DBWR writes before LGWR wirtes?

Hi,
Under what circumstances does DBWR write a block a dirty block out before LGWR writes to the logs?
Thanks
Dilip.

Here is a great blog post regarding many of the parts of Oracle's architecture:
http://blogs.ittoolbox.com/bi/confessions/archives/post-index-how-oracle-works-10605
Check it out for your answer.

Similar Messages

Has anyone else ever had a problem where you had to perform 2 datasocket writes before the datasocket read would pick up the change?

I have a local VI that is simply a control that writes to the datasocket server whenever the control value changes.(the dataitem on the server is permanent - its initialized and never released by the server)
In the same local VI I have a datasocket read polling a different dataitem on the server.
The remote machine has a VI that reads the permanent dataitem on the server once per second.
For some reason, after adding the local VI mentioned above, the remote VI stopped picking up the first change in the permanent variable.
I'd start both the local and remote VIs...
Then change the local control and the remote V
I would not update - as if the datasocket write(upon adjusting the control) did not take place. So I'd change the control value again - this time the remote VI would update to this new value. And from here on out - the remote VI would update correctly. This problem only occurs when the local VI is first started up.
What in the heck is going on?

dingler44 wrote:
>
> Has anyone else ever had a problem where you had to perform 2
> datasocket writes before the datasocket read would pick up the change?
>
> I have a local VI that is simply a control that writes to the
> datasocket server whenever the control value changes.(the dataitem on
> the server is permanent - its initialized and never released by the
> server)
> In the same local VI I have a datasocket read polling a different
> dataitem on the server.
> The remote machine has a VI that reads the permanent dataitem on the
> server once per second.
> For some reason, after adding the local VI mentioned above, the
> remote VI stopped picking up the first change in the permanent
> variable.
> I'd start both the local and remote VIs...
> Then change the local control and the remote VI would not update -
> as if the datasocket write(upon adjusting the control) did not take
> place. So I'd change the control value again - this time the remote
> VI would update to this new value. And from here on out - the remote
> VI would update correctly. This problem only occurs when the local VI
> is first started up.
>
> What in the heck is going on?
Gorka is right, this came up on Info-LV a few days ago. Someone
described a similar problem. I replied that I had seen similar
behaviour, reported it to NI, and they verified a bug. There is no fix
yet, but they are aware of it and will fix it. No anticipated release
date for the fix.
Regards,
Dave Thomson
David Thomson 303-499-1973 (voice and fax)
Original Code Consulting [email protected]
www.originalcode.com
National Instruments Alliance Program Member
Research Scientist 303-497-3470 (voice)
NOAA Aeronomy Laboratory 303-497-5373 (fax)
Boulder, Colorado [email protected]

Checkpoint not complete;;cannot allocate new log ;;; PLZ HELP ME

Hi all,
We are working on Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 on a Redhat Linux Server platform.
We are facing the following problem in the alert.log file :
Wed Aug 22 02:58:57 2007
Thread 1 cannot allocate new log, sequence 43542
Checkpoint not complete
Current log# 1 seq# 43541 mem# 0: /u01/oradata/DB01/redo01.log
Thread 1 advanced to log sequence 43542
Current log# 4 seq# 43542 mem# 0: /u01/oradata/DB01/redo04.log
Current log# 4 seq# 43542 mem# 1: /u01/oraindx/DB01/redo04.log
Wed Aug 22 03:00:00 2007
Thread 1 advanced to log sequence 43543
Current log# 5 seq# 43543 mem# 0: /u01/oradata/DB01/redo05.log
Current log# 5 seq# 43543 mem# 1: /u01/oraindx/DB01/redo05.log
Wed Aug 22 03:01:00 2007
Thread 1 cannot allocate new log, sequence 43544
Checkpoint not complete
Current log# 5 seq# 43543 mem# 0: /u01/oradata/DB01/redo05.log
Current log# 5 seq# 43543 mem# 1: /u01/oraindx/DB01/redo05.log
Thread 1 advanced to log sequence 43544
Current log# 6 seq# 43544 mem# 0: /u01/oradata/DB01/redo06.log
Current log# 6 seq# 43544 mem# 1: /u01/oraindx/DB01/redo06.log
Wed Aug 22 03:01:26 2007
Thread 1 advanced to log sequence 43545
Current log# 2 seq# 43545 mem# 0: /u01/oradata/DB01/redo02.log
Thread 1 advanced to log sequence 43546
Current log# 3 seq# 43546 mem# 0: /u01/oradata/DB01/redo03.log
Thread 1 advanced to log sequence 43547
Current log# 1 seq# 43547 mem# 0: /u01/oradata/DB01/redo01.log
Wed Aug 22 03:01:38 2007
Thread 1 cannot allocate new log, sequence 43548
Checkpoint not complete
Current log# 1 seq# 43547 mem# 0: /u01/oradata/DB01/redo01.log
I know that this message indicates that Oracle wants to reuse a redo log file, but
the current checkpoint position is still in that log. In this case, Oracle must
wait until the checkpoint position passes that log. Because the
incremental checkpoint target never lags the current log tail by more than 90%
of the smallest log file size, this situation may be encountered :
1-if DBWR writes too slowly,
or
2-if a log switch happens before the log is completely full,
or
3-if log file sizes are too small.
I read some posts in this forum regarding this error but sincerly i don't know how to find the exact cause of this error? Maybe Should I add new redo files or one new redo group? I don't know how to resolve it :( ;;;
such as I have 6 redo files 3 of them 5MB size and the others 3 files 10MB size ;
Thank you,
Regards,
Message was edited by:
HAGGAR

