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Capturing packets with the GigaStor
216 GigaStor (23 Feb 2018) — Archive/Non-authoritative version
By default the GigaStor uses a dynamic sampling ratio for statistics. This can
be changed in the GigaStor Control Panel > Settings > General tab to a fixed
sampling ratio of 1, 100, or whatever you wish.
Using dynamic sampling allows the GigaStor to make decisions about how
sampling for statistics should be accomplished. The GigaStor makes its decisions
based on the amount of memory available in the statistics queue buffer and
the amount of packets coming into the capture card. All statistical processing is
handled in the statistics queue buffer (stored in RAM) and the size of this buffer
is very significant for probe instances providing statistics information.
If you set GigaStor Packet Sampling to a