Log Buffer to disk?

Hi,
When the information in the Log Buffer is saved to disk?
Thanks,
Felipe

Hi Felipe,
Let’s conduct a brief summary about the redo process. When Oracle blocks are changed, including undo blocks, oracle records the changes in a form of vector changes which are referred to as redo entries or redo records. The changes are written by the server process to the redo log buffer in the SGA. The redo log buffer is then flushed into the online redo logs in near real time fashion by the log writer LGWR.
The redo logs are written by the LGWR when:
•     When a user issue a commit.
•     When the Log Buffer is 1/3 full.
•     When the amount of redo entries is 1MB.
•     Every three seconds
•     When a database checkpoint takes place. The redo entries are written before the checkpoint to ensure recoverability.
Remember that redo logs heavily influence database performance because a commit cannot complete until the transaction information has been written to the logs. You must place your redo log files on your fastest disks served by your fastest controllers. If possible, do not place any other database files on the same disks as your redo log files. Because only one group is written to at a given time, there is no harm in having members from several groups on the same disk.
To avoid losing information that could be required to recover the database at some point, Oracle has an archive (ARCn) background process that archives redo log files when they become filled. However, it’s important to note not all Oracle Databases will have the archive process enabled. An instance with archiving enabled, is said to operate in ARCHIVELOG mode and an instance with archiving disabled is said to operate in NO ARCHIVELOG mode.
You can determine with mode or if archiving is been used in your instance either by checking the value for the LOG_ARCHIVE_START parameter in your instance startup parameter file (pfile or spfile – This parameter is deprecated on version 10g), by issuing an SQL query to the v$database (“ARCHIVELOG” indicates archiving is enabled, and “NOARCHIVELOG” indicates that archiving is not enabled) or by issuing the SQL ARCHIVE LOG LIST command.
SQL> Select log_mode from v$database;
LOG_MODE
ARCHIVELOG
SQL> archive log list
Database log mode                    Archive Mode
Automatic archival                    Enabled
Archive destination                   USE_DB_RECOVERY_FILE_DEST
Oldest online log sequence       8
Next log sequence to archive   10
Current log sequence               10The purpose of redo generation is to ensure recoverability. This is the reason why, Oracle does not give the DBA a lot of control over redo generation. If the instance crashes, then all the changes within SGA will be lost. Oracle will then use the redo entries in the online redo files to bring the database to a consistent state. The cost of maintaining the redolog records is an expensive operation involving latch management operations (CPU) and frequent write access to the redolog files (I/O).
Regards,
Francisco Munoz Alvarez
www.oraclenz.com

Similar Messages

To where does the LGWR write information in redo log buffer ?

Suppose my online redo logfiles are based on filesystems .I want to know to where the LGWR writes information in redo log buffer ? Just write to filesystem buffer or directly write to disk ? And the same case is associated with the DBWR and the datafiles are based on filesystems too ?

It depends on the filesytem. Normally there is also a filesystem buffer too, which is where LGWR would write.Yes but a redo log write must always be a physical write.
From http://asktom.oracle.com/pls/ask/f?p=4950:8:15501909858937747903::NO::F4950_P8_DISPLAYID,F4950_P8_CRITERIA:618260965466
Tom, I was thinking of a scenario that sometimes scares me...
**From a database perspective** -- theoretically -- when data is commited it
inevitably goes to the redo log files on disk.
However, there are other layers between the database and the hardware. I mean,
the commited data doesn't go "directly" to disk, because you have "intermediate"
structures like i/o buffers, filesystem buffers, etc.
1) What if you have commited and the redo data has not yet "made it" to the redo
log. In the middle of the way -- while this data is still in the OS cache -- the
OS crashes. So, I think, Oracle is believing the commited data got to the redo
logs -- but is hasn't in fact **from an OS perspective**. It just "disapeared"
while in the OS cache. So redo would be unsusable. Is it a possible scenario ?
the data does go to disk. We (on all os's) use forced IO to ensure this. We
open files for example with O_SYNC -- the os does not return "completed io"
until the data is on disk.
It may not bypass the intermediate caches and such -- but -- it will get written
to disk when we ask it to.
1) that'll not happen. from an os perspective, it did get to disk
Message was edited by:
Pierre Forstmann