Make DBWR write more aggressively - as you are on 9i the parameter I would use is FAST_START_MTTR_TARGET=(how long you want recovery to take in seconds), the lower that number, the more aggressively DBWR has to write to keep up with the target, the advantage of this is that by the time LGWR comes to overwrite the redo log file, the chances are that DBWR has already written the "high scn#" (and beyond) from the checkpoint q.
-- This disadvantage is that you will get more I/O to your disks.
2. Create more redo log file groups - this will give DBWR more time to write before LGWR tries to overwrite a particular redolog file, again the chances are that the extra (6th) or (7th) group will give CKPT enough time to completely checkpoint beyond the "highest scn#" before that group is again required..
Which one to go for.. well that's up to you and your setup, if you have an I/O bound system then 2. would be better for you, as 1. will just increase your I/O problem, however if physical space is an issue and I/O isn't then 1. might be better (with the added advantage that instance recovery will also be faster).
Sorry for the training session, but as with everything to do with Oracle, there is rarely one solution that apply to everyone...
Gopu

DBWR / LGWR PROCESSES

Hi,
On what basis dbwr processes and lgwr processes should be increased. Please give me clear understanding of the same. How to identify need of dbwr and lgwr processes should be increased as well how many it should be increased

check :
http://www.dba-oracle.com/t_dbwr_database_writer_tuning_tips.htm
lgwr can be increase pr not

At what point oracle write "Checkpoint not complete" in alert log ?

DB version: 10.2.0.4/RHEL 5.8
During a batch run, we encountered lots of 'Checkpoint not complete' errors in alert log.
Later we discovered that ORLs were sized only 50mb and this DB had only 3 redo log group. Since this is the potential cause, we are going create at least 10 Redo log groups and increase Redo log size to 500MB.
But , I want to know what exactly causes oracle to write "Checkpoint not complete" in alert log ?
For the purpose of this discussion, I am assuming we have only 1 ORL per redo log group. Is my below assumption correct ?
ORL1
|----------------> ORL1 file got full, so, LGWR starts writing to ORL2 file. Checkpoint occurs at log switch
|                    DBWR writes modified blocks associated with the redo entries in ORL1 to datafiles
|
V
ORL2
|----------------> ORL2 file got full, LGWR wants to start writing to ORL3 file. Checkpoint is initiated at log switch.
|                    But checkpoint can't be finished due to unknowN reasons
|
V
ORL3
|---------------->

