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Flashback Logging Internals

Julian Dyke

Independent Consultant

Web Version - December 2007

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Agenda

Extended Clusters versus Fast Start Failover Flashback Database Flashback Logging Internals

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Extended Clustersversus

Fast Start Failover

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Extended ClustersOverview

Storage Network

Public Network

Database Database

Storage Networks

Site2Site1

Private Network

Instance 2

Node 2

Quorum

Site3

Instance 1

Node 1

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Extended ClustersOverview

Currently the Holy Grail of high availability

RAC nodes located at physically separate sites In-built disaster recovery

In the event of a site failure, database is still available

Active / Active configuration Users can access database via either site

Storage is duplicated at each site

Can use ASM or vendor-supplied storage technology to ensure all writes are replicated to storage on each site

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Extended ClustersAdvantages and Disadvantages

Advantages

Disaster recovery - all changes written to both sites

Active / Active - both sites available

Disadvantages

Complexity

Cache fusion traffic between sites

Requires Enterprise Edition licences + RAC option

Cost of inter-site fibre network

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Fast Start FailoverOverview

Storage Network

Public Network

Database Database

Storage Networks

Site2 - StandbySite1 - Primary

Private Network

Instance 2

Node 2

Quorum

Site3

Instance 1

Node 1

Observer

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Fast Start FailoverOverview

Target standby database must be nominated

Failure of primary database can be detected and automatically failed over to nominated standby database

Primary database can potentially be reinstated automatically

Requires flashback logging

Requires DGMGRL configuration

Must configure MAXIMUM AVAILABILITY protection mode Standby database archive log destination must be

configured as LGWR SYNC

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Fast Start FailoverAdvantages & Disadvantages

Advantages

No interconnect network required between sites

No fibre network required between sites

RAC licences not required if each site is a single-instance

Disadvantages

Active / Passive

Requires Enterprise Edition licence

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Fast Start FailoverObserver

Requires third independent site with: Oracle client installation (administrative user) Oracle Net configuration to primary and standby

On third site: DGMGRL starts observer Observer monitors state of primary database If primary database fails observer initiates failover to target

standby database

Observer checks if standby database can still see primary database before initiating failover

Performance impact of observer process on primary / standby is minimal

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FlashbackDatabase

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Flashback DatabaseIntroduction

Introduced in Oracle 10.1

Uses past block images to back out changes to a database

Allows database to be recovered to a previous time to correct problems caused by: logical data corruptions user errors

Amount of time required to flashback a database is proportional to how far back database must be reverted

Time to restore and recover entire database could be much longer

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Flashback DatabaseIntroduction

During normal database operation, Oracle occasionally logs past block images in flashback logs

Flashback logs are written sequentially not archived

Oracle automatically creates, resizes and deletes flashback logs in the flash recovery area

DBA should be aware of flashback logs To monitor performance To decide how much space to allocate to flash recovery

area

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Flashback DatabaseFlashing Back

Before images are used to restore database to a point in the past Forward recovery is then used to bring the database to a

consistent state

Oracle returns datafiles to previous point in time Not auxiliary files such as initialization parameter files

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Flashback DatabaseApplications

Flashback recovery of database to earlier SCN Testing Application / User errors

Recovery through resetlogs

Opening standby database with write access

Fast start failover

Automatic reinstantiation of old primary following fast start failover to standby

Alternative to delayed redo application for physical or logical standby databases

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Flashback DatabaseWhat do we already know?

Introduced in Oracle 10.1

Requires flash recovery area

Maintains before image logs for block changes

Records are appended to flashback logs

Uses RVWR background process

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Flashback DatabaseWhat don't we know?

Are index blocks logged? Is undo logged? Is temporary segments logged? What happens when a segment is deleted Is a block logged every time it is changed? If not, how does Oracle know? What when an object leaves the buffer cache Is there any control structure What about multiple block sizes? How does it work in RAC? What about contention - latches? Undocumented parameters? When is flashback overwritten?

