Endace DAG 7.1S, User Manual

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EDM 01-17 DAG 7.1S Card User Guide
High Load
Performance
Overview
As the DAG 7.1S 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
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. If not the buffer will eventually fill and packet records will be lost.
If the user process is writing records to hard disk, it may be necessary to
use a faster disk or disk array. If records are being processed in real-time,
a faster host CPU may be required.
In Linux and Free BSD, when the PC buffer fills, the following message
displays on the PC screen:
kernel: dagN: pbm safety net reached 0xNNNNNNNN
The same message is also printed to
log /var/log/messages
. In addition,
when the PC buffer fills the “Data Capture” LED on the card will flash or
flicker, or may go OFF completely.
In Windows no screen message displays to indicate when the buffer is
full. Please contact Endace Customer Support at
support@endace.com for
further information on detecting buffer overflow and packet loss in
Windows
Detecting Packet Losses
Once the buffer fills, any new packets arriving will be discarded by the
DAG card until some data is read out of the buffer to create free space.
You can detect any such losses by observing the Loss Counter
(lctr
field) of the Extensible Record Format [ERF]. See Chapter 6: Data
Formats later in this User Guide for more information on the Endace ERF.
©2006 27 Version 2: May 2006