Your assumption is only partly right.
I would illustrate it like
ORL1
|
|----------------> ORL1 file got full, so, LGWR starts writing to ORL2 file. Checkpoint occurs at log switch
|                    DBWR starts writing modified blocks associated with the redo entries in ORL1 to datafiles
|
V
ORL2
|----------------> ORL2 file got full, LGWR starts writing to ORL3 file.
|                    Checkpoint for ORL2 is initiated at log switch.
|
V
ORL3
|
|----------------> ORL3 file (the last member) also got full very quickly . LGWR wants to start the 'new cycle' by
                    writing (Reusing) to ORL1. But the checkpoint intiated by the log switch of ORL1 from the previous cycle is
                    not complete yet !! .
Basically you get this error when LGWR attempts to reuse an online redo log file (ORL1 in the above example) and finds that it cannot.
This is because the remaining ORL files (ORL2 and ORL3) got fully written before DB writer finished checkpointing the modified blocks associated with ORL1 yet.
Until the checkpoint of ORL1 is complete, the DB effectively hangs and user sessions have to wait until LGWR can safely reuse ORL1
Yes a larger redo log size of 10 groups can help. But make sure the I/O subsytem where ORLs are stored has no latency issues.

LGWR and DBWn

I learnt from the books that LGWR writes every three seconds
1. Can we change this time?
2. Is it so that the Log_Buffer size should be just large enough to accommodate redo entries generated for 3 seconds because after that it will flush the contents
Re: Concepts of checkpoint, dbwr, lgwr processes
Also want to know whether the commit includes flushing of all the redo log buffer to redo logs or just the redo entries related to commit?

Also want to know whether the commit includes
flushing of all the redo log buffer to redo logs or
just the redo entries related to commit?These paragraph in concept answers your question.
When a user issues a COMMIT statement, LGWR puts a commit record in the redo log buffer and writes it to disk immediately, along with the transaction's redo entries. The corresponding changes to data blocks are deferred until it is more efficient to write them. This is called a fast commit mechanism. The atomic write of the redo entry containing the transaction's commit record is the single event that determines the transaction has committed. Oracle returns a success code to the committing transaction, although the data buffers have not yet been written to disk.
Note:
Sometimes, if more buffer space is needed, LGWR writes redo log entries before a transaction is committed. These entries become permanent only if the transaction is later committed.
When a user commits a transaction, the transaction is assigned a system change number (SCN), which Oracle records along with the transaction's redo entries in the redo log. SCNs are recorded in the redo log so that recovery operations can be synchronized in Real Application Clusters and distributed databases.
In times of high activity, LGWR can write to the redo log file using group commits. For example, assume that a user commits a transaction. LGWR must write the transaction's redo entries to disk, and as this happens, other users issue COMMIT statements. However, LGWR cannot write to the redo log file to commit these transactions until it has completed its previous write operation. After the first transaction's entries are written to the redo log file, the entire list of redo entries of waiting transactions (not yet committed) can be written to disk in one operation, requiring less I/O than do transaction entries handled individually.
Therefore, Oracle minimizes disk I/O and maximizes performance of LGWR. If requests to commit continue at a high rate, then every write (by LGWR) from the redo log buffer can contain multiple commit records.

Urgent: Question DBWn & LGWR

I have some question regarding oracle database architecture. Reply me as soon as possible i have exams ahead!!!!!
Q1 DBWR Process writes dirty bufers to Data files. Consider a situation in which DBWR writes uncommitted data to data files and instance then crashes then how its
recovery will happen next time when instance starts?
Q2 LGRW write log entries to Log files..similarly if LGRW writes uncommited redo log entries to redo log files and then instnce crashes what will happen?
Q3 How SCN is tackled at instance recovery procedure? means how SCN help identify creeps?
Q4 How at instance recovery phase oracle db come to know that data in data files belongs to uncommitted transaction thus needs to be undo, and these redo log entries in redo log files are part of committed transaction so the must be rolled forward.

