Question: I see high
"SQL*Net message to client" messages in my AWR report.
What are "SQL*Net message to client" waits and how
do I tune SQL*Net message to client waits? Is this a TCP performance issue?
Answer: The SQL*Net message to
client may indicate a network-related issue that causes clients too long to
get data from the database server. Thus, it can be a TCP issue, but it
is not limited to that.
Also see my notes on the opposite event,
SQL*Net message from
client.
Common causes of a high SQL*Net message to client
might include TCP/IP bottlenecks or TNS parameter issues:
- High network latency: Check with
netstat to ensure that your TCP/IP does not have bottlenecks.
The
SQL*Net message to client
Oracle metric indicates
the server (foreground process) is sending a message to the
client.
Network bottlenecks are very common in distributed
systems and those with high network traffic. They are
manifested as SQL*Net wait events:
% Total
Event Waits Time (cs) Wt Time
--------------------------------- ------------ ------------ -------
SQL*Net more data to client 3,914,935 9,475,372 99.76
db file sequential read 1,367,659 6,129 .06
db file parallel write 7,582 5,001 .05
rdbms ipc reply 26 4,612 .05
db file scattered read 16,886 2,446 .03
Also see my notes on
“SQL*Net message
to client” and network throughput.
Tanel Poder has a great note that explains why “SQL*Message from client” is
a good indicator of throughput (e.g. by TCP/IP based database links), but that
“SQL*Net Message” wait events cannot be used to measure network latency because of the
architecture of the Transparent Network Substrate (TNS):
“So, if you’re sending loads of
data over a slow link or mis-configured TCP connection, the “SQL*Net message to
client” wait time can be used as a low-confidence indicator of your SQL*Net
throughput (in conjunction with “bytes sent via SQL*Net to client”), but never a
measure of network latency!”
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