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

Similar Messages

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

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

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

Buffer, library,dictionary, shared pool, redo log buffer chache hit ratios

Can please one provide information and sql queries to calculate Buffer, library,dictionary, shared pool, redo log buffer chache hit ratios and if any other ratio for investigation of performance issues in oracle 10g database (10g.1 and 10g.2 both). thanks in advance.

In and by themselves most of the standard ratio calculations are useless to misleading. All the ratios should always be considered in relation to other data such as total requests for a resource, existence of any outlying values in the associated events, etc ....
The proper warning being given then you can find most of the standard ratios mentioned with SQL for their calculation in the Performance and Tuning manual for your version of Oracle.
HTH -- Mark D Powell --

High redo log buffer wait

Hi,
I can see "high redo log buffer wait" event. The instance spent 23% of its resources waiting for this event. Any suggestion to tune redo log buffer?
DB version : 10.2.0.4.0
Os : AIX
SQL> SELECT name, value FROM v$sysstat WHERE name = 'redo log space requests';
NAME VALUE
redo log space requests 3542
SQL> sho parameter buffer
NAME TYPE VALUE
buffer_pool_keep string
buffer_pool_recycle string
db_block_buffers integer 0
log_buffer integer 14238720
use_indirect_data_buffers boolean FALSE
SQL> select GROUP#,BYTES from v$log;
GROUP# BYTES
1 1073741824
4 1073741824
3 1073741824
2 1073741824
SQL> show parameter sga
NAME TYPE VALUE
lock_sga boolean FALSE
pre_page_sga boolean FALSE
sga_max_size big integer 5G
sga_target big integer 5G
Thanks

Gowin_dba wrote:
I can see "high redo log buffer wait" event. The instance spent 23% of its resources waiting for this event. Any suggestion to tune redo log buffer?
SQL> SELECT name, value FROM v$sysstat WHERE name = 'redo log space requests';
NAME VALUE
redo log space requests 3542 How are you getting from 3,542 "redo log space requests" to 23% of the instance resources waiting for "high redo log buffer wait" (which is not a wait event that can be found in v$event_name in any version of Oracle) ?
"redo log space requests" is about log FILE space, by the way, not about log BUFFER space.
Regards
Jonathan Lewis

REDO LOG BUFFER

Whenever a DML like Insert statement is issued it gets written to the Database buffer cache first by the server process(dedicated server).
Which process writes this DML activity to the redo log buffer ?
I guess DML is first written to the redolog files and only after that the same DML is committed to the data files.Is this correct ?
Can get any references to read on how any activity/DML is processed with a Oracle architecture perspective.
Thanks

Yes. Only the server process for that session knows what changes were made to the buffer cache. So it is the only one that can write the change vectors to the redo log buffer.
Hemant K Chitale

Where can I find redo log buffer advice

Hi,
Our customer needs the information about redo log buffer. But In administrator--database configuration--memory parameters item of grid control 10g, I can only get the information about buffer cache & shared pool, as well as the corresponding advice. I cannot find information about redo log buffer in this page. I wonder why the information about redo log buffer is not included in this page.Where can I find it?

The Log Buffer is part of your Intialization Parameters.
So, from EM, you can find the information you need when you select the Database > Adminstration > All Initialization Parameters (under Database Configuration)

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

Redo log buffer is in cretical position

hi Experts,
please try to solve my query, here in my system redo log buffer shows(alert monitoring) 99<4000 and message is '4000 redo entries per redo log space requests'
so i think i need to increase log_buffer parameter value upto required level ,then i entered in database
splplus / as sysdba
and i try to check my file is in which type (spfile or in pfile) by executing command
" SHOW PARAMETER pfile"
it shows
name--- spfile
type---string
value---/oracle/qty/102_64/dbs/spfileqty.ora
when i Excute 'SHOW PARAMETER spfile' also the same result
now in here i have doubt please clarify me
1) my file is spfile or pfile ?
2) how can i increase my parameter value (alter system set log_buffer = xxx scope=pfile (or) spfile )
3) is that my process correct for that error
please clarify me
thanks & regards

Hi,
As per my knowledge, Oracle 10g by default starts with SPFILE and if you are setting the parameter with alter command then yes the scope should be SPFILE. After that when you schedule any DB related activity(backup, update statistics etc.,) it will create pfile from spfile.
Before doing any changes take the backup of the existing files both (pfile and spfile) at os level.
Regards,
Sharath

Redo Log Buffer 32.8M, Seems to Big?

I just took over a database (Mainly used for OLTP on 11gR1) and I am looking at the log_buffer parameter it is set to 34412032 (32.8M). Not sure why it is so high.
select
    NAME,
    VALUE
from
    SYS.V_$SYSSTAT
where
    NAME in ('redo buffer allocation retries', 'redo log space wait time');
redo buffer allocation retries     185
redo log space wait time          5180(database has been up for 7.5 days)
Any opinions on this? I Normally keep try to stay below 3M and have not really seen it above 10M.