Yes, it is possible that there are changes present in the redo log that are not reflected in the datafiles. That's OK, because Oracle does not necessarily write to the datafile at commit time. Oracle knows what Redo entries need to be applied to the datafiles because the Control File has record of the last checkpoint position in the Redo log. From this, it knows that all Redo entries before that checkpoint time need not be reapplied to the datafiles, because checkpoint means it was written to disk.
So, at recovery time, Oracle may need to reflect into the datafiles those changes found in the log. This is called rolling forward.
And yes, it is also possible that there are changes in the data files that have not yet been committed. (This is because Oracle goes ahead and makes the changes to blocks (and datafiles, if checkpointed) even though they've not been committed, and relies on the Undo data to undo these if another session reads the data). Uncommitted changed may also be introduced during the roll-forward step just done. So, during instance recovery, these uncommitted changes are rolled back. Picture it as Oracle replaying what's been recorded persistently (on disk) before the crash. The control file knows the last checkpoint, and starts there, looking in the redo log. It applies all changes it finds there (and some of these changes actually apply to the Undo ts). Now it's applied changes that should have been recorded in the datafiles but had not yet gotten a chance to, and also some changes that had gotten written to the datafile, but was never committed. So, then it issues a rollback for each uncommitted transaction, undoing anything that was never committed.
Keep in mind that the definition of a transaction being committed is that it has been assigned an SCN, which has been recorded in the Online Redo Log File, AND all the Redo to represent that change has also been written to the Online Redo Log File. I.E. in sqlplus, when you type "commit", control is not returned to you until these 2 things have happened.
This looks verbose at first, but it really does make sense. Take a look at the online docs... the Concepts, Backup /Recovery, and the Admin Guide - search for recovery, scn, etc.
Tom Best

LGWR / Group commit - 2 commits at the same time

Hello guys,
while talking with another DBA about commits and wait events... we came to the following question:
"What happens, if 2 transactions are commit at the same time?"
I found the following 2 things at the web and one does support my opinion and the other not.
1) http://www.ixora.com.au/tips/tuning/log_buffer_size.htm
=> If several commits occur in distinct transactions before LGWR wakes up, then the commit markers are all flushed to disk in a single sync write. This is sometimes called a group commit
2) http://www.pythian.com/blogs/162/quantifying-commit-time
=> And worst of all, you will serialize at the entire instance on it. If a commit has caused a “log file sync”, another session will have to wait for the already started flush, and then wait for it’s data to be flushed in effect waiting twice the time.
Now we get the following 2 scenarios:
1) Transaction A commits and posts the LGWR to flush (come up)
2) Transaction B commits at the the same time and the LGWR is not up until yet
3) LGWR comes up and flushes both transactions in one part
1) Transaction A commits and posts the LGWR to flush (come up)
2) LGWR is up and flushing
3) Transaction B commits, but the LGWR is still flushing
As i understand in both cases transaction A and B will see the wait event "log file sync".
But what happens to transaction B in the second example:
Does it wait until LGWR finished and will post the LGWR again or does the LGWR know that another transaction was committing in the meanwhile, while he was flushing the log buffer to disk and start flushing the buffer again?
If the transaction B will post the LGWR again, it have to wait twice and the "log file sync" will take the double time?
This situation is really hard to simulate in a test environment, so i need your experience and observation :-)
Regards
Stefan

Hello Jonathan,
thanks for your quick reply.
So A has a (relatively) short 'log file sync' wait, and B has a longer wait which includes some of the time spent while the log writer was clearing the buffer for A - but it is still just one wait.That is the point i was going to - let's take the following wait times on the transactions:
log file parallel write - 5 ms
log file sync - 7 ms (includes 5ms log file parallel write and 2 ms wake up and post activities)
So in the second case:
Session A will take 7 ms (2 ms wake up / post activities and 5 ms parallel write)
Session B will take 13 ms (7 ms waiting for transaction A and 5 ms parallel write and 1 ms post)
I know that these time values are very hypothetical, but if the LGWR checks if there are some pending request we will save the time for the wake up.
Regards
Stefan

Using SO Write to play a continuous waveform using PC sound card. (Circular Buffers?))

How can I play a continuous, repeating waveform using SO VI's (that isn't choppy)? I gather that I need to buffer the waveform, and I see examples using AO VI's, but how do you buffer using SO Write? What are Circular Buffers and might I use this technique to solve my problem?
Windows ME
Dell Inspiron 5000e
LabView 6i

