my notes on
Oracle network tuning tips!
The 'sQL*Net message to client? Oracle metric indicates the server
(foreground process) is sending a message to the client, and it can be used to
identify throughput issues over a network, especially distributed databases with
slow database links.
The SQL*Net more data to client event happens when Oracle writes
multiple data buffers (sized per
SDU) in a single
logical network call.
Network bottlenecks are very common in distributed systems
and databases with high network traffic:
Top 5 Wait Events
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
"SQL*Message from client" is a good indicator of throughput (e.g. by TCP/IP based
database links), but 'sQL*Net Message? wait events cannot be used to
measure network latency because of the architecture of the Transparent Network
"SQL*Net message to client" wait time can be used as a low confidence
indicator of throughput when data is transmitted over a slow link or a
poorly or incorrectly configured TCP connection. The throughput is
directly related to "bytes sent via SQL*Net to client". This is not,
however, an accurate assessment or quantification of network latency.
v$sesstat or v$mystat can be queried to get the statistics on
bytes and roundtrips handled through SQL*Net.
'sQL*Net message to client?
cannot be used for network latency measurements based on the way TCP stack works and how Oracle
uses it. "SQL*Net message to client" measures only how much time it took
for the response message to be placed into the TCP send buffer on the
Once the response message is in the TCP send buffer, the Oracle server
process continues and then begins its wait for the next "SQL*Net message to
client". Delivery of the packet from this point in time is the
responsibility of the TCP stack to deliver the packet. The Oracle
server has no mechanism for measuring when the message packet actually
arrived; therefore, it is not possible to correlate this process to network
It is also important to note that there are differences between "SQL*Net
Message to Client" and "SQL*Net more data to client" wait events.
Initially, the first session data unit (SDU) buffer containing the return
data is sent and written to the TCP buffer as a "SQL*Net message to client"
Sometimes Oracle will need to write more data for a call than will fit
into the first SDU buffer. In these circumstances, any additional
writes for that call are sent as a "SQL*Net more data to client" wait event.
The relationship of the frequency or number of "SQL*Net message to client"
waits to "SQL*Net more data to client" waits is dependent on the following:
- Oracle Net session data unit size
- Per call data amounts returned to the client
For more, see my notes on
monitoring network latency with Oracle.
Also see my related notes: