Dbwr_io_slaves or db_writer_processes

Im having a little trouble understanding the situations in which using dbwr_io_slaves would be better than db_writer_processes, it would be nice if someone could explain it clearly or point me to documentation that explains it cleary.
I have a secondary question which im pretty sure is related, which is, one of the databases that I deal with has a large number of dirty buffers (ie: > 600,000) at the moment the only way I know to clear this and reduce the number is to restart the database from time to time (this is not something I want to be doing and need a better solution).
Some background information:
The database gets information loaded into it in batches on a nightly basis, the server the database runs on has two dual core cpu's with 8gb of ram, there are also a couple of other databases running on these systems as well.

With DBWR I/O Slaves, you have one dbwr processes that gathers dirty blocks from the cache and shares them out to the I/O slaves to write. This emulates asynchronous I/O for operating systems that don't have it, and probably isn't needed on any modern system.
With multiple database writers you break the cache into smaller pieces (working data sets) and each DBWn is responsible for keeping part of the cache clean - this improves scalability for extreme cases but if you read Kevin Closson's blog (The index http://kevinclosson.wordpress.com/kevin-closson-index/general-performance-and-io-topics/ has a number of posts on over-configuring dbwr processes) you will see why this isn't often necessary.
Your 600,000 dirty blocks is a big number - how big is the entire buffer cache, and how are you checking that number ? One possibility for the size is that consistent read copies of dirty current blocks don't seem to get their dirty bit cleared on some versions of Oracle. If you refine you test to show the block state, you may find that most of the dirty blocks are CR copies.
Regards
Jonathan Lewis
http://jonathanlewis.wordpress.com
http://www.jlcomp.demon.co.uk
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    제품 : ORACLE SERVER
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    http://www.fors.com/velpuri2/PERFORMANCE/ASYNC
    hare krishna
    Alok

  • Db_writer_processes vs dbwr_io_slaves

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  • Oracle db_writer_processes

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    Note: 67422.1 - Init.ora Parameter "DB_WRITER_PROCESSES" Reference Note
    Regards,
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  • Increase the db_writer_processes

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    http://kevinclosson.wordpress.com/2007/08/10/learn-how-to-obliterate-processor-caches-configure-lots-and-lots-of-dbwr-processes/

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    「非同期I/O」という意味が理解出来ていないので、噛み砕いた表現をして頂けると嬉しいです。。。
    宜しくお願い致します。

    Q1.そもそもI/O SLAVEとはなんでしょうか?非同期I/Oという記述もあったのですがこちらも意味が分かりません。非同期I/Oの技術的な説明はこちらが良いですかね。。
    <<http://lab.klab.org/files/alm/20070806/aio.pdf>>
    I/O SLAVEはアプリケーションから渡されたDISK I/OをDBWRが一つのプロセス上で全て処理するのではなく、それぞれのI/O SLAVEに渡す事で、アプリケーション側からはDBWRはあたかも並列で非同期にI/Oを処理しているかのように見せる技術と理解してます。
    Q2.「BUFFERのFLUSHなどのDBWRとして~」という記述について
      結局はDBWR_IO_SLAVESを設定してもしなくてもDBライターの動きは同じなのでしょうか?DISK I/OのみをSLAVEに実行させる、とあります。
    BUFFERのFLUSH処理等は設定していてもしていなくても変わらないが、DISK I/Oの動作はSLAVE経由になるので変わる、という理解です。
    Q3.DBWR_IO_SLAVESを設定するメリットは何になるでしょうか?非同期I/Oが使えない環境において、DBWRのDISK I/O処理がボトルネックになっている場合に、DISK I/O処理を分散させる事でそのボトルネックを解消する事ができると考えてます。
    基本的に11.2.0.3が入るLinuxであれば、libaioが入っていると思いますので、非同期I/Oが可能だと考えてます。
    <<http://docs.oracle.com/cd/E16338_01/server.112/b56317/appc_linux.htm#BABIIHEJ>>

  • DB_WRITER_PROCESSES in Oracle 10.2.0.4 HP UX

    Hi all,
    I noticed that asynchronous i/o cannot be used with regular filesystem on HP UX (only raw devices).
    As far as I know what I can do is to configure multiple db writers with DB_WRITER_PROCESSES parameter.
    DB has CPU_COUNT in 16, but 2 db writers (default value = CPU_COUNT / 8).
    Will I have any benefit by increasing DB_WRITER_PROCESSES?
    Or only if there is a bottleneck in writing dirty buffers?
    Thanks