Hi MGS!
MGS writes:
> How can I play a continuous, repeating waveform using SO VI's (that
> isn't choppy)? I gather that I need to buffer the waveform, and I see
> examples using AO VI's, but how do you buffer using SO Write? What
> are Circular Buffers and might I use this technique to solve my
> problem?
>
I've tried to do the same, i.e. synthesize a wave for continuous,
non-choppy output, and have some un-solved problems.
A buffer is sent to SO Write, which is then transfered to the device
driver and the sound output hardware. The problem is how to refill the
buffer that is sent SO Write, before the internal buffers of SO Write
and/or the device driver or hardware empties their buffer, which would
result in silence. We need a warning that the buff
er is almost empty,
so that more samples can be constructed. I've found no way to do that
using LabView 5.1.
However, we now how fast samples are consumed, so they can be produced
at the same rate, or perhaps a little faster. If they are produced
much faster, the memory will fill up, but if the difference is small,
the program can run for quite a while. It was a year ago, or so, that
I did this, so I don't remember the details.
A circular buffer can be used in a multi-thread producer-consumer
program. I've done it in C and C++, from some tutorial I found on
threads, or maybe it was from the OSS (open sound system) for
Linux. It is a buffer with a write position and a read position. The
addresses wraps around, so it works a bit like a conveyor belt. The
writer must check that the buffer is non-full, and the reader that it
is non-empty. I'm not sure how to implement it i LabView, or if it's needed.
Helge Stenstrom
[email protected]

Motive of checkpoint and SCN using with DBWr and LOGWr processes ??

What checkpoint has to do with log writer process i am not getting exactly ?..
Like see i fire 1 update query and apparently it is generating some redo blocks which in turn will come to my redo log files now in tihs whole cycle where the checkpoint will occur and why??
1)My update query
2) take locks
3)generate redo
4)generate undo
5)Blocks are modified but they are still in redo log buffer...
now this blocks eventually comes to redo log files in this whole way where check pointing take place and why??
checkpoint also takes place when Datablocks are flushed to datafiles again the same reason why??
Same way around the same question the what checkpointing has to do with DBWr process also i am not clear...
Apart from this whole picture SCN is generated when user issue comitts..and we can say SCN can be used to identify that transaction is committed or not.?
So what is the motive of SCN to update in Control file...MAy b to get the latest transaction committed..??
Sorry one thread with so much questionss..but this all things are creating a fuzzy picture i want to make it clear thnx for your help in advance ..
I read documentation but they havent mentioned in depth for checkpointing..??
THANKS
Kamesh
Edited by: 851733 on Apr 12, 2011 7:57 AM

851733 wrote:
What checkpoint has to do with log writer process i am not getting exactly ?..And where exactly did you read that it has anything to do with it? How did you come up to the relation anyways? The time checkpointing would come into the play with the log files would be when there would be a log switch and this would induce a checkpoint, causing/triggering the DBWR to write the dirty buffers to the datafile and allowing the redo log group to be reused. That's about it.
Like see i fire 1 update query and apparently it is generating some redo blocks which in turn will come to my redo log files now in tihs whole cycle where the checkpoint will occur and why??
1)My update query
2) take locks
3)generate redo
4)generate undo
5)Blocks are modified but they are still in redo log buffer...
now this blocks eventually comes to redo log files in this whole way where check pointing take place and why??Read my reply above, at the time of writing the change vectors in the log file, there won't be any checkpointing coming into the picture.
checkpoint also takes place when Datablocks are flushed to datafiles again the same reason why??Wrong, the checkpoint event would make the dirty buffers written to the dataflile. Please spend some time reading the Backup and Recovery guide and in that, instance recovery section. In order to make sure that there wont be much time spent in the subsequent instance recovery, it would be required to move the dirty buffers periodically to the data file. THis would be caused by the incremental checkpoint . Doing so would constantly write the content out of the buffer cache thus leaving few buffers only as the candidate for the recovery in the case of the instance crash.
Same way around the same question the what checkpointing has to do with DBWr process also i am not clear...Read the oracle documentation's Concept guide again and again as long as it doesn't start getting in sync in with you(and it may take time). One of the events , when DBWR writes , is the occurance of the Checkpoint. Whenever there would be a checkpoint, the DBWR would be triggered to write the buffers (dirty) to the datafile.
Apart from this whole picture SCN is generated when user issue comitts..and we can say SCN can be used to identify that transaction is committed or not.? Not precisely since there would be a SCN always there , even when you query , for that too. But yes, with the commit, there would be a commit SCN that would be generated including a commit flag entered in the redo stream telling that the transaction is finally committed. The same entry would be updated in the transcation table as well mentioning that the tranaction is committed and is now over.
So what is the motive of SCN to update in Control file...MAy b to get the latest transaction committed..??Where did you read it?
Sorry one thread with so much questionss..but this all things are creating a fuzzy picture i want to make it clear thnx for your help in advance ..
I read documentation but they havent mentioned in depth for checkpointing..??
Read the book, Expert one on one by Tom Kyte and also, from documentation, version 11.2's Concept guide. These two would be more than enough to get the basics correct.
HTH
Aman....

