Transcript Flashback Logging Internals

Flashback Logging
Internals
Julian Dyke
Independent Consultant
Web Version - December 2007
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Agenda
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Extended Clusters versus Fast Start Failover
Flashback Database
Flashback Logging Internals
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Extended Clusters
versus
Fast Start Failover
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Extended Clusters
Overview
Public Network
Instance 1
Private Network
Quorum
Node 1
Instance 2
Node 2
Site3
Storage
Storage
Network
Networks
Database
Site1
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Database
Site2
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Extended Clusters
Overview
5

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
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Storage is duplicated at each site
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Can use ASM or vendor-supplied storage technology to
ensure all writes are replicated to storage on each site
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Extended Clusters
Advantages and Disadvantages
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Advantages
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Disaster recovery - all changes written to both sites
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Active / Active - both sites available
Disadvantages
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Complexity
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Cache fusion traffic between sites
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Requires Enterprise Edition licences + RAC option
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Cost of inter-site fibre network
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Fast Start Failover
Overview
Public Network
Instance 1
Private Network
Observer
Quorum
Node 1
Instance 2
Node 2
Site3
Storage
Storage
Network
Networks
Database
Site1 - Primary
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Database
Site2 - Standby
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Fast Start Failover
Overview
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
Target standby database must be nominated
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Failure of primary database can be detected and automatically
failed over to nominated standby database
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Primary database can potentially be reinstated automatically
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Requires flashback logging
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Requires DGMGRL configuration
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Must configure MAXIMUM AVAILABILITY protection mode
 Standby database archive log destination must be
configured as LGWR SYNC
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Fast Start Failover
Advantages & Disadvantages
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9
Advantages
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No interconnect network required between sites
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No fibre network required between sites
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RAC licences not required if each site is a single-instance
Disadvantages
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Active / Passive
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Requires Enterprise Edition licence
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Fast Start Failover
Observer
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
Requires third independent site with:
 Oracle client installation (administrative user)
 Oracle Net configuration to primary and standby
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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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Flashback
Database
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Flashback Database
Introduction
12

Introduced in Oracle 10.1
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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 Database
Introduction
13

During normal database operation, Oracle occasionally logs
past block images in flashback logs
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Flashback logs are
 written sequentially
 not archived
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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 Database
Flashing Back
14

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 Database
Applications
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
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 Database
What do we already know?
16

Introduced in Oracle 10.1
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Requires flash recovery area
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Maintains before image logs for block changes
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Records are appended to flashback logs
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Uses RVWR background process
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Flashback Database
What don't we know?
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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 Area
Prerequisites

Archiving must be enabled
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Flash recovery area must be configured using
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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 Database
Parameters

One supported parameter:
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DB_FLASHBACK_RETENTION_TARGET
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Specifies upper limit on how far back in time database may be
flashed back
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Specified in minutes
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Default value is 1440 minutes (24 hours)

Affects number of flashback logs retained in flash recovery
area
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Flashback Database
Configuration

To enable flashback logging database must be mounted but
not open
SQL> STARTUP MOUNT
SQL> ALTER DATABASE FLASHBACK ON;
SQL> ALTER DATABASE OPEN;

To disable flashback logging use:
SQL> ALTER DATABASE FLASHBACK OFF;

To check if flashback is currently enabled:
SQL> SELECT flashback_on FROM v$database;
FLASHBACK_ON
-----------YES
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Flashback Database
System Change Numbers and Times

To check current SCN use:
SQL> SELECT current_scn FROM v$database;

To check oldest SCN that can be flashed back to use:
SQL> SELECT oldest_flashback_scn FROM v$flashback_database_log;
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To check oldest time that can be flashed back to use:
SQL> ALTER SESSION SET nls_date_format = 'DD-MON-YYYY HH24:MI:SS';
SQL> SELECT oldest_flashback_time FROM v$flashback_database_log;
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Flashback Database
Operation