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Flash Recovery AreaPrerequisites

Archiving must be enabled

Flash recovery area must be configured using

DB_RECOVERY_FILE_DEST_SIZE - size of flashback recovery area in bytes

DB_RECOVERY_FILE_DEST - location of flashback recovery area

For example:

SQL> ALTER SYSTEM SET db_recovery_file_dest_size = 10G;

SQL> ALTER SYSTEM SET db_recovery_file_dest = '/oradata/recovery';

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Flashback DatabaseParameters

One supported parameter:

DB_FLASHBACK_RETENTION_TARGET

Specifies upper limit on how far back in time database may be flashed back

Specified in minutes

Default value is 1440 minutes (24 hours)

Affects number of flashback logs retained in flash recovery area

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Flashback DatabaseConfiguration

To enable flashback logging database must be mounted but not open

SQL> STARTUP MOUNTSQL> ALTER DATABASE FLASHBACK ON;SQL> ALTER DATABASE OPEN;

To check if flashback is currently enabled:

SQL> SELECT flashback_on FROM v$database;

To disable flashback logging use:

SQL> ALTER DATABASE FLASHBACK OFF;

FLASHBACK_ON------------YES

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Flashback DatabaseSystem Change Numbers and Times

To check current SCN use:

SQL> SELECT current_scn FROM v$database;

To check oldest SCN that can be flashed back to use:

SQL> SELECT oldest_flashback_scn FROM v$flashback_database_log;

To check oldest time that can be flashed back to use:

SQL> ALTER SESSION SET nls_date_format = 'DD-MON-YYYY HH24:MI:SS';

SQL> SELECT oldest_flashback_time FROM v$flashback_database_log;

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Flashback DatabaseOperation

To flashback the database use the following syntax:

SQL> FLASHBACK [ STANDBY ] DATABASE [ database ]{ TO { { SCN | TIMESTAMP } expr | RESTORE POINT restore_point }| TO BEFORE { SCN | TIMESTAMP } expr | RESETLOGS} };

Database must be mounted and not open to flashback

For example

SQL> SHUTDOWN IMMEDIATESQL> STARTUP MOUNTSQL> FLASHBACK DATABASE TO SCN 461918;

Flashback complete.

SQL> ALTER DATABASE OPEN READ ONLY;

SQL> ALTER DATABASE OPEN RESETLOGS

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Flashback DatabaseRestrictions

Cannot flash back to an SCN ahead of the current SCN

Cannot flash back to a time in the future

Database must be opened with read write access Cannot open read only

Database must be opened with RESETLOGS

Cannot flash back if datafile resized (shrunk) during flashback period

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Flashback DatabaseDynamic Performance Views

V$FLASHBACK_DATABASE_LOG

OLDEST_FLASHBACK_SCN NUMBER

OLDEST_FLASHBACK_TIME DATE

RETENTION_TARGET NUMBER

FLASHBACK_SIZE NUMBER

ESTIMATED_FLASHBACK_SIZE

NUMBER

V$FLASHBACK_DATABASE_STAT

BEGIN_TIME DATE

END_TIME DATE

FLASHBACK_DATA NUMBER

DB_DATA NUMBER

REDO_DATA NUMBER

ESTIMATED_FLASHBACK_SIZE

NUMBER

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Flashback DatabaseDynamic Performance Views

V$FLASHBACK_DATABASE_LOGFILE

NAME VARCHAR2(13)

LOG# NUMBER

THREAD# NUMBER

SEQUENCE# NUMBER

BYTES NUMBER

FIRST_CHANGE# NUMBER

FIRST_TIME DATE

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FlashbackLoggingInternals

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Flashback Log FilesLocation and Naming

Stored in Flash Recovery Area (mandatory) Subdirectory is <database_name>/flashback

Use Oracle-Managed Files (OMF) (mandatory) For example

o1_mf_3504ofnh_.flb o1_mf_350g3r24_.flb o1_mf_350jl666_.flb

Used sequentially Can be reused

Generated when required Dropped when space required in flash recovery area

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Flashback Log FilesSizing

Flashback log size same as database block size e.g. 4096 or 8192

Initial size is 1001 x block size determined by

_flashback_log_size (defaults to 1000) additional block for file header

e.g 1001 x 8192 = 8200192 bytes

Subsequent size reduces to 3989504 probably determined by

size of flashback generation buffer (3981204) additional block for file header

note there is a rounding error here

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Flashback Log FilesControlfile DumpsSQL> ALTER SESSION SET EVENTS 'immediate trace name controlf level 3';

*******************************************************************FLASHBACK LOGFILE RECORDS*******************************************************************FLASHBACK LOG FILE #4: (name #12) /oradata/recovery/PROD/flashback/o1_mf_350kw47d_.flbThread 1 flashback log links: forward: 5 backward: 3size: 486 seq: 4 bsz: 8192 nab: 0x1e7 flg: 0x0 magic: 3 dup: 1Low scn: 0x0000.00071169 05/20/2007 14:05:08High scn: 0x0000.00071980 05/02/2007 15:16:48