ORACLE8에서의 DBWR (DBWR_IO_SLAVES와 DB_WRITER_PROCESSES)

제품 : ORACLE SERVER
작성날짜 : 2002-08-12
Oracle 8에서의 DBWR (dbwr_io_slaves와 db_writer_processes)
Oracle 7에서의 db_writers는 master-slave processing을 통해, async I/O를
simulate하기 위해 사용되었다고 볼 수 있다. Oracle 8에서 DBWR의 write
processing에 더 나은 성능을 제공하기 위해 복수 개의 database writer를 사용
하는 방법은 다음과 같이 두가지로 나눌 수 있다.
1. DBWR IO slaves (dbwr_io_slaves)
Oracle7에서의 mulitple DBWR process들은 단순히 slave process로서, asynch
I/O call을 수행할 수는 없었다. Oracle 8.0.3부터, slave database writer code
가 kernal에 포함되었고, slave process의 async I/O가 가능하게 되었다. 이것은
init.ora file 내에 dbwr_io_slaves라는 parameter를 통해 가능하며, IO slave가
asynchronous I/O가 가능하여 I/O call 이후에 slave가 block되지 않아 더 나은
성능을 제공한다는 것을 제외하고는 Oracle7과 매우 유사하다. slave process는
instance 생성 시기가 아닌 database open 시에 start되기 때문에 oracle process
id가 9번부터 할당되며, o/s에서 확인되는 process 이름도 ora_i10n_SID와 같은
형태가 된다.
dbwr_io_slaves=3으로 지정한 경우, 아래와 같은 oracle background process가
구동되며, ora_i101_V804, ora_i102_V804, ora_i103_V804이 dbwr의 slave
process들이다.
tcsol2% ps -ef | grep V804
usupport 5419 1 0 06:23:53 ? 0:00 ora_pmon_V804
usupport 5429 1 1 06:23:53 ? 0:00 ora_smon_V804
usupport 5421 1 0 06:23:53 ? 0:00 ora_dbw0_V804
usupport 5433 1 0 06:23:56 ? 0:00 ora_i101_V804
usupport 5423 1 0 06:23:53 ? 0:00 ora_arch_V804
usupport 5431 1 0 06:23:53 ? 0:00 ora_reco_V804
usupport 5435 1 0 06:23:56 ? 0:00 ora_i102_V804
usupport 5437 1 0 06:23:56 ? 0:00 ora_i103_V804
usupport 5425 1 0 06:23:53 ? 0:00 ora_lgwr_V804
usupport 5427 1 0 06:23:53 ? 0:00 ora_ckpt_V804
2. Multiple DBWR (db_writer_processes)
multiple database writer는 init.ora file내의 db_writer_processes라는
parameter에 의해 구현되며, 이것은 Oracle 8.0.4부터 제공되었다. 이것은
기존의 master-slave 관계가 아닌 진정한 의미의 복수개의 database writer를
사용하는 것이며, database writer process들은 PMON이 start된 후에 start되어
진다.
이름은 ora_dbwn_SID 형태이며, 아래에 db_block_lru_latches=2,
db_writer_processes=2로 지정한 경우 구동된 oracle background process들의
