Why checkpoint increase SCN?

the database is not used now,and I uses command
"alter system checkpoint" repeatly,and every time
checkpoint,the SCN will increase,but there is nobody
using the database,and there is no transaction to
commit,why will SCN increase? From oracle doc,every
time commit,the SCN will increase,under what
situation will SCN alse increase?

under what situation will SCN alse increaseThe recovery process uses SCN to identify a particular state of the database.
Oracle sometimes says SCN stands for System Commit Number (e.g. error message for ORA-8209) but this is misleading because it can also be incremented by other things. These things are not well documented, but they seem to be related to recursive data dictionary SQL (which makes sense). One such is block cleanout. Another is checkpointing, as you've noticed. I believe activity by stuff like AQ, DBMS_JOB, etc will increment the SCN.
There is an aside in Metalink note #28929.1 which states rather tantalizingly:
In some situations, the SCN increment during startup may permit the database to open.
Does this mean STARTUP also increments the SCN? Something for me to test when I get home this evening.
Does anybody out there have a definitive list?
Cheers, APC

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Checkpoint and SCN

Hi,
When executing below command, I found check point occur and SCN is changed...
alter system switch logfile;
While executing below commnd checkpoint is also ocuured but SCN not changed..
alter system checkpoint;
As both the execusion, checkpoint occur and SCN is written.. can anyone tell why this happened?
Can anyone explain the relationship between checkpoint and SCN?
Thanks,
Tina K.

Lets keep it simple and straight, SCN generate when ever there is change (ie change vector).
checkpoint -Simple write dirty block to datafile and update control file too.
( check point keep database safe(=easy recoverable )from instance failure , power failure..)
Just think u have 2 online redo logfile , you switch the logfile so all changes is there in logfile 1 is wriiten to datafile Right, now logfile 2 is current logfile and power went off. When u start again database oracle try to check/match SCN of all datafiles from control file , if any databfiles header has less SCN it will read chnage vector from online redo and apply chnages and update header.
SCN is base for all recovery and work around the change vector.
I know i am very poor in explaining things , if you want to know more insight story read book "Backup and Recovery by Rama -Oracle Press" it has very good details.
Cheer,
Virag Sharma
http://virag.sharma.googlepages.com/

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....

Incremental checkpoint and SCN

Hi,
I am getting messages of incremental checkpoint in my alert logs with some scn.
>
Completed checkpoint up to RBA [0x125de6.2.10], SCN: 445135162445
>
Does this mean that all dirty blocks which have had their initial changes before this SCN(445135162445) will be written to disk so that instance recovery can begin from the SCN from which checkpoint has completed.Or is it the other way like the incremental checkpoint has occured at scn 445135162445.
Sekar

user13485610 wrote:
As per my knowledge, the checkpoint is classified as below (correct me if I am wrong somewhere)
Checkpoint types can be divided as INCREMENTAL and COMPLETE.
Also COMPLETE CHECKPOINT can be divided further into
PARTIAL and FULL.
It would be convenient to have a reference to the documents where you picked up this information. There may be further reading in them that clarifies the meaning. The terms have been around for a long time, of course, but it's always hard to get any sort of definitive description together - in your case, for example, you don't make any comment about which checkpoints lead to high priority writes and which to low, but the description of any type of checkpoint isi incomplete without some reference to the write priority.
As far as classifying checkpoints by name - I'm not too concerned that there is still some confusion in the different way that people name or categorise them, provided that they can describe what's going on to ensure that there is no ambiguity. In this context I think there are only options to consider:
a) does the particular type of checkpoint walk along the checkpoint queue (CKPTQ) in order to pick the blocks that need to be written to disc.
b) does the particular type of checkpoint use a different queue (such as an object queue or file queue) to pick the blocks that need to be written to disc.
c) is there any other mechanism for picking the blocks to be written - such as walking the LRU and identifying all dirty blocks.
To my mind, an incremental checkpoint should probably have a definition that says it walks the checkpoint queue.
I dislike the term "complete" if it then leads to the option for "partial" - how much clarity can you read into the statement "at this point Oracle does a partial complete checkpoint" (or should that be a "complete partial checkpoint") - but I can understand the need for a term of that sort to distinguish a checkpoint that is based on one of the other queues.
But my doubt is mentioned below.
2.At the time of log switch - Sometimes log switches may trigger a complete checkpoint , if the
next log where the log switch is to take place is Active.
Why is this behaves in this fashion? (Any internal thoughts on this please)This, in part, is why I'd like to see the reference document - I think that the term "complete" may have been given a different meaning at this point. If the logfile you want to use is still active checkpoint activity MUST take place urgently, but it need only be a checkpoint that walks the CKPTQ up to the point where the content of the target redo log can be discarded. This is no different from any other checkpointing due to log file switch - but it could have a higher degree of urgency. (The need to differentiate this special case on log file switch probably came about at the time that Oracle stopped triggering an automatic checkpoint at every log file switch.)
Regards
Jonathan Lewis

