Interpreting writes from v$sysstat
- From: "Milen Kulev" <makulev@xxxxxxx>
- To: oracle-l-bounce@xxxxxxxxxxxxx
- Date: Fri, 19 Sep 2008 11:57:49 +0200
Hello Listers,
I have the following problem:
I am sampling v$sysstat each 5 seconds (similarly to the snapper Tool from
Tanel Poder ->
http://blog.tanelpoder.com/2007/12/06/oracle-session-snapper-v106-released/).
I have found that the discrepancy between statistics 'physical write total
bytes ' and
'physical write bytes ' is suspiciously high.
According to the documentation
(https://students.kiv.zcu.cz/doc/oracle/server.102/b14237/dynviews_2136.htm)
the definitions of both system statistics are:
- 'physical write total bytes ' -> Total size in bytes of disk reads by all
database instance activity including
application reads, backup and recovery, and other utilities.
The difference between this value and "physical read bytes" gives the total
write size
in bytes by non-application workload.
- 'physical write bytes' -> 'Total size in bytes of all disk writes from the
database application activity (and not other kinds of instance activity)'
The real data (from a production database):
physical write total bytes -> Change rate = 379,494/second (379kb/sec)
physical write bytes -> Change rate = 26,214/second (26kB/sec)
My questions were:
1) Should sampling on v$sesstat suffice to find which process/es is/are writing
actively. I suppose yes,
but I want just to be sure that I am on the right track
2) According to the documentation (see above) 'physical write total bytes '
contains
'physical write bytes' + RMAN (none active at the moment) + background process
activity (not noticeable)
+ exp/imp (none) + expd/impd (none) + recursive (und SYS) SQL activity.
If it so, which process is consuming the bandwidth of
379kb/sec - 26kB/sec = 353kb/sec ? The bandwidth and performance is not the
problem, I just want
to be sure that I am interpreting statistics correctly...
3) How to interpret the sentence "disk writes from the database application
activity" ? Are these
only writes generated by INSERT/UPDATE/DELETE/TRUNCATE user sql statements ?
Users are performing only selects. I can not see any (noticeable, from
V$session_wait) LGWR or DBWR activity.
OS = SUSE 10 x86
Database = 10.2.0.4, no RAC
Any suggestions are welcome.
Best Regards. Milen
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