예이다. 여기에서 ora_dbw0_V804, dbw1_V804이 dbwr process들이다. 만약
db_writer_processes를 지정하지 않으면 기본값은 1인데 이 때도 Oracle7과 같이
ora_dbwr_SID 형태가 아닌 ora_dbw0_SID 형태의 process가 구동된다.
usupport 5522 1 0 06:31:39 ? 0:00 ora_dbw1_V804
usupport 5524 1 0 06:31:39 ? 0:00 ora_arch_V804
usupport 5532 1 0 06:31:39 ? 0:00 ora_reco_V804
usupport 5528 1 0 06:31:39 ? 0:00 ora_ckpt_V804
usupport 5530 1 0 06:31:39 ? 0:00 ora_smon_V804
usupport 5526 1 0 06:31:39 ? 0:00 ora_lgwr_V804
usupport 5520 1 0 06:31:39 ? 0:00 ora_dbw0_V804
usupport 5518 1 0 06:31:38 ? 0:00 ora_pmon_V804
db_writer_processes로 지정된 각 writer process는 하나의 latch set에 할당된다.
그러므로 db_writer_processes를 db_block_lru_latches으로 지정되는 LRU latch의
갯수와 같은 값으로 지정하는 것이 권장할 만하며, 단 CPU의 갯수를 초과하는 것은
바람직하지 않다.
[참고] 현재까지 init.ora file내에 구동되는 dbwr의 갯수는 db_block_lru_latches
parameter에 의해 제한된다. 즉 db_writer_processes 값을 db_block_lru_latches
보다 크게 하여도 db_block_lru_latches로 지정
된 수의 dbwr process가 기동된다.
Oracle8에서 DBWR I/O slave나 복수개의 DBWR를 제공하는 방법 중 좋은 점은
이 기법을 제공하는 것이 kernal 안에 포함되어 기존의 OSD layer로 구현되었던
것보다 port specific한 부분이 없고 generic하다는 것이다.
3. 두 가지 방법 중 선택 시 고려사항
이러한 두가지 형태의 DBWR 기법이 모두 도움이 되기는 하지만, 일반적으로
어느 것을 사용할 것인지는 OS level에서 asynchronous I/O가 제공하는지와
CPU 갯수에 의존한다. 즉, system이 복수 개의 CPU를 가지고 있으면
db_writer_processes를 사용하는 것이 바람직하며, aync I/O를 제공하는 경우
두 가지 모두 사용에 효과를 얻을 수 있다. 그런데 여기서 주의할 것은
dbwr_io_slaves가 약간의 overhead가 있다는 것이다.
slave IO process를 가능하게 하면, IO buffer와 request queue의 할당을 위해
부가적인 shared memory가 필요하다.
multiple writer processes와 IO slave는 매우 부하가 많은 OLTP 환경에서
적합하며, 일정 수준 이상의 성능을 요구할 때만 사용하도록 한다. 예를 들어
asynch I/O가 사용 가능한 경우, I/O slave도 사용하지 않고 하나의 DBWR만을
asynch I/O mode로 사용하는 것이 충분하고 바람직할 수 있다. 현재의 성능을
조사하고 bottleneck이 되는 부분이 DBWR 부분인지 정확히 조사한 후 사용하여야
한다.
[참고] 이 두 parameter를 함께 사용하면 dbwr_io_slaves만 효과가 있다.
이것은 dbwr_io_slaves는 master dbwr process를 db_writer_proceses에 관계 없이
하나만 가지도록 되어 있기 때문이다.