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
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For example
SQL> SHUTDOWN IMMEDIATE
SQL> STARTUP MOUNT
SQL> FLASHBACK DATABASE TO SCN 461918;
Flashback complete.
SQL> ALTER DATABASE OPEN READ ONLY;
SQL> ALTER DATABASE OPEN RESETLOGS
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Flashback Database
Restrictions
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
Cannot flash back to an SCN ahead of the current SCN
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Cannot flash back to a time in the future
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Database must be opened with read write access
 Cannot open read only
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Database must be opened with RESETLOGS
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Cannot flash back if datafile resized (shrunk) during flashback
period
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Flashback Database
Dynamic Performance Views
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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
© 2007 Julian Dyke
BEGIN_TIME
DATE
END_TIME
DATE
FLASHBACK_DATA
NUMBER
DB_DATA
NUMBER
REDO_DATA
NUMBER
ESTIMATED_FLASHBACK_SIZE
NUMBER
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Flashback Database
Dynamic Performance Views
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V$FLASHBACK_DATABASE_LOGFILE
NAME
VARCHAR2(13)
LOG#
NUMBER
THREAD#
NUMBER
SEQUENCE#
NUMBER
BYTES
NUMBER
FIRST_CHANGE# NUMBER
FIRST_TIME
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DATE
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Flashback
Logging
Internals
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Flashback Log Files
Location and Naming
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Stored in Flash Recovery Area (mandatory)
 Subdirectory is <database_name>/flashback
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Use Oracle-Managed Files (OMF) (mandatory)
For example
 o1_mf_3504ofnh_.flb
 o1_mf_350g3r24_.flb
 o1_mf_350jl666_.flb
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
Used sequentially
 Can be reused

Generated when required
 Dropped when space required in flash recovery area
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Flashback Log Files
Sizing
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
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 Files
Controlfile Dumps
SQL> 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_.flb
Thread 1 flashback log links: forward: 5 backward: 3
size: 486 seq: 4 bsz: 8192 nab: 0x1e7 flg: 0x0 magic: 3 dup: 1
Low scn: 0x0000.00071169 05/20/2007 14:05:08
High scn: 0x0000.00071980 05/02/2007 15:16:48
FLASHBACK LOG FILE #5:
(name #13) /oradata/recovery/PROD/flashback/o1_mf_350p2jz0_.flb
Thread 1 flashback log links: forward: 6 backward: 4
size: 486 seq: 5 bsz: 8192 nab: 0x1e7 flg: 0x0 magic: 5 dup: 1
Low scn: 0x0000.00071980 05/20/2007 15:16:48
High scn: 0x0000.0007247b 05/02/2007 16:43:13
Current Logfile
FLASHBACK LOG FILE #6:
(name #14) /oradata/recovery/PROD/flashback/o1_mf_350v4kz1_.flb
Thread 1 flashback log links: forward: 1 backward: 5
size: 486 seq: 4 bsz: 8192 nab: 0xffffffff flg: 0x0 magic: 4 dup: 1
Low scn: 0x0000.0007247b 05/20/2007 16:43:13
High scn: 0xffff.ffffffff 05/02/2007 00:00:00
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Flashback Logging
Recovery Writer Process
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Flashback uses the recovery writer (RVWR) background
process
 Copies flashback blocks from flashback generation buffer
to flashback logs
SELECT description
FROM v$bgprocess
WHERE name = 'RVWR';
DESCRIPTION
--------------Recovery Writer
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
Checks for records in flashback generation buffer every 3
seconds
 Waits on rdbms ipc message
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In Linux records written to disk using pwrite64
 Multi block writes (8192 byte records)
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Flashback Logging
Recovery Writer Process

Recovery process structure is linked into SGA global area
SELECT addr FROM x$ksbdp
WHERE ksbdpnam = 'RVWR';
ADDR
--------------2000D860
SELECT ksmfsnam,ksmfstyp
FROM x$ksmfsv
WHERE ksmfsadr = '2000D860';
KSMFSNAM KSMFSTYP
-------- ------krfwrp_ ksbdp
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ksbdp structure for RVWR background process is krfwrp_
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Flashback Generation Buffer
Sizing
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Flashback uses a flashback generation buffer
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Size of generation buffer is recorded in V$SGASTAT
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Size is determined by _flashback_generation_buffer_size
 defaults to 4194304
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To verify size of buffer use
SELECT bytes FROM v$sgastat
WHERE pool = 'shared pool'
AND name = 'flashback generation buff';
BYTES
---------4194304
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Flashback Generation Buffer
Granules

