Re: Broadcast on commit and log file sync
- From: Lei Zeng <leizeng2003@xxxxxxxxx>
- To: "saurabhmanroy@xxxxxxxxx" <saurabhmanroy@xxxxxxxxx>, oracle-l <oracle-l@xxxxxxxxxxxxx>
- Date: Fri, 24 Feb 2012 15:04:09 -0800 (PST)
For RAC environement with BOC SCN, the 'log file sync' time involves two
pieces: redo buffer write to log file, and SCN broadcast acknowledge back time
redo write time - Total elapsed time of the write from the redo log buffer to
the current redo log file in 10s of milliseconds
redo size - Total amount of redo generated in bytes
Lei
DBspeed http://www.dbspeed.com/product.html
________________________________
From: saurabh manroy <saurabhmanroy@xxxxxxxxx>
To: oracle-l <oracle-l@xxxxxxxxxxxxx>
Sent: Friday, February 24, 2012 1:56 AM
Subject: Broadcast on commit and log file sync
Hello All,
I am seeing the following message in lgwr tracefile along with 'log write
time wait time' warnings:
Warning: log write broadcast wait time 3000ms
This is a RAC database (Version 11.1.0.7.9) We are using default BOC SCN scheme.
My question is: In RAC, Can this wait time exacerbate problems of log
file sync waits ? Any known conditions ?
There are few bugs (e.g. 7716356) on MOS pointing to increase in log
file sync waits due to BOC scheme, but i am unable to draw a clear
picture in my mind about the whole mechanism of BOC and how can it
impact log file sync.
Any comments are welcome.
Regards,
Saurabh Manroy
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A description of the life cycle of 'log file sync' wait :
1. A user sessions issues a commit or rollback and starts waiting on log file
sync.
2. LGWR gather redo information to be written to redo log files, issues IO and
queues BOC to an LMS process and posts LMS process.
3. LGWR waits for redo buffers to be flushed to disk and SCN to be ACK'd
4. Completion of IO and receiving ACK from LMS signal write complete. LGWR then
posts foreground process to continue.
5. Foreground process wakes up and log file sync wait ends.
You can check the AWR or v$SYSSTAT for the following stat names.
DBspeed is more easy to examin how their values change over a time frame.
http://www.dbspeed.com/index.html
'AWR instance & service statistics' section, 'AWR instance activity statistics
by stat name' report
redo write broadcast ack count - Number of times a commit broadcast
acknowledgment has not been received by the time when the corresponding log
write is completed.
redo write broadcast ack time - Total amount of the latency associated with
broadcast on commit beyond the latency of the log write (in microseconds).
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