Redo log buffer question

hi masters,
this seems to be very basic, but i would like to know internal of this process.
we all know that LGWR writes redo entries to online redo log files on disk. on commit SCN is generated and tagged to transaction. and LGWR writes this to online redo log files.
but my question is, how these redo entries comes to redo log buffer??? look all required data is fetched into buffer cache by server process. it is modified there, and committed. DBWR writes this to datafiles, but at what point, which process writes this committed transaction (i think redo entry) into log buffer cache????
does LGWR do this?? what internally happens exactly???
if you can plz focus some light on internals, i will be thankfull....
thanks and regards
VD

Hi Vikrant,
DBWR writes this to datafiles, but at what point, which process writes this committed transaction (i think redo entry) into log buffer cache????Remember that, Before DBWR Acts on flushing the dirty Blocks to Data files, Before this server process, makes sure that LGWR finishes the writing Redo Log Buffer to Online Redo Log files. Since as per ORACLE Architecture poting of Recovering data till point of time @ Crash is important and this is achieved by Online Redo Logs files.
Rest how the data is Updated in to the Redo Log Buffer Aman had stated the clear steps.
- Pavan Kumar N

I/O Error when writing access Log buffer to file. error number: 28

Hi,
Oracle OracleAS Web Cache 10.1.2.3.0, Build 10.1.2.3.0 080201 is writing events like this:
[alert 13215] I/O Error when writing access Log buffer to file. error number: 28
I've looked for this alert and i've found this description:
WXE-13215 I/O Error when writing access Log buffer to file. error number: %d
Severity: alert
Cause: I/O error happened when OracleAS Web Cache tried to write to the access log file.
Action: Check the status of access log file. For example, see if the disk is full
Anybody knows what "error number: 28" stands for? The logs are sended to other server and it seems that the disk size is ok.
thanks!

I tested again...it seems LabVIEW doesn't like the way I wrote the measurement files. I attached a set of VIs (ZIP-file) to give you a clue how it was done. When I try to save and read a file containing just plain DBLs from a 1D-Array (see attached file generating VI) it works perfectly well for large file sizes.
So there might get something messed up by the way LabVIEW saved the measured data. You'll notice, that I (accidentally) left the "prepend array string size" input of "write to binary file" function unwired which means that the size information will be written as a type of header. Maybe this is the reason that it doesn't work as expected...
Attachments:
VibrationMeasurement.zip ‏76 KB
generate_test_bin_data.vi ‏12 KB

What does redo log buffer holds, changed value or data block?

Hello Everyone,
i am new to database side and have one query as i know redo log buffer contain change information , my doubt is does it store the value only or the changed data block? because if we can see data buffer cache size is more as it holds data block and redo log buffer size is less .

The Redo Log buffer contains OpCodes that represent the SQL commands, the "address" (file,block,row) where the change is to be made and the nature of the change.
It does NOT contain the data block.
(the one exception is when you run a User Managed Backup with ALTER DATABASE BEGIN BACKUP or ALTER TABLESPACE BEGIN BACKUP : The first time a block is modified when in BEGIN BACKUP mode, the whole block is written to the redo stream).
The log buffer can be and is deliberately smaller than the blocks buffer cache. Entries in the redo log buffer are quickly written to disk (at commits, when it is 1/3rd or 1MB full, every 3seconds, before DBWR writes a modified data block).
Hemant K Chitale

About Log Buffer writhing..

Hi,
Wondering how Log Buffer behaves...
1. If timesten is configured as single node(no replication and cache option), When does log buffer data write to log file?
By running some test, it seems like only at checkpoint action it is being write.
Is there any other way to write log buffer data beside with checkpoint action?
2. Which process is doing writing action from log buffer to log file?
Is it the same process that uses with replication and cache option?
3. In TimesTen Manual ...
asynchronous replication "TimesTen Data Manager writes the transaction update records to the transaction log buffer."
Return Twosafe "The master replication agent writes the transaction records to the log and inserts a special precommit log record before the commit record."
Then ... does it mean that log buffer write process is different according to replication type?
4. I am assuming Log_Buffer_Wait from 'monitor;' output is the waiting time of Log Buffer writing time to logfile...
If it is correct, is the possibility of Log_Buffer_Wait occurance increases when log buffer size is large with no replication option?
w
I am willing to hear for above.
Thank you,

TimesTen generates log records for the purposes of redo and undo. Log records are generated for pretty much any change to persistent recoverable data within TimesTen. Log records are first written into the in memory log buffer and then are written to disk by a dedicated flusher thread that runs within the sub-daemon process that is assigned to the datastore. The log flusher thread runs continuously when there is data to be flushed and when there is any significant write workload on the system log data will reach disk very shortly after it has been placed in the buffer. Under very light write workload it may talke a little longer for the data to reach disk.
There is a single logical log buffer (size determined by LogBufMB) which in TimesTen 11g, is divided into multiple physical buffers (strands) for increased concurrency of logging operations ( number of strands determined by LogBufParallelism).
Several of your observations are not correct; I would like to understand hwat tests you performed to arrive at these conclusions:
1. Yes, the log buffer is flushed during a checkpoint operation but in fact it is also being flushed continuously at all times by the log flusher thread.
2. You can force the buffer to be flushed at any time simply by executing a durable commit within the datastore. A durable commit flushes all log starnds synchronously to disk and does not return until the writes have completed successfully and been acknowledged by the storage hardware.
3. The text that you quote from the replication guide is ambiguous and could be better phrased. When it talks about 'writing to the log' it means placing records in the in-memory log buffer. The presence or absence of replication does not fundamentally change the way logging works though the replication agent, when active, typically perfoms a durable commit every 100 ms. Also, in some replication modes, additional durable commits may be executed by the replication agent before sending a block of replicated transactions.
4. The LOG_BUFFER_WAITS field in SYS.MONITOR counts the number of times that application transactions have been blocked due to there being no free space in the log buffer to receive their log records. This is due to some form of logging bottleneck. By far the most common reason is that the log buffer is undersized. the default size if only 64 MB and this is far too small for any kind of write intensive workload. For write intensive workloads a significantly larger log buffer size is needed (max size allowed is 1 GB).
5. The field LOG_FS_WRITES in SYS.MONITOR counts the number of physical writes that the log flusher thread has performed to the logs on disk. The flusher will typically write a lot of data in a single write (when under heavy load). Flusher writes are filesystem block aligned.
Hope that helps clarify things.
Chris

Best way to move redo log from one disk group to another in ASM?

Hi All,
Our db is 10.2.0.3 RAC db. And database servers are window 2003 server.
We need to move more than 50 redo logs (some are regular and some are standby) which are not redundant from one disk group to another. Say we need to move from disk group 1 to 2. Here are the options we are thinking about but not sure which one is the best from easiness and safety prospective.
Thank you very much for your help in advance.
Shirley
Option 1:
1)     shutdown immediate
2)     copy log files from disk group 1 to disk group2 using RMAN (need to research on this)
3)     startup mount
4)     alter database rename file ….
5)     Open database open
6)     delete the redo files from disk group 1 in ASM (how?)
Option 2:
1)     create a set of redo log groups in disk group 2
2)     drop the redo log groups in disk group 1 when they are inactive and have been archived
3)     delete the redo files associated with those dropped groups from disk group 1 (how?) (According to Oracle menu: when you drop the redo log group the operating system files are not deleted and you need to manually delete those files)
Option 3:
1)     create a set of redo members in disk group 2 for each redo log group in disk group 1
2)     drop the redo log memebers in disk group 1
3)     delete the redo files from disk group 1 associated with the dropped members

Absolutely not, they are not even remotely similar concepts.
OMF: Oracle Managed Files. It is an RDMBS feature, no matter what your storage technology is, Oracle will take care of file naming and location, you only have to define the size of a file, and in the case of a tablespace on an OMF DB Configuration you only need to issue a command similar to this:
CREATE TABLESPACE <TSName>; So the OMF environment creates an autoextensible datafile at the predefined location with 100M by default as its initial size.
On ASM it should only be required to specify '+DGroupName' as the datafile or redo log file argument so it can be fully managed by ASM.
EMC. http://www.emc.com No further commens on it.
~ Madrid
http://hrivera99.blogspot.com

Log buffer overflow

I have been receiving the flex log buffer overflow error for a long time. I don't believe it is causing any problem but I'm not sure.
I have Iplanet Web Server 4.1 on Solaris 2.6.
I have changed the LogFlushInterval from the default 30 seconds to 5 seconds.
I am logging a great deal of information
My questions are...
should I be concerned ?
when I get that error is the buffer being immediately dumped to the log file ?
am I losing any log information ?
can I increase the buffer size ?
should I reduce the LogFlushInterval any more ?
Thanks

The error message indicates that an access log entry exceeded the maximum of 4096 bytes and was truncated. You should check the access log file for suspicious entries.
Adjusting LogFlushInterval won't affect this problem, and unfortunately there's no way to increase flex log buffer size.

