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Tuning 'log file sync'

One of our database experiences heavy waits on 'log file sync'.
Top waits from V$SYSTEM_EVENT are given below. 'Log file sync'
exceeds 'db file sequential read' and 'db file scattered read'.
Metalink Note 223117.1 says that they key to understanding
'log file sync' is to compare average times waited for 'log file sync'
and 'log file parallel write': if they are similar then
it is I/O problem, but if they are different then "the delay is
caused by the other parts of Redo Logging mechanism
that occur during a COMMIT/ROLLBACK (and are not I/O-related)".
How to find out what are these "other parts"?

This is 9.2.0.5.0 on Tru64.

Thanks


                            Time  Average
                          Waited     Wait
Event                      (sec)    (sec)
------------------------  -------  ------
SQL*Net message from c    9836478     .19
rdbms ipc message          798023     .30
PX Idle Wait               763886    2.15
enqueue                    387408     .71
log file sync              237811     .21
pmon timer                 190927    3.11
smon timer                 186319    5.18
db file sequential read     85030       0
db file parallel write      57577     .20
local write wait            41371     .42
rdbms ipc reply             32636     .33
log file parallel write     27843     .03
latch free                   8538     .01

0
9/1/2005 10:54:25 PM
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"Vsevolod Afanassiev" <vafanassiev@yahoo.com> wrote in message 
news:1125615265.135322.10050@f14g2000cwb.googlegroups.com...
> One of our database experiences heavy waits on 'log file sync'.
> Top waits from V$SYSTEM_EVENT are given below. 'Log file sync'
> exceeds 'db file sequential read' and 'db file scattered read'.
> Metalink Note 223117.1 says that they key to understanding
> 'log file sync' is to compare average times waited for 'log file sync'
> and 'log file parallel write': if they are similar then
> it is I/O problem, but if they are different then "the delay is
> caused by the other parts of Redo Logging mechanism
> that occur during a COMMIT/ROLLBACK (and are not I/O-related)".
> How to find out what are these "other parts"?
>
> This is 9.2.0.5.0 on Tru64.
>
> Thanks
>
>
>                            Time  Average
>                          Waited     Wait
> Event                      (sec)    (sec)
> ------------------------  -------  ------
> SQL*Net message from c    9836478     .19
> rdbms ipc message          798023     .30
> PX Idle Wait               763886    2.15
> enqueue                    387408     .71
> log file sync              237811     .21
> pmon timer                 190927    3.11
> smon timer                 186319    5.18
> db file sequential read     85030       0
> db file parallel write      57577     .20
> local write wait            41371     .42
> rdbms ipc reply             32636     .33
> log file parallel write     27843     .03
> latch free                   8538     .01
>


You do seem to have some unusual
activity there.  Most surprising is the
smon timer, at an average of five seconds
when the common smon timeout is every
five minutes.

Second on the list is the very low value for
the average wait for SQL*Net message from client:
Connor's observation that you are doing too
many commits may be the basic problem - do
you have several 'batch' processes that are running
round and round in circles doing tiny bits of work
in each cycle and then committing.

Your write times (db file parallel at 200 ms
and log file parallel at 30 ms) are slow, which
suggests you have an I/O problem. Again this
may be a side effect of your coding strategy,
exaggerated by an aggressive MTTR.

Log file sync is often a consequence of
log writer having trouble - but in this case,
as you point out, the log file sync average
is much higher than the log writer average,
so it is POSSIBLE  that there is another cause.

As others have pointed out, increasing the
log buffer size can increase the average
length of log file syncs - on the other hand,
decreasing the log buffer can leave you
suffering from log buffer space waits - but
the figures do not suggest that your 2M log
buffer is too large.

You don't have any significant time waiting on
'LGWR waiting for redo', which can occur in
a highly concurrent system as LGWR waits
for other sessions to fill the log buffer that they
have allocated but not yet copied into.

So the thought I would pursue is that the log writer
is having trouble notifying the correct sessions that
the log buffer has been written. Are you seeing many
timeouts on log file sync ? How busy is the CPU ? How
many processes Oracle processes are running on the
machine ? How much non-Oracle CPU is being used ?


As a side-note, the average wait for enqueues is large.
Take a look at v$enqueue_stats to find out which
types of enqueue are suffering most waits and losing most
time - this may be relevant.  I have seen three cases of
unusual log file sync times so far in 9.2 - one of them
related to frequent truncation of global temporary tables,
and the enqueue times may be giving us a hint that this
is your problem.



-- 
Regards

Jonathan Lewis

Now waiting on the publishers:    Cost Based Oracle - Volume 1

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated 8th July 2005





0
jonathan5683 (1392)
9/5/2005 7:34:47 AM
Late feedback that may be relevant on this one.

I recently saw a cluster of examples about unusual
log file sync/log file parallel write waits - (yours was
one of them) and decided that something odd was
happening that needed further investigation.

