544 tim=1425991835507684 WAIT #140627501114184: nam=’db file sequential read’ ela= 375 file#=6 block#=748 blocks=1 obj#=192544 tim=1425991835508101 WAIT #140627501114184: nam=’db file sequential read’ ela= 463 file#=6 block#=749 blocks=1 obj#=192544 tim=1425991835508619 WAIT #140627501114184: nam=’db file sequential read’ ela= 685 file#=6 block#=750 blocks=1 obj#=192544 tim=1425991835509400 WAIT #140627501114184: nam=’db file sequential read’ ela= 407 file#=6 block#=751 blocks=1 obj#=192544 tim=1425991835509841 *** 2015-03-10 12:50:35.509 ksqrcl: FB,16,18002c2 ksqrcl: returns 0
Note: we acquire the lock (ksqgtl), read 16 blocks by “db file sequential read”, write them to the flashback log (buffer), format them in memory, release the lock (ksqrcl). That lock can be held for quite a long time – in this case 13 milliseconds. Fortunately, as the output above shows us, the single block reads after the first one may all be accelerated by O/S prefetching: your timings may vary.
The higher the level of concurrent activity the more likely it is that processes will collide trying to format the same 16 blocks (the lock is exclusive, so the second will request and wait, then find that the blocks are already formatted when it finally gets the lock). This brings me to the special case where waits for the FB enqueue waits might have a noticeable impact … if you’re running parallel DML and Oracle decides to use “High Water Mark Brokering”, which means the parallel slaves are inserting data into a single segment instead of each using its own private segment and leaving the query co-ordinator to clean up round the edges afterwards. I think this is most likely to happen if you have a tablespace using fairly large extents and Oracle thinks you’re going to process a relatively small amount of data (e.g. small indexes on large tables) – the trade-off is between collisions between processes and wasted space from the private segments.
키워드에 대한 정보 enq fb contention
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