Will any of DDL command trigger a data flush from the data buffer to disk?

Will any of DDL command trigger a data flush from the data buffer to disk?---No.164

I mean if I issue the DDL commands Such as DROP, TRUNCAE, CREATE, Can these commands trigger a data flush action?

Redo Log Buffer sizing problem

My pc has 512mb RAM and i was trying to increase the redo log buffer size. Initially the log_buffer size was 2899456 bytes. So i tried to increase it to 3099456 by issuing the command:
ALTER SYSTEM SET LOG_BUFFER=3099456 SCOPE=SPFILE;
And i issued SHUTDOWN IMMEDIATE. Upon restarting my database, when i queried SHOW PARAMETERS LOG_BUFFER . The value has been changed to 7029248 bytes not 3099456 which i wanted. How did this happen?

1.) We are all volunteers.
2.) It was only 5 hours between posts and you're complaining that there are no answers?
3.) You didn't bother to mention platform or Oracle version, even after being specifically asked for it? Which part of "What is your Oracle version?" do you not understand? And yes, the platform may be useful too....
From memory, there could a couple of things going on. First off, starting in 9i, Oracle allocates memory in granules, so, allocating chunks smaller than granule size can result in being rounded up to granule size. Second, on some platforms, Oracle protects the redo buffer with "guard pages", i.e., extra memory that serves simply to try to prevent accidental memory overflows from corrupting the redo buffer.
If you want a specific answer, or at least a shot at one, post:
1.) Oracle version (specific version: 8.1.7.4, 9.2.0.8, 10.2.0.3, etc).
2.) Platform
3.) O/S and version
4.) Current SGA size
Reposting the same question, or threatening to do so, will get you nowhere.
-Mark

Redo log backup to disk is failed

Hi,
My Archive log backup to disk is failed
BR0002I BRARCHIVE 7.00 (13)
BR0006I Start of offline redo log processing: aeamsbyx.svd 2009-05-04 15.15.07
BR0477I Oracle pfile E:\oracle\DV1\102\database\initDV1.ora created from spfile E:\oracle\DV1\102\database\spfileDV1.ora
BR0101I Parameters
Name                           Value
oracle_sid                     DV1
oracle_home                    E:\oracle\DV1\102
oracle_profile                 E:\oracle\DV1\102\database\initDV1.ora
sapdata_home                   E:\oracle\DV1
sap_profile                    E:\oracle\DV1\102\database\initDV1.sap
backup_dev_type                disk
archive_copy_dir               W:\oracle\DV1\sapbackup
compress                       no
disk_copy_cmd                  copy
cpio_disk_flags                -pdcu
archive_dupl_del               only
system_info                    SAPServiceDV1 SAP2DQSRV Windows 5.2 Build 3790 Service Pack 1 Intel
oracle_info                    DV1 10.2.0.2.0 8192 21092 71120290
sap_info                       46C SAPR3 DV1 W1372789206 R3_ORA 0020109603
make_info                      NTintel OCI_10103_SHARE Apr 5 2006
command_line                   brarchive -u / -c force -p initDV1.sap -sd
BR0013W No offline redo log files found for processing
BR0007I End of offline redo log processing: aeamsbyx.svd 2009-05-04 15.15.11
BR0280I BRARCHIVE time stamp: 2009-05-04 15.15.11
BR0004I BRARCHIVE completed successfully with warnings
I have checked the target directory nothing is backed up and gone through few SAP notes 10170, 17163, 132551, 490976 and 646681 but nothing is helped.
And anohter question is in DB13 Calander --> Schedule an action pattern maximum I can backup only 1 month redo logs. But I have 3 months redo log files are there. How can I back up those files.
Our environment is SAP R/3 4.6C, windows 2003 and Oracle 10.2.0.2.0
Please some one help me on this.
Thanks and Regards
Satya

Update your BRTools. They are very old.
Check that your DB is in archive-log-mode. If no enable it.
testing backup
- run a online backup
- run "sqlplus / as sysdba"
- SQL> alter system switch logfile; ... this switches the current online log... a new log will written to oraarch.
- run a archivelog backup
... now you should have complete db-backup with minimum 1!!! archive log.
Now you can delete old redologs from oraarch.
If this doesn't work and your database is in archive-log-mode:
- shutdown sap and oracle
- MOVE all redologs from oraarch to a other location manually... no more files should be on oraarch
- run a offline backup
If the offline backup was running successfully you can delete the prior moved redologs. The backup is consistent and the redologs will no more required.
- start oracle and sap
Oracle should now write new redologs to oraarch. Test online backup!
Edited by: Thomas Rudolph on May 6, 2009 10:16 PM
Edited by: Thomas Rudolph on May 6, 2009 10:17 PM