Flashback generation buffer appears to be limited to a single
granule
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If granule size is less than _flashback_generation_buffer_size
 buffer size will be rounded down
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For example for a 4mb granule size:
SELECT bytes FROM v$sgastat
WHERE pool = 'shared pool'
AND name = 'flashback generation buff';
BYTES
---------3981204
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Granule size can be controlled using _ksmg_granule_size
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Flashback Generation Buffer
Location

To determine location of flashback generation buffer use:
ALTER SYSTEM SET EVENTS 'immediate trace name global_area level 2';
ksbdp krfwrp_ [2000D860, 2000D88C) = 0000007B 2AE1C924 00000000 00000000 ...
Dump of memory from 0x2000D870 to 0x2000D88C
2000D870 52575652 00000200 00006723 0005A080 [RVWR....#g......]
2000D880 00000001 199DC5EA 00040081
KSBDPPRO = 0X2AE1C924
Location of RVWR
KSBDPSER = 1
KSBDPERR = 0
background
KSBDPNAM = 'RVWR'
process
KSBDPFLG = 2
krfwb krfwbf_ [2000D8DC 2000D970) = 000001E5 00002000 003C7288 00001FE8 ...
Dump of memory from 0x2000D8CC to 0x2000D9F0
2000D8C0
27834200
Location of
2000D8D0 003CBD94 000001E6 000001E6 00000003
2000D8E0 29A1B71C 00000002 00037D60 00000001
flashback
etc..
generation buffer
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In this 32 bit example location is 0x27834200
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Flashback Generation Buffer
Shared Pool Reserved Area
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Size of flashback generation buffer is affected by shared pool
reserved area
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By default 5% of each granule is allocated to shared pool
reserved area
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For example our flashback generation buffer is 0x27834200
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Granule size is 4MB
SELECT MAX(baseaddr), gransize
FROM x$ksmge
WHERE baseaddr <= '27834200';
MAX(BASEADDR) GRANSIZE
------------------------27800000
4194304
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SELECT ksmchptr,ksmchsiz
FROM x$ksmspr
WHERE ksmchptr >= '27800000'
AND ksmchptr < '27C00000';
KSMCHPTR
KSMCHSIZ
----------------------27800038
24
27800050
212888
27833FE8
24
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Flashback Logging
Latches
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
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 Files
Dumps
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The following dumps are undocumented
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All flashback records for a thread can be dumped using:
SQL> ALTER SYSTEM DUMP FLASHBACK THREAD <thread_number>
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In a single instance database thread_number will always be 1
All flashback records for a specific flashback logfile can be
dumped using
SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number>
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Flashback logfiles are numbered from 1 upwards
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Flashback Log Files
Dumps
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All flashback records for a specific record type can be
dumped using:
SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number>
TYPE <type>;
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All flashback records for a specific database block number
can be dumped using:
SQL> ALTER SYSTEM DUMP FLASHBACK LOGFILE <log_file_number>
DBA <absolute_file_number> . <block_number>;


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>
LOGICAL;
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Flashback Log Files
Dumps

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 Database
Dumps
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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 data
Hex dump of block: st=0, typ_found=1
Dump of memory from 0xB56CDC00 to 0xB56CFC00
B56CDC00 0000A206 00406EFC 00034D8E 06010000 [[email protected]......]
B56CDC10 0000F52B 00000001 0000023D 00034D8C [+.......=....M..]
<hex block dump>
.....
<symbolic block dump>
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Flashback Records
Record Types