Sky13 wrote:
I just took over a database (Mainly used for OLTP on 11gR1) and I am looking at the log_buffer parameter it is set to 34412032 (32.8M). Not sure why it is so high.
In 11g you shouldn't set the log_buffer parameter - let Oracle set the default.
The value is derived from the setting for the CPU count and the transactions parameter, which may be derived from sessions, which may be derived from processes. Moreover, Oracle is going to allocate at least a granule (which may be 4MB, 8MB, 16MB, 64MB or 256MB depending on the size of the SGA, so you are unlikely to save memory by reducing the log buffer size.
Here's a link to a discussion which shows you how to find out what's really behind that figure.
Re: Archived redo log size more less than online redo logs
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
Author: <b><em>Oracle Core</em></b>

Multiplexing Redo Log Files question

If you are running RAC on ASM on a RAID system, is this required? We are using an HP autoraid which mirrors at the block level and in the documentation about Multiplexing Redo Log Files it says that you do it to protect against media failure. The autoraid that we are using gives us multiple levels of redundancy against media failure so I was wondering if Multiplexing would be adding more overhead than is needed. Thanks for your input.

ASM is quite compex and I'm not going to outline all the advantages or reasons for ASM, but under ASM you can drop and add devices to maintain your capacity needs online without loosing data, which you cannot do using RAID, which requires a re-initialize, for example, regardless of redundancy. Please see the documentation. ASM, like pretty much everything Oracle will add complexity and you will have to check your requirements. ASM is however pretty much the standard. If you use external RAID, make sure your storage is not using RAID 5 or 0. Regarding logical errors, you could for example overwrite or delete a file by mistake, in which case file redundancy does not protect you. If you are looking for reasons or ways not to use ASM, I'm sure you will find them, but what's the point?

REDO LOG GROUP QUESTION

I was reading an article and it said
the distance(in bytes) between the checkpoint position in a redolog group and the end of the current redolog group can never be more then 90 % of the size of the smallest redo log group
Can someone elaborate this

I'm not sure what you want to elaborate on, but yes, it's true. If your redo logs are 100MB in size, and nothing else causes a checkpoint to take place, you'll have a checkpoint issued when you hit the 90MB mark. The idea is simply that you don't want to sit there doing nothing at all and then bang! the logs switch and you have to go hell-for-leather performing a massive checkpoint, all the while praying more log switches don't mean that you're threatening to catch up with yourself (at which point you'd have the 'thread unable to advance to log...' problem). By implementing the 90% rule, the idea is that your log switch, at worst, will cause a "10%-sized" checkpoint, which should be bearable.
Of course, the situation is made more complex by the fact that other things DO kick in and cause their own checkpoints, so the interaction between -for example, FAST_START_MTTR_TARGET and the 90% rule can get, er, 'interesting'.

DB Cache Full or Redo Log Full?

Is there any way that Oracle can write to datafiles in the middle of a transaction?
Iam reading, processing and writing very large sized lobs which gives error that "no free buffers available in buffer pool".
When in lobs, a lob is not written until the whole tranaction finishes - but in my case the lob size is large than the size of the data buffer cache.
The error is "ORA-00379: no free buffers available in buffer pool DEFAULT for block size 8K"
Exact question I would like to know now is that which buffer is full; data_buffer_cache or the redo log buffer?
If data_buffer cache, then is there a mechanism which allows to write data to dtafiles in the middle of a transaction as i have to do processing with lobs - which are 3 to 4 times the size of the db cache size.
I am referring to the same problem outlined in an earlier thread.
Thanks

Is there any way that Oracle can write to datafiles
in the middle of a transaction?
r.- Oracle writes to the datafiles only commited transactions according to some elements
Iam reading, processing and writing very large sized
lobs which gives error that "no free buffers
available in buffer pool".
r.- You have to increase the size of the buffer Pool
When in lobs, a lob is not written until the whole
tranaction finishes - but in my case the lob size is
large than the size of the data buffer cache.
The error is "ORA-00379: no free buffers available in
buffer pool DEFAULT for block size 8K"
Exact question I would like to know now is that which
buffer is full; data_buffer_cache or the redo log
buffer?
data_buffer_cache. In what version you are ?
If data_buffer cache, then is there a mechanism which
allows to write data to dtafiles in the middle of a
transaction as i have to do processing with lobs -
which are 3 to 4 times the size of the db cache
size.
r.- Oracle does not write to the datafiles in that way
I am referring to the same problem outlined in an
earlier thread.
Thanks Joel PÃ©rez
http://www.oracle.com/technology/experts

Redo log space requests and Enqueue Waits

Hi all,
I am seeing an increase on the Enqueue Waits and Redo Log Space Request from 58, 274 to 192, 1245 in two weeks time respectively.
The DB is a production database and runs on an HP cluster with 4X1G ram and 550mghz cpu.
There are four Redo Log files with 200M (2 members each)which I have increased to 400M over this past weekend.
I have included below the memory structure details:
Redo Log Summary
Total System Global Area 1646094824 bytes
Fixed Size 104936 bytes
Variable Size 408989696 bytes
Database Buffers 1228800000 bytes
Redo Buffers 8200192 bytes
My question is that, who do I stop it from growing further and passing the 1:5000 ratio ?
At the moment the ratio is in the range of 1:186194.
Your input is much appreciated.
Cheers,
Seyoum.

Here is some information from Oracle's Peformance Tuning Guide.
The V$SYSSTAT statistic redo log space requests indicates how many times a server process had to wait for space in the online redo log, not for space in the redo log buffer. A significant value for this statistic and the wait events should be used as an indication that checkpoints, DBWR, or archiver activity should be tuned, not LGWR. Increasing the size of log buffer does not help.

Redo log buffer question

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