Redo log backup on disk

Hi
I want to take Redo log backup on disk. what changes i have to made in init<SID>.sao file.
Regards
Vikram

Hi Vikram,
If you use brtools then even without parameter change in init<SID>.sap also you can take backup.You have to provide media as disk at the time of backup then the backup will be stored in the default ....../<SID>/sapbackup.
You you want to take backup to some other place then you have to change the parameter
archive_copy_dir=<dirctory where you want to take the backup>
But if you want to take using db13 then you have to change the parameter for backup media to disk
backup_dev_type = disk
archive_copy_dir = /oracle/<SID>/sapbackup
Regards
Ashok Dalai
Edited by: Ashok Dalai on Aug 3, 2009 8:35 AM

What exactly is Redo log buffer?

I know that Redo log buffer is a part of SGA and it sotores each and every change in it. But i want to know whether it stores all the updates and other changes as it is stored in DB Buffer Cache.? Or if not what exactly is sotored in it and when...?
null

Hi,
Redo Log Buffers are part of SGA and they store each and every entry that is made in the DB.
This is also stored in the Redo Log FIles. This information is used during recovery of a Crashed DB.
A Redo Log does not Store the Data but oinly the Stmt. that was executed in the DB.
A DB Buffer Stores data and not the command.
If u need more information Pls Refer to The Oracle 8 Concepts on the Oracle Documentation.
Hope this helps.
Regards,
Ganesh R
null

Tuning : log buffer space in 11gr2

Hi,
version : 11202 on hpux
awr Top 5 events shows :
Top 5 Timed Foreground Events :
Event Waits Time(s) Avg wait (ms) % DB time Wait Class
log buffer space 12,401 29,885 2410 55.83 Configuration
My log_buffer size is :
SQL> show parameter log_buffer
NAME                                 TYPE        VALUE
log_buffer                           integer     104857600And the sga values are :
SQL> show parameter sga
NAME                                 TYPE        VALUE
sga_max_size                         big integer 15G
sga_target                           big integer 15GI wanted to know if there are guide lines for tuning log_buffer space .
Can just double it from 100m to 200m ?
Thanks

Yoav wrote:
Top 5 Timed Foreground Events :
Event                    Waits    Time(s)       Avg wait (ms) % DB time    Wait Class
log buffer space      12,401 29,885            2410              55.83     Configuration My log_buffer size is :
SQL> show parameter log_buffer
NAME                                 TYPE        VALUE
log_buffer                           integer     104857600I wanted to know if there are guide lines for tuning log_buffer space .
Can just double it from 100m to 200m ?
You're the second person this week to come up with this issue.
The ONLY sensible guideline for the log buffer is to let it default until you have good reason to change it. Reasons for change (even to the point of modifying a hidden parameter) are really application dependent.
The last couple of times something like this has come up the issue has revolved around mixing very large uncommitted changes with large numbers of small transactions - resulting in many small transactions waiting for the log writer to complete a large write on behalf of the large transaction. Does this pattern describe your application environment ?
For reference - how many public and how many private redo strands do you have, and how many have been active. (See http://jonathanlewis.wordpress.com/2012/09/17/private-redo-2/ for a query that shows the difference).
Regards
Jonathan Lewis

Pros and cons between the large log buffer and small log buffer?

pros and cons between the large log buffer and small log buffer?
Many people suggest that small log buffer (1-3MB) is better because we can avoid the waiting events from users. But I think that we can also have advantage with the bigger on...it's because we can reduce the redo log file I/O...
What is the optimal size of the log buffer? should I consider OLTP vs DSS as well?

Hi,
It's interesting to note that some very large shops find that a > 10m log buffer provides better throughput. Also, check-out this new world-record benchmark, with a 60m log_buffer. The TPC notes that they chose it based on the cpu_count:
log_buffer = 67108864 # 1048576x cpuhttp://www.dba-oracle.com/t_tpc_ibm_oracle_benchmark_terabyte.htm

Log Buffer to disk?

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