The results are written up here:
    http://www.jlcomp.demon.co.uk/log_file_writes.html

The bottom line is this:
Bug 2669566 - log writer does not report
log file parallel write wait times properly,
so a high average wait time for log file SYNC
may be telling you that lgwr has an I/O problem,

    The bug is in 9.2 - it's fixed in 10.1
    It's not going to be fixed in 9.2


-- 
Regards

Jonathan Lewis

http://www.jlcomp.demon.co.uk/cbo_book/ind_book.html
Cost Based Oracle - Volume 1: Fundamentals
On-shelf date: Nov 2005

http://www.jlcomp.demon.co.uk/faq/ind_faq.html
The Co-operative Oracle Users' FAQ

http://www.jlcomp.demon.co.uk/appearances.html
Public Appearances - schedule updated 4th Sept 2005







"Vsevolod Afanassiev" <vafanassiev@yahoo.com> wrote in message 
news:1125615265.135322.10050@f14g2000cwb.googlegroups.com...
> One of our database experiences heavy waits on 'log file sync'.
> Top waits from V$SYSTEM_EVENT are given below. 'Log file sync'
> exceeds 'db file sequential read' and 'db file scattered read'.
> Metalink Note 223117.1 says that they key to understanding
> 'log file sync' is to compare average times waited for 'log file sync'
> and 'log file parallel write': if they are similar then
> it is I/O problem, but if they are different then "the delay is
> caused by the other parts of Redo Logging mechanism
> that occur during a COMMIT/ROLLBACK (and are not I/O-related)".
> How to find out what are these "other parts"?
>
> This is 9.2.0.5.0 on Tru64.
>
> Thanks
>
>
>                            Time  Average
>                          Waited     Wait
> Event                      (sec)    (sec)
> ------------------------  -------  ------
> SQL*Net message from c    9836478     .19
> rdbms ipc message          798023     .30
> PX Idle Wait               763886    2.15
> enqueue                    387408     .71
> log file sync              237811     .21
> pmon timer                 190927    3.11
> smon timer                 186319    5.18
> db file sequential read     85030       0
> db file parallel write      57577     .20
> local write wait            41371     .42
> rdbms ipc reply             32636     .33
> log file parallel write     27843     .03
> latch free                   8538     .01
> 


0
jonathan5683 (1392)
9/15/2005 7:22:42 AM
Reply:

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error: expected '=', ',', ';', 'asm' or '__attrib
Hi I'm trying to compile an ADC Driver & come acrosss the following error. I've no experience writing drivers before, and hence have no clue how to fix it. Hope someone out there has encountered the problem & suggesst a fix for the same. The Error is I get is : qadc.c: At top level: qadc.c:97: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'qadc_read' make: *** [qadc.o] Error 1 [root@localhost qadc]# ########################################################################### ADC Driver Code ########################################################################### #define MODULE #define __KERNEL__ #include <linux/config.h> #include <linux/module.h> #include <linux/kernel.h> /* printk */ #include <linux/fs.h> / #include <linux/errno.h> /* error codes */ #include <linux/types.h> /* size_t */ #include <linux/proc_fs.h> /* proc file system */ #include <linux/fcntl.h> #include <asm/system.h> /* cli, flags */ #include <asm/uaccess.h> /* copy from/to user */ /*Registers to get qadc access*/ volatile unsigned short * qadcmcr = (unsigned short *)0x40190000; volatile unsigned short * qacr0 = (unsigned short *)0x4019000a; volatile unsigned short * qacr1 = (unsigned short *)0x4019000c; volatile unsigned short * qacr2 = (unsigned short *)0x4019000e; volatile unsigned short * qasr0 = (unsigned short *)0x40190010; volatile unsigned short * qasr1...

logical to 'on' / 'off'
Hi, is there a function implemented doing this conversion? my Problem is, that I want to use the following code: set(handles.edit_curr_trq_sl,'Enable',get(hObject,'Value')) where get(hObject,'Value') gives the state of a checkbox thank you! function [str]=tf2oo(logic) switch logic case 0 str='off'; case 1 str='on'; end%switch end%function tf2oo() while i do not know a built in function, I use my own:) meisterbartsch wrote: > > > function [str]=tf2oo(logic) > switch logic > case 0 > str='off'; &g...

Difference between 'is' and '=='
Hey guys, this maybe a stupid question, but I can't seem to find the result anywhere online. When is the right time to use 'is' and when should we use '=='? Thanks alot~ mwql: >Hey guys, this maybe a stupid question, but I can't seem to find the >result anywhere online. When is the right time to use 'is' and when >should we use '=='? http://docs.python.org/ref/comparisons.html -- Ren´┐Ż Pijlman mwql wrote: > Hey guys, this maybe a stupid question, but I can't seem to find the > result anywhere online. When is the right time to ...

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