Why does increasing logical processors in Hyper-V for virtual machine increase performance.

To my understanding, virtual machines (assuming relative weight is even) get an equal share of the processor. When the VM is given some processing time, it shouldn't matter if the VM is seeing a single core for it to use or multiple cores, since
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Checkpoint & SCN

Hi,
I have 1 small doubt related to checkpoint and SCN,
I have scheduled a hot backup at 4AM in the morning & ends at 7 AM in morning,
before and after there are 10 log switches,
But when i checked the at 3 pm , the view v$datafile shows the checkpoint time at 2:31 AM
why is it so? if a checkpoint occurs at the redo log file switch.
It means that there will be last checkpoint after 7AM or latest redo log switch..
Please correct me if Im wrong
Edited by: user10745179 on Sep 28, 2010 3:33 AM

Hi Aman,
thanks for ur reply ,
but then what happens at the begin tablespace backup mode, at that time checkpoint occurs then why it is not updated
Please tell if im wrong?
Please tell me wht exactly happens?
When the checkpoint will occur for other tahn clean shutdown?
Edited by: user10745179 on Sep 28, 2010 4:43 AM
Edited by: user10745179 on Sep 28, 2010 4:44 AM

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In order to avoid board clutter and confusion, we ask that you please refrain from cross and double posting.
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ORA-19648: datafile 132: incremental-start SCN equals checkpoint SCN

I re-ran full backup but still cumulative backups failing with this error, looking for quick commands to fix this
RMAN-00571: ===========================================================
RMAN-00569: =============== ERROR MESSAGE STACK FOLLOWS ===============
RMAN-00571: ===========================================================
RMAN-03009: failure of backup command on T1 channel at 04/12/2010 20:55:16
ORA-00604: error occurred at recursive SQL level 1
ORA-01427: single-row subquery returns more than one row
ORA-19648: datafile 132: incremental-start SCN equals checkpoint SCN
ORA-19640: datafile checkpoint is SCN 4361058556501 time 02/08/2008 20:32:22

19645, 00000, "datafile %s: incremental-start SCN is prior to creation SCN %s"
// *Cause: The incremental-start SCN which was specified when starting an
// incremental datafile backup is less than the datafile's
// creation SCN.
// *Action: Specify a larger incremental-start SCN.

Checkpoint entries in my alert log - needhelp to decipher

hi guys,
I have the following in my alert log. Can someone please help decipher to let me know why Checkpoint is not being complete?
thanks
Beginning log switch checkpoint up to RBA [0x240.2.10], SCN: 0x0000.00255e4d
Thread 1 advanced to log sequence 576
Current log# 2 seq# 576 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 576 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Thread 1 cannot allocate new log, sequence 577
Checkpoint not complete
Current log# 2 seq# 576 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 576 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Tue Sep 2 16:22:39 2008
Completed checkpoint up to RBA [0x240.2.10], SCN: 0x0000.00255e4d
Tue Sep 2 16:22:39 2008
Beginning log switch checkpoint up to RBA [0x241.2.10], SCN: 0x0000.00255ed6
Thread 1 advanced to log sequence 577
Current log# 1 seq# 577 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.1a.log
Current log# 1 seq# 577 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.1b.log
Tue Sep 2 16:22:46 2008
Completed checkpoint up to RBA [0x241.2.10], SCN: 0x0000.00255ed6
Tue Sep 2 16:22:48 2008
Beginning log switch checkpoint up to RBA [0x242.2.10], SCN: 0x0000.00255f57
Thread 1 advanced to log sequence 578
Current log# 2 seq# 578 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 578 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Thread 1 cannot allocate new log, sequence 579
Checkpoint not complete
Current log# 2 seq# 578 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 578 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Tue Sep 2 16:22:51 2008
Completed checkpoint up to RBA [0x242.2.10], SCN: 0x0000.00255f57