    >
    If bottleneck exists it would be at the disk end & not the CPU end.
    Server processes are at least 1000 times faster than mechanical disk drives.
    a single DB_WRITER could keep 10 disk controllers busy & not be a bottleneck.Well, then why does Oracle default to 1 process every 8 cpus?
    DBWn also handles checkpoints, file open synchronization, and logging of Block Written records, according to [url http://docs.oracle.com/cd/E11882_01/server.112/e25513/bgprocesses.htm#BBBDIIHC]the docs. On hp-ux Oracle does file handles oddly - see MOS How to Disable Asynch_io on HP to Avoid Ioctl Async_config Error Errno = 1 [ID 302801.1] if you (the OP) haven't already.
    So on hp-ux you have a potential problem of the architecture assuming asyncronicity but the dickering between dbw, lgwr and ckpt turning into bickering. Just because something is 1000 times faster doesn't mean it isn't doing 1000 more operations. In real life, you wind up with Oracle telling you to ignore Private strand flush not complete errors, because there are no actual lgwr problems unless switches are slow.

  • Db_writer_process and async IO??

    we have ORACLE 10Gr2 on Redhar AS LInux server version 4.X. I checked one of database init.ora have following setup:
    db_writer_processes = 2
    filesystemio_options = asynch
    DISK_ASYNCH_IO = true
    TAPE_ASYNCH_IO = true
    Based on what I know "db_writer_processes " and "filesystemio_options = asynch" are exclusive. Can anyone tell me database will use whiche one?
    Thanks.

    Based on what I know "db_writer_processes " and "filesystemio_options = asynch" are exclusiveWhere did you get the information these two parameters are exclusive?
    HTH -- Mark D Powell --

  • Initial Parameter DB_WRITER_PROCESSES Doesn't work

    I set the initial parameter DB_WRITER_PROCESSES as 5 in init.ora file. But when I restarted the database, I found the parameter show as 1 and only DBW0 existed.
    I don't know the reason, would you please help me?
    Thanks and Best Regards,
    Su Qian

    Did you get a message in the alert log about starting dbwr processes failed?No, I havn't gotten the message.
    Also, having multiple writers on a single cpu system could bring performance down. I think one rule of thumb is to have 4 cpus per writer.Thanks for your message. I will not re-set the parameter.
    First, I'd like to let you know that the environment of my production database are as follows:
    OS: HP-UX
    Database Version: Oracle8 Enterprise Edition Release 8.0.4.2.1 - Production
    The setting of DB Block: 2K
    The main problem I come across is when running application the response speed of DB is very slow.
    The first step I adopted was checking free space. After checking I thought it was sufficient. But when I adopted the second step checking v$system_event, I found the wait time of following events is high.
    EVENT TOTAL_WAITS TOTAL_TIMEOUTS TIME_WAITED AVERAGE_WAIT
    db file parallel write 471 1 128370 272.547771
    log file parallel write 30307 0 51440 1.69729765 (For reference)
    pmon timer 37189 36806 11111071 298.773051
    rdbms ipc message 142737 112253 66295957 464.462312
    rdbms ipc reply 296 0 7167 24.2128378
    smon timer 372 369 11082933 29792.8306
    write complete waits 75 37 5149 68.6533333
    According to the above message, I decided to tune I/O first. Is my thought right?
    If dbwr is rather slow, these events should give som hints: free buffer waits, rdbms ipc reply, log file switch (checkpoint incomplete).Yes, when I had a look at the alert.log I found the error message: checkpoint incomplete. So I created a new log group.Perhaps the problem was too few log groups and not dbwr. If you do not have high free buffer waits dbwr should be ok :)I agree with you. I havn't high free buffer waits, so the main problem could be too few log groups. But I don't understand two points as follows.
    1. Last Friday I had 4 log groups and the size of every group is 2M. when getting the message "Thread 1 cannot allocate new log, sequence 87816 Checkpoint not complete", I created a new log group of 2M. Normally, this operation should improve performance, but when I observed the statistics I found the average wait time of "db file parallel write" became more longer. (Please refer to the following list)
    EVENT TOTAL_WAITS TIME_WAITED AVERAGE_WAIT DATE LOG_GROUPS
    db file parallel write 471 128370 272.547771 2002-08-05 5
    db file parallel write 798 88568 110.987469 2002-08-03 4
    2. I think the shortage of log groups is caused by the archive of log group don't complete before overwirting. From the alert.log file I found the time of log switch is about 20 minutes. From my viewpoint 20 minutes is enough for log archive. Why is the log groups still short?
    In addition, according to the setting of checkpoint parameters I think the checkpoint should occur only at the time of log switch.
    NAME TYPE VALUE
    db_block_checkpoint_batch integer 8
    log_checkpoint_interval integer 1000000
    log_checkpoint_timeout integer 0
    db_block_max_dirty_target integer 4294967294
    db_block_buffers integer 153600 (For reference)
    Do you know the meaning db_block_checkpoint_batch? (I can't find it in the document of 8i, because it is obsoleted.)
    From the alert.log file, I got the following errors.
    1. Fri Aug 2 17:10:36 2002
    Errors in file /lims/oradata/lab1/udump/ora_10976.trc:
    ORA-00600: internal error code, arguments: [1237], [], [], [], [], []
    (I got the error message occasionally.)
    2. Fri Aug 2 16:57:37 2002
    ORA-1652: unable to extend temp segment by 256 in tablespace TEMP
    (I don't understand it clearly, because the size of temporary tablespace is 400M. I think it is large enough.)     
    3. Fri Jul 26 12:00:17 2002
    Thread 1 cannot allocate new log, sequence 87816
    Checkpoint not complete
    Current log# 1 seq# 87815 mem# 0: /u02/oradata/lab1/db/redolab101.log
    Looking forward your reply.
    Thanks and Best Regards,
    Su Qian