FLASHBACK LOG FILE #5: (name #13) /oradata/recovery/PROD/flashback/o1_mf_350p2jz0_.flbThread 1 flashback log links: forward: 6 backward: 4size: 486 seq: 5 bsz: 8192 nab: 0x1e7 flg: 0x0 magic: 5 dup: 1Low scn: 0x0000.00071980 05/20/2007 15:16:48High scn: 0x0000.0007247b 05/02/2007 16:43:13

FLASHBACK LOG FILE #6: (name #14) /oradata/recovery/PROD/flashback/o1_mf_350v4kz1_.flbThread 1 flashback log links: forward: 1 backward: 5size: 486 seq: 4 bsz: 8192 nab: 0xffffffff flg: 0x0 magic: 4 dup: 1Low scn: 0x0000.0007247b 05/20/2007 16:43:13High scn: 0xffff.ffffffff 05/02/2007 00:00:00

Current Logfile

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Flashback LoggingRecovery Writer Process

Flashback uses the recovery writer (RVWR) background process Copies flashback blocks from flashback generation buffer

to flashback logs

Checks for records in flashback generation buffer every 3 seconds Waits on rdbms ipc message

In Linux records written to disk using pwrite64 Multi block writes (8192 byte records)

SELECT descriptionFROM v$bgprocessWHERE name = 'RVWR';

DESCRIPTION---------------Recovery Writer

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Flashback LoggingRecovery Writer Process

Recovery process structure is linked into SGA global area

SELECT ksmfsnam,ksmfstyp FROM x$ksmfsvWHERE ksmfsadr = '2000D860';

SELECT addr FROM x$ksbdpWHERE ksbdpnam = 'RVWR';

ADDR---------------2000D860

ksbdp structure for RVWR background process is krfwrp_

KSMFSNAM KSMFSTYP-------- -------krfwrp_ ksbdp

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Flashback Generation BufferSizing

Flashback uses a flashback generation buffer

Size of generation buffer is recorded in V$SGASTAT

Size is determined by _flashback_generation_buffer_size defaults to 4194304

To verify size of buffer use

SELECT bytes FROM v$sgastatWHERE pool = 'shared pool'AND name = 'flashback generation buff';

BYTES----------4194304

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Flashback Generation BufferGranules

Flashback generation buffer appears to be limited to a single granule

If granule size is less than _flashback_generation_buffer_size buffer size will be rounded down

For example for a 4mb granule size:

SELECT bytes FROM v$sgastatWHERE pool = 'shared pool'AND name = 'flashback generation buff';

BYTES----------3981204

Granule size can be controlled using _ksmg_granule_size

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Flashback Generation BufferLocation

To determine location of flashback generation buffer use:

ALTER SYSTEM SET EVENTS 'immediate trace name global_area level 2';

krfwb krfwbf_ [2000D8DC 2000D970) = 000001E5 00002000 003C7288 00001FE8 ...Dump of memory from 0x2000D8CC to 0x2000D9F02000D8C0 27834200 2000D8D0 003CBD94 000001E6 000001E6 000000032000D8E0 29A1B71C 00000002 00037D60 00000001 etc..

Location of flashback

generation buffer

ksbdp krfwrp_ [2000D860, 2000D88C) = 0000007B 2AE1C924 00000000 00000000 ...Dump of memory from 0x2000D870 to 0x2000D88C2000D870 52575652 00000200 00006723 0005A080 [RVWR....#g......]2000D880 00000001 199DC5EA 00040081 KSBDPPRO = 0X2AE1C924 KSBDPSER = 1 KSBDPERR = 0 KSBDPNAM = 'RVWR' KSBDPFLG = 2

Location of RVWR background

process

In this 32 bit example location is 0x27834200

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Flashback Generation BufferShared Pool Reserved Area

Size of flashback generation buffer is affected by shared pool reserved area

By default 5% of each granule is allocated to shared pool reserved area

For example our flashback generation buffer is 0x27834200

Granule size is 4MB SELECT ksmchptr,ksmchsiz FROM x$ksmsprWHERE ksmchptr >= '27800000'AND ksmchptr < '27C00000';

SELECT MAX(baseaddr), gransize FROM x$ksmgeWHERE baseaddr <= '27834200';

MAX(BASEADDR) GRANSIZE-------------------------27800000 4194304

KSMCHPTR KSMCHSIZ-----------------------27800038 2427800050 21288827833FE8 24

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Flashback LoggingLatches

The following latches are used by flashback logging flashback allocation flashback mapping flashback copy flashback sync request flashback FBA barrier flashback SCN barrier hint flashback FBA barrier flashback hint SCN barrier