http://www.fors.com/velpuri2/PERFORMANCE/ASYNC
hare krishna
Alok

Nightmare with Log Writer, active files please explain !

Hi
Can anybody explain how many redo log groups server may need in no-archiving mode ?
The documentation from Oracle web site says:
If you have enabled archiving, Oracle cannot re-use or overwrite an active online log file until ARCn has archived its contents. If archiving is disabled, when the last online redo log file fills, writing continues by overwriting the first available active file.
From this it looks only 2 groups are needed. I dont get how is it possible to overwrite active file ??? I think I should be written by dbw and become inactive first. Is that the reason I get "checkpoint has not completed" warnings in the log and pure performance ?
Andrius

I believe the minimum required by oracle is 2 groups.
Obviously, this won't cut it in ARCHIVELOG mode for most databases. But then, you were referring to NOARCHIVELOG mode. I tend to go with 3 in this type of scenario.
As for the 2nd part. Only one redo log is 'ACTIVE' at a time. When a log switch occurs, it goes to the next INACTIVE redo log and starts writing to that. Thus, overwriting what was previously in it. DBWn doesn't write redo entries, that is handled by LGWR. As far as Oracle8i, the only part DBWn had on redo logs was triggering log writting by LGWR.

Blue Ray Writer for iMac

I need a Blue Ray writer for my new iMac.
I’ve not used a Blue Ray writer before and I’m not sure if the write - read rates justify going to the expense of a USB3 device?
I would appreciate any words of wisdom

The short answer is... Yes.
For the long answer, click on the following link...
Blu-ray buying guide
http://www.macworld.com/article/1142800/bluray.html

How is doing the write within RAC /ASM environment

Hi All;
Within Oracle RDBMS with cooked file system setup environment, we know or been told that dbw is doing the write. My question is who is doing the read/write job in ASM setup environment? Instance with ASM setup environment you can find dbw and asm_dbw processes. I am wander who is doing what? If you can share some light about it that will be greatly appreciated.

Some important ASM background processes
DBWR This process manages the SGA buffer cache in the ASM instance.
DBWR writes out dirty buffers (changed metadata buffers) from the ASM
buffer cache to disk.
PING The PING process measures network latency and has the same
functionality in RDBMS instances.
SMON This process is the system monitor and also acts as a liaison to the
Cluster Synchronization Services (CSS) process (in Oracle Clusterware) for
node monitoring.
ARBx These are the slave processes that do the rebalance activity (where x
is a number).
KATE The Konductor or ASM Temporary Errands (KATE) process is used
to process disks online. This process runs in the ASM instance and is started
only when an offlined disk is onlined.
CKPT The CKPT process manages cross-instance calls (in RAC).
GMON This process is responsible for managing the disk-level activities
(drop/offline) and advancing diskgroup compatibility.
MARK The Mark Allocation Unit (AU) for Resync Koordinator (MARK)
process coordinates the updates to the Staleness Registry when the disks go
offline. This process runs in the RDBMS instance and is started only when
disks go offline in ASM redundancy diskgroups.
HTH
Antonio NAVARRO

Data Loss when a database crashes

Hi Experts,
I was asked the question of "how much data is lost when you pull the plug off an oracle database all of a sudden".. and my answer was "all the data in the buffers that have not been committed". We know that you can have committed data sitting in the redo logs (that have not been written to the datafiles) that the instance will use for recovery once the instance is restarted again; however, this got me thinking and asking the question of how much uncommitted data is actually sitting in memory that potentially can be lost if the instance goes down all of a sudden?
With the use of sga_target and sga_max_size, the memory allocation for the cache buffer will vary from time to time.. So, is it possible to quantify the amount of lost data at all (in byts, kb, mb..etc)?
For example if the sga is set to 1gb (sga_max_size=1000mb), check point every 15mins (as we can't predict/know how often the app commits).. assume a basic transaction size for any small to medium size database. Redo logs set to 50mb (even though this doesn't come into play at this stage).
I would be really interested in your thoughts and ideas please.
Thanks

All Oracle Data Manipulation Language (DML) and Date Definition Language (DDL) statements must record an entry in the Redo log buffer before they are being executed.
The Redo log buffer is:
•     Part of the System Global Area (SGA)
•     Operating in a circular fashion.
•     Size in bytes determined by the LOG_BUFFER init parameter.
Each Oracle instance has only one log writer process (LGWR). The log writer operates in the background and writes all records from the Redo log buffer to the Redo log files.
Well, just to clarify, log writer is writing committed and uncommitted transactions from the redo log buffer to the log files more or less continuously - not just on commit (when the log buffer is 10mb full, 1/3 full, every 3 seconds or every commit - whichever comes first - those all trigger redo writes).
The LGWR process writes:
•     Every 3 seconds.
•     Immediately when a transaction is committed.
•     When the Redo log buffer is 1/3 full.
•     When the database writer process (DBWR) signals.
Crash and instance recovery involves the following:
•     Roll-Forward
The database applies the committed and uncommitted data in the current online redo log files.
•     Roll-Backward
The database removes the uncommitted transactions applied during a Roll-Forward.
What also comes into play in the event of a crash is MTTR, for which there is a advisory utility as of Oracle 10g. Oracle recommends using the fast_start_mttr_target initialization parameter to control the duration of startup after instance failure.
From what I understand, uncommitted transactions will be lost, or more precisely undone after an instance crash. That's why it is good practice to manually commit transactions, unless you plan to use SQL rollback. Btw, every DDL statement and exit from sqlplus implies an automatic commit.
Edited by: Markus Waldorf on Sep 4, 2010 10:56 AM

DBWR writes before LGWR wirtes?

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