  • DB_WRITER_PROCESSES

    What is the maximum number of DB_WRITER_PROCESSES can allow?
    How can we increase the number of DB_WRITER_PROCESSES?
    Is there any performance issue allowing more DB_WRITER_PROCESSES?
    Regards

    I just want to note the fact that there are some arguments about the appropriate count of dbwr.
    These days, almost every enterprise level file system(whatever it is called) provides high performance asynchronous I/O capability. Do multiple dbwr processes have meaning here? Especially when your I/O request is handled at asynchronous way?
    What do you think about this?

  • In synch_io mode is db_writer_processes=cpus ok or do I need dbrw_io_slaves

    Using 11.2.0.3.0 on sun unix sprac server, 8 cpus. Due to OS bug we can not run in asynch i/o mode (we set disk_asynch_io = false). Our next window for OS fix is June.
    in meantime DBA says that setting db_writer_processes = # of cpus(8) is all we need. i mentioned that docs say that dbrw_io_slaves should be set to > 0 but he says that that setting is depreciated and not needed that db_writer_processes is all we need. In fact he says that he doesn't think that going back to async i/o is going to make much difference. now management is thinking if it aint broke why fix it, why risk OS upgrade if things aren't broken. I brought that on our db log file synchs are 100 to 1 to db sequential and scattered reads and that that is not good, and that we are about to hit our busiest time.
    So is db_writer_processes setting all we need to deal with running in synch I/O mode or will setting dbrw_io_slaves be a plus. Unfortunately we do hav a window to test different settings and their impact on performance. I respect the people here and so do my bosses.

    Hi,
    Could you please use the code tags to format your code? Read about how to do that in the [url https://forums.oracle.com/forums/help.jspa]FAQs. It makes it a lot easier for everyone and you will get more responses.
    You mean the top 5 sessions over the last 3 days for the log file sync wait event? I assume you've said 3 days because you are looking at the v$system_event view which is cumulative since startup? Please confirm.
    The response times look pretty bad for the log file sync event - I'm looking at the average_wait time there, which is in milliseconds. On my system, for example, we have 1ms response time and we generate 5MB of REDO a second. Obviously there are many factors to consider but you wnat to get it as low as possible.
    How does the load on the disks look? Do you have your REDO logs on the same drives as your data files?
    Feel free for someone else to correct me if you think I'm wrong on this but my thoughts are that you have two issues.
    1) Async vs sync for I/O. I would say async is better but you'd have to test it for your application and load profile
    2) High log file sync wait event, which is synchronous because of what I mentioned earlier so I don't rhink running in async mode would help here
    Do you have an AWR report you can paste out here (using the code tags)? Not the whole thing, just the key parts at the top of the report.
    Rob

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