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Don Burleson Blog 







LNS wait on SENDREQ tips

Oracle Database Tips by Donald BurlesonApril 19, 2015

Question:  I am running Oracle 10gr2 with a standby database on Data Guard using ASYNC redo log transport.  I am getting these LNS event in my AWR report, LNS Wait events.

%Time                        Total     Wait    wait      Waits
Event                        Waits     -outs   Time (s)  (ms)    /txn
----------------------- -------------- ------ ----------- ------- ---
LNS wait on SENDREQ         312            .0     197     632      0.0
LNS ASYNC end of log    143,963         100.0   1,545      11      5.8

I have a slow network with an average packet time of 25 milliseconds with busy updates at a rate of 250 block changes per second.  During the period of high LNS wait on SENDREQ, my database "hangs", not allowing anybody to sign-in.  What is a wait on the Log Network Server (LNS) process, and how do I fix this issue?

Answer:  The LNS wait on SENDREQ can happen with either the SYNC or ASYNC LGWR attributes. 

When using ASYNC transport mode in Oracle 10g r2 and beyond, Oracle recommends allowing for sufficient I/O bandwidth for LNS read I/Os to the online redo logs of the production database.

You can choose the LGWR attributes for synchronous (LogXptMode = 'SYNC') or asynchronous mode (LogXptMode = 'ASYNC').  See these important notes on Data Guard synchronous (SYNC) vs asynchronous (ASYNC) modes.

You can dynamically change the LGWR attributes with the "edit database" command:

edit database 'stdbydb' set property LogXptMode=SYNC

edit database 'primdb'  set property LogXptMode=ASYNC

The LNS wait on SENDREQ occurs more frequently on a busy network when the LNS process is stuck, waiting for the RFS to send an ack from the standby server.  The root cause of a LNS wait on SENDREQ is a busy network, and this error is most likely to appear when the system is during high-volume updates (heavy DML). 

Log transport steps

Here are the log transport steps and the associated waits for each log transport step.  Step 1 is manifested as a log file parallel write wait, while all subsequent steps are the "LNS wait on SENDREQ':

0 - The LGWR process writes from the online redo log filesystem to the archived redo log directory.  This can be seen as a "log file parallel write" wait event.

1 - Using an FTP-type mechanism, the LNS process detects the completion of a redo log being written to the local archived redo log directory. 

2 - Once the redo has completed archiving, the LNS process communicates with the remote file service (RFS) process) to manage the transport of the flat file to the remote standby server, waiting for a system acknowledgement (an "ack") from the remote file service (RFS) process.

3 - Once the ack is received from  standby, the LNS process notifies the log writer process (LGWR) and the commit occurs.

4 - Upon arrival at the standby server, the redo updates are applied to the standby database, thereby reproducing the updates from the originating server.

For more details on the LNS wait for SENDREQ, see MOSC note 233491.1.

Possible solutions for LNS wait on SENDREQ

Possible solutions for a hung database on LNS wait on SENDREQ include:

0 - Change net_timeout:  If using SYNC mode in 10gr2 and beyond, consider reducing the value of the net_timeout attribute.  The value of net_timeout indicates number of seconds that LGWR on the production database waits for Oracle*Net to respond to a LGWR request.

1 - Try setting COMMIT WRITE NOWAIT.  This will allow transactions to  continue processing without waiting for LGWR to post a message back stating that all redo changes are on disk.

2 - Spread-out the DML load over a longer period of time, by adjusting the schedule of any batch jobs

3 - Get a faster network transport such as dark fibre.

4 - Change the size of the online redo logs to make the archived redo log smaller, thereby creating smaller archived redo logs.  This will create more frequent, smaller redo log transports that will complete faster.

5 - Tune the TNS layer, perhaps trying the tcp.nodelay parameter in the protocol.ora file

How the LNS wait on SENDREQ occurs

The Data Guard redo log transport may be synchronous mode (LogXptMode = 'SYNC') or asynchronous mode (LogXptMode = 'ASYNC'). 

The Async LNS process is a background daemon process that periodically checks for new redo entries from the log writer (LGWR) process.  The ASYNC wait is a normal idle time event when you are using Data Guard asynchronous redo log transport.

The Log Network Server (LNS) is a background process that manages all archived redo log flat file transports to a standby server.  This query display data from the


The "LNS wait on SENDREQ" is the sum of network time and RFS I/O time.  To see the network time, take the value for LNS wait on SENDREQ and subtract the "RFS write" wait event time from the standby server.  You can also use the UNIX/Linux netstat utility to measure packet shipment latency.

The Oracle documentation notes these wait events for destinations configured with the LGWR SYNC and ASYNC Attributes

Waits with the LGWR SYNC Attribute:

Wait Event Monitors the Amount of Time Spent By . . .

LGWR wait on LNS

The LGWR process waiting to receive messages from the LNSn process.

LNS wait on ATTACH

All network servers to spawn an RFS connection.


All network servers to write the received redo data to disk as well as open and close the remote archived redo log files.

LNS wait on DETACH

All network servers to delete an RFS connection.

Wait with the LGWR ASYNC attribute:

Wait Event Monitors the Amount of Time Spent By . . .

LNS wait on DETACH

All network servers to delete an RFS connection.

LNS wait on ATTACH

All network servers to spawn an RFS connection.


All network servers to write the received redo data to disk as well as open and close the remote archived redo log files.

True ASYNC Control FileTXN Wait

The LNSn process to get hold of the control file transaction during its lifetime.

True ASYNC Wait for ARCH log

The LNSn process waiting to see the archived redo log (if the LNSn process is archiving a current log file and the log is switched out).

Waiting for ASYNC dest activation

The LNSn process waiting for an inactive destination to become active.

True ASYNC log-end-of-file wait

The LNSn process waiting for the next bit of redo after it has reached the logical end of file.

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