Aman.... wrote:
Oracleguy,
You switched in 3 minutes only,
Tue Sep 2 16:22:48 2008*
Beginning log switch checkpoint up to RBA http://0x242.2.10, SCN: 0x0000.00255f57
Thread 1 advanced to log sequence 578
Current log# 2 seq# 578 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 578 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Thread 1 cannot allocate new log, sequence 579
Checkpoint not complete
Current log# 2 seq# 578 mem# 0: /ora2/oradata/9.2.0.7/TESTDB/redoTESTDB.2a.log
Current log# 2 seq# 578 mem# 1: /ora3/oradata/9.2.0.7/TESTDB/redoTESTDB.2b.log
Tue Sep 2 16:22:51 2008+
Completed checkpoint up to RBA http://0x242.2.10, SCN: 0x0000.00255f57
Its pretty small time for log checkpoint to get complete. You need to make your log files bigger and add new members in them so that thereis sufficient time for checkpoint to get complete.
What's your current redo log configuration?
Aman....hi aman, did you mean 3 minutes, or 3 seconds?
I have two groups, and the log files are at 10M
thanks

Difference between switch logfile and checkpoint

Hi All,
what is the difference between
Forcing log file switches:
ALTER SYSTEM switch logfile;
and
ALTER SYSTEM checkpoint;
when i should use one and not use the other
Thanks very much

I don't know that checkpoint will increase the scn. In a way it can be called correct but there are multitudes of SCN floating around. For example, for the matching of the datafiles and controlfles, there are two scn's which are used, stop scn and checkpoint scn. Than for the block, there can be , commit scn , transaction scn , High scn and low scn. There is one counter that is there for both checkpoint and SCN and checkpoint number is generated from SCN itself. I am sure that there is a missing link here some where but still, to some extent this is correct.
Checkpointing is an event which happens at certain conditions and SCN is always there, just like in a clock, there is always a time.
HTH
Aman....

How to increase No. of Connections in Oracle DB?

Hi,
As my team using more no. of connections, I need to increase the number of connections.
Kindly guide me to check the current number of connections and how to change / increase that?
As I don't have knowledge on this, I am looking the detailed input on this.
Kindly guide me, How to check the no. of connections in DB and how to increase that?
Thanks in advance,
Orahar.

Orahar wrote:
As my team using more no. of connections, I need to increase the number of connections.For what type of sessions? That determines whether dedicated or shared server sessions on the Oracle server can be used. Shared server scales a lot better than dedicated servers, but requires specific factors to be taken into consideration (e.g. the move of session UGA from the PGA to the SGA, dispatchers required, etc).
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FAST_START_MTTR_TARGET and checkpoint

DB version : 11.2
If I set
FAST_START_MTTR_TARGET=0
LOG_CHECKPOINT_INTERVAL=0
LOG_CHECKPOINT_TIMEOUT=1800Then how often does checkpoint occurs ? -- ignoring Manual log switch and ALTER SYSTEM CHECKPOINT command

TeslaMan wrote:
DB version : 11.2
If I set
FAST_START_MTTR_TARGET=0
LOG_CHECKPOINT_INTERVAL=0
LOG_CHECKPOINT_TIMEOUT=1800Then how often does checkpoint occurs ? -- ignoring Manual log switch and ALTER SYSTEM CHECKPOINT commandThese parameter affects the checkpoints, but if you are thinking that why checkpoint occurs when I am having above setting then, you are bit confused, because happening of checkpoint is another thing and having the values of the above parameter is different. When your current redo log becomes full, then there will be logswitch and logswitch is the cause of checkpoint. Checkpoint automatically occurs at a log switch.
Now if the question is why there is so much log switch, then simple answer is generation of more redo, more transactions. Further question turns into different direction that why there is more redo generation and its answer is far away from this current one.
Regards
Girish Sharma

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Why checkpoint increase SCN?

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