By default each latch only has one child except flashback copy latch maximum number of copy latches may be determined

by _flashback_copy_latches

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Flashback Log FilesDumps

The following dumps are undocumented

All flashback records for a thread can be dumped using:

SQL> ALTER SYSTEM DUMP FLASHBACK THREAD <thread_number>

SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number>

In a single instance database thread_number will always be 1

All flashback records for a specific flashback logfile can be dumped using

Flashback logfiles are numbered from 1 upwards

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Flashback Log FilesDumps

All flashback records for a specific record type can be dumped using:

All flashback records for a specific database block number can be dumped using:

By default block dumps etc are included in the dump file To dump a summary of records in the flashback log use:

SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number> DBA <absolute_file_number> . <block_number>;

SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number> LOGICAL;

SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number> TYPE <type>;

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Flashback Log FilesDumps

Example of header

DUMP OF FLASHBACK LOG FILE 9 FILE HEADER: Compatibility Vsn = 169869568=0xa200100 Db ID=308670124=0x1265eeac, Db Name='FLASH' Activation ID=308689068=0x126638ac Control Seq=318=0x13e, File size=972=0x3cc File Number=9, Blksiz=8192, File Type=8 FLASH BACK FLASHBACK HEADER: Flashback Block Header: Seq: 9 Block: 1 Cks: 0x22b Flag: 0x1 Lst: 0 description:"Thread 0001, Seq# 0000000009, SCN 0x00000003a2d7" thread: 1 seq: 9 version 0 nab: 0x3cd reset logs count: 0x25102f2c scn: 0x0000.00000001 formatted blocks: 972 usable blocks: 972 magic: 5 previous magic: 0 flags: 0x0 Low scn: 0x0000.0003a2d7 05/07/2007 10:31:48 High scn: 0x0000.000401d3 05/26/2007 16:59:06 Last Marker: fba: (lno 0 thr 0 seq 0 bno 0 bof 0)

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Flashback DatabaseDumps

Example of block image

**** Record at fba: (lno 9 thr 1 seq 9 bno 966 bof 692) **** RECORD HEADER: Type: 1 (Block Image) Size: 28 RECORD DATA (Block Image): file#: 1 rdba: 0x00406efc Next scn: 0x0000.00000000 [0.0] Flag: 0x0 Block Size: 8192 BLOCK IMAGE: buffer rdba: 0x00406efc scn: 0x0000.00034d8e seq: 0x01 flg: 0x06 tail: 0x4d8e0601 frmt: 0x02 chkval: 0xf52b type: 0x06=trans dataHex dump of block: st=0, typ_found=1Dump of memory from 0xB56CDC00 to 0xB56CFC00B56CDC00 0000A206 00406EFC 00034D8E 06010000 [.....n@..M......]B56CDC10 0000F52B 00000001 0000023D 00034D8C [+.......=....M..]

<hex block dump>

.....

<symbolic block dump>

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Flashback RecordsRecord Types

Every flashback record has a type

Type Description

1 Block Image

2 Marker

3 Skip

4 Set

5 4 Byte Skip

6 Empty Block Image

7 Begin Crash Recovery Record

8 Drop File

9 Drop Tablespace

10 Add File

Type Description

11 Add Tablespace

12 Resize File

13 Convert Plugin

14 Rename Tablespace

15 TSPITR

16 Resetlogs

17 Absolute Set

18 Primary Switchover

19 Standby Switchover

20 Incarnation Change

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Flashback LoggingRVWR Background Process Dumps

Some additional RVWR background process dumps can be executed from ORADEBUG

Dumping session must attach to RVWR process Either use operating system process id

$ ps -ef | grep rvwr | grep -v greporacle 11055 1 0 16:04 ? 00:00:00 ora_rvwr_PROD

SQL> ORADEBUG SETOSPID 11055;

Or use Oracle process id

SQL> SELECT pid FROM v$process WHERE addr IN(

SELECT paddr FROM v$bgprocess WHERE name = 'RVWR');

SQL> ORADEBUG SETORAPID 20;

PID---20

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Flashback LoggingRVWR Background Process Dumps

To dump flashback generation status use:

SQL> ORADEBUG DUMP FLASHBACK_GEN 1

To dump flashback logfile headers use:

SQL> ORADEBUG DUMP FBHDR 1

To dump all logical flashback records in the current flashback incarnation use:

