Endace Measurement Systems.
http://www.endace.com
EDM01.05-10r1 DAG 4.3S Card User Manual
25
Revision 6. 22 September 2005.
5.1 Starting Capture Session
, continued
Procedure
, continued
Process Description
Starting a capture
session.
Once the capture parameters are configured, a
capture session is started by:
dagsnap –d dag0 –v –o tracefile0 &
dagsnap –d dag1 –v –o tracefile1
Option
-v
provides user information during
capture; it can be omitted for automated trace
runs.
If the
–o tracefile
parameter is not specified
the tool writes to stdout, which can be used to
pipeline
dagsnap
with other tools from
dagtools
package.
By default dagsnap runs forever. Dagsnap can be
stopped with a signal:
killall dagsnap
or key strokes CTL + C
Dagsnap can also be configured to run for a
fixed number of seconds and then exit using the
–s
option.
5.2 High Load Performance
Description
As the DAG card captures packets from the network link, it writes a
record for each packet into a large buffer in the host PC’s main memory.
Avoiding
packet loss
In order to avoid packet loss, the user application reading the record, such
as
dagsnap
, must be able to read records out of the buffer faster than they
arrive. Otherwise the buffer eventually fills, and packet records are lost.
For Linux and FreeBSD, when the PC buffer becomes full, the message:
kernel: dagN: pbm safety net reached 0xNNNNNNNN”
is displayed on the PC screen, and printed to
log /var/log/messages
.
The “Data capture” LED also goes out. This may be visibly indicated as
flashing or flickering.
Continued on next page
