Re: Question about log file sync in DataGuard
Date: Fri, 12 Oct 2012 13:09:28 -0700 (PDT)
Message-ID: <1350072568.749.YahooMailNeo_at_web121605.mail.ne1.yahoo.com>
I can't honestly say that LNS wait on SENDREQ is a 'subset' of LGWR-LNS wait on channel but they are related; the former records time for opening, writing redo to and closing the redo logs on the standby and the latter records wait time for the receive messages from the standby (acknowledgements that the redo was received and written remotely or that the write failed). If the network is slow then I would imagine that both waits would record high wait times. One suggested method of mitigating such waits is to set the SDU (Session Data Unit) in the listener.ora to the maximum of 32767 presuming you're using a static SID (configuring the SID in the listener.ora) rather than using dynamic registration. Another method, if you're using Data Guard Broker, is to set the SEND_BUF_SIZE and RECV_BUF_SIZE parameters in the sqlnet.ora file to site-specific values. As an example:
SEND_BUF_SIZE”000000
RECV_BUF_SIZE”000000
This would send larger packets on the network to reduce send/receive traffic.
Of course I can't speak for your installation/configuration without further details. David Fitzjarrell
From: Michael Schmitt <mschmitt_at_uchicago.edu> To: David Fitzjarrell <oratune_at_yahoo.com>; "oracle-l_at_freelists.org" <oracle-l_at_freelists.org> Sent: Friday, October 12, 2012 1:45 PM
Subject: RE: Question about log file sync in DataGuard
Hi David,
Yes the standby redo logs are necessary on the standby server in 11.1.0.7 as well. Since we are in LGWR SYNC mode there is no doubt some of our time is waiting on the network and also the file I/O on the standby server. I was suspecting this time was represented mainly by "LNS wait on SENDREQ". One of the things I was unsure about, is if "LNS wait on SENDREQ" is just a subset of "LGWR-LNS wait on channel"
Thanks,
Mike
From: David Fitzjarrell [mailto:oratune_at_yahoo.com]
Sent: Friday, October 12, 2012 2:37 PM
To: Michael Schmitt; oracle-l_at_freelists.org
Subject: Re: Question about log file sync in DataGuard
I'm not sure about 11.1.0.7 but in 11.2 it's necessary to have standby redo logs on the standby server; the waits could be network related as Oracle syncs not only the local redo logs but also the remote logs on the standby.
David Fitzjarrell
From: Michael Schmitt <mschmitt_at_uchicago.edu<mailto:mschmitt_at_uchicago.edu>>
To: "oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>" <oracle-l_at_freelists.org<mailto:oracle-l_at_freelists.org>>
Sent: Friday, October 12, 2012 12:33 PM
Subject: Question about log file sync in DataGuard
Hello,
I am hoping the list can help me better understand the waits I am seeing for log file sync in a Data Guard environment.
This is for an 11.1.0.7 database running on Linux and running in max availability mode (LGWR SYNC)
We consistently see "log file sync" as the top wait event in AWR reports taking 30-40% of the DB time.
A two hour report today showed 1,938 seconds for log file sync.
The report also indicates the following background wait events.
LGWR-LNS wait on channel: 707(s)
LNS wait on SENDREQ: 680(s)
log file parallel write: 667(s)
I am trying to get a better understanding of the break down on the "log file sync". My understanding is that 667(s) of the "log file sync" time is spent on "log file parallel write" waiting for the LGWR on the primary database to write to the redo logfile on the primary server. That would leave ~ 1271(s) time of the reported "log file sync" left. Are the "LGWR-LNS wait on channel" and "LNS want on SENDREQ" basically reporting on the same thing (sending data to standby and waiting for confirmation) or are those two distinct functions? I was thinking that they might be the same function and that approximately 564(s) of the "log file sync" time was just multiple sessions reporting the "log file sync" wait.
Any advice on better understanding this would be appreciated
Mike
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