SQL> ORADEBUG DUMP FBINC 1

To include before images in the above dump use:

SQL> ORADEBUG DUMP FBINC 2

SQL> ORADEBUG DUMP FBTAIL 1

To dump the last 2000 flashback records use:

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Flashback LogPhysical Structure

Block size determined by db_block_size parameter

Block 0 contains file header

Remaining blocks have 16 byte block headerIncludes check sum

Block Header

FileHeader

STOP

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Flashback RecordsLogical Structure

Added sequentially to flashback logs

Consists of a header and an optional body

If present body is written first followed by header

For all record types Header includes type and length Structure is read backwards

Logical records can cross physical record boundaries

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Flashback RecordsLogical Structure

Record# 1

Record# 2

Record# 3

Record# 4

Record# 5

Body

Body

Body

Header

Body

Header

Header

Header

Header

STOP

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Flashback RecordsPhysiological StructureFile Header

FlashbackRecords

Empty Space

STOP

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Flashback RecordsBlock Images

For block images Body is a copy of the data block Used for data blocks, undo blocks Not compressed Flashback records are always larger than single block Include 28 byte header

Common block types appearing as block images include Data and index blocks (trans data) Segment headers Undo headers Undo blocks (manual and automatic) Local tablespace bitmap blocks Automatic segment space management bitmap blocks

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Flashback LogsFlashback log tail

New flashback records are always appended beyond the flashback log tail

Flashback database commands start at the flashback log tail and work forwards

SQL> ALTER SESSION SET EVENTS 'immediate trace name controlf level 2';

****************************************************************CHECKPOINT PROGRESS RECORDS****************************************************************THREAD #1 - status:0x2 flags:0x0 dirty:15low cache rba:(0xd.1f33.0) on disk rba:(0xd.1f42.0)on disk scn: 0x0000.0004087e 05/26/2007 18:11:01resetlogs scn: 0x0000.00000001 05/05/2007 23:07:24heartbeat: 623592856 mount id: 310450827Flashback log tail log# 1 thread# 1 seq 10 block 309 byte 0

For example:

To check flashback log tail use:

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krfwb krfwbf_ [2000D8BC, 2000D9F0) = 000001E5 00002000 003C7288 00001FE8Dump of memory from 0x2000D8CC to 0x2000D9F02000D8C0 28434200 [.BC(]2000D8D0 003CBD94 000001E6 000001E6 00000003 [..<.............]2000D8E0 2A61B71C 00000002 003C7288 00000001 [..a*.....r<.....]2000D8F0 00000001 002A21F0 00000002 00000000 [.....!*.........]2000D900 00000001 00000000 00000000 00000002 [................]2000D910 00000000 002A01D4 003C6C3C 00000001 [......*.<l<.....]2000D920 00000000 00000000 00000002 00000001 [................]2000D930 00000000 00000000 00000152 00000002 [........R.......]2000D940 0000000A 00000135 00000001 000002B4 [....5...........]2000D950 000001E5 00000000 00000000 00000000 [................]2000D960 00000000 24ACC246 00000030 00000001 [....F..$0.......]2000D970 00000001 0000000A 00000009 000003E8 [................]2000D980 00002000 00000000 2A4976FC 2BBBA220 [. .......vI* ..+]2000D990 2A567EEC 00000047 0000000A 00000001 [.~V*G...........]2000D9A0 00000000 00000001 00000009 00000000 [................]2000D9B0 00000001 00000080 00000800 00000000 [................]2000D9C0 00000000 00000000 00000000 00000000 [................]2000D9D0 00000001 00000000 00000000 00000000 [................]2000D9E0 001E5ECA 002A2050 00000000 00000000 [.^..P *.........]

Flashback Logs Flashback log tail

Current pointer is also maintained in SGA. For example:

SQL> ALTER SESSION SET EVENTS 'immediate trace name global_area level 2';

Block Number0x135=309

Log Number0x1=1

Sequence Number0xA = 10

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Flashback LoggingConclusions

Very similar design to LGWR Changes initially written to memory buffer RVWR subsequently flushes flashback records to disk

Requires memory buffer Defaults to 4MB Efficient multi-block disk writes

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Flashback LoggingConclusions

All blocks are logged when they first become dirty Includes data, indexes, undo, segment headers, bitmaps Subsequent changes not necessarily logged No separate structure identified so probably uses flags in

buffer headers to monitor which blocks have been logged Flag may be reset when DBWR flushes dirty block to disk

Amount of flashback redo log generated roughly equivalent to value of physical writes statistics

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Thank you for listening

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