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Every flashback record has a type
Type
Description
Type
1
Block Image
11
Add Tablespace
2
Marker
12
Resize File
3
Skip
13
Convert Plugin
4
Set
14
Rename Tablespace
5
4 Byte Skip
15
TSPITR
6
Empty Block Image
16
Resetlogs
7
Begin Crash Recovery Record
17
Absolute Set
8
Drop File
18
Primary Switchover
9
Drop Tablespace
19
Standby Switchover
10
Add File
20
Incarnation Change
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Description
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Flashback Logging
RVWR Background Process Dumps


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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 grep
oracle 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'
);
PID
--20
SQL> ORADEBUG SETORAPID 20;
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Flashback Logging
RVWR 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

To dump the last 2000 flashback records use:
SQL> ORADEBUG DUMP FBTAIL 1
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Flashback Log
Physical Structure

Block size determined by db_block_size parameter

Block 0 contains file header

Remaining blocks have 16 byte block header
Includes check sum
File
Header
44
STOP
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Block
Header
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Flashback Records
Logical Structure
45

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 Records
Logical Structure
Record# 1
Body
Header
Record# 2
Record# 3
Body
Header
Body
Header
Record# 4
Header
Record# 5
Body
Header
46
STOP
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Flashback Records
Physiological Structure
File Header
Flashback
Records
Empty Space
47
STOP
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Flashback Records
Block Images
48

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 Logs
Flashback 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
To check flashback log tail use:
SQL> ALTER SESSION SET EVENTS 'immediate trace name controlf level 2';

For example:
****************************************************************
CHECKPOINT PROGRESS RECORDS
****************************************************************
THREAD #1 - status:0x2 flags:0x0 dirty:15
low cache rba:(0xd.1f33.0) on disk rba:(0xd.1f42.0)
on disk scn: 0x0000.0004087e 05/26/2007 18:11:01
resetlogs scn: 0x0000.00000001 05/05/2007 23:07:24
heartbeat: 623592856 mount id: 310450827
Flashback log tail log# 1 thread# 1 seq 10 block 309 byte 0
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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';
krfwb krfwbf_ [2000D8BC, 2000D9F0) = 000001E5
Dump of memory from 0x2000D8CC to 0x2000D9F0
2000D8C0
28434200
2000D8D0 003CBD94 000001E6 000001E6 00000003
2000D8E0 2A61B71C 00000002 003C7288 00000001
2000D8F0 00000001 002A21F0 00000002 00000000
2000D900 00000001 00000000 00000000 00000002
2000D910 00000000 002A01D4 003C6C3C 00000001
2000D920 00000000 00000000 00000002 00000001
2000D930 00000000 00000000 00000152 00000002
2000D940 0000000A 00000135 00000001 000002B4
2000D950 000001E5 00000000 00000000 00000000
2000D960 00000000 24ACC246 00000030 00000001
2000D970 00000001 0000000A 00000009 000003E8
2000D980 00002000 00000000 2A4976FC 2BBBA220
2000D990 2A567EEC 00000047 0000000A 00000001
2000D9A0 00000000 00000001 00000009 00000000
2000D9B0
00000001
00000080 00000800
00000000
Sequence
Number
Block Number
2000D9C0 00000000 00000000 00000000 00000000
0xA = 10
0x135=309
2000D9D0 00000001 00000000 00000000 00000000
2000D9E0 001E5ECA 002A2050 00000000 00000000
50
© 2007 Julian Dyke
00002000 003C7288 00001FE8
[.BC(]
[..<.............]
[..a*.....r<.....]
[.....!*.........]
[................]
[......*.<l<.....]
[................]
[........R.......]
[....5...........]
[................]
[....F..$0.......]
[................]
[. .......vI* ..+]
[.~V*G...........]
[................]
[................]
Log Number
[................]
0x1=1
[................]
[.^..P *.........]
juliandyke.com
Flashback Logging
Conclusions
51

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
© 2007 Julian Dyke
juliandyke.com
Flashback Logging
Conclusions
52

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
© 2007 Julian Dyke
juliandyke.com
Thank you for listening
[email protected]
53
© 2007 Julian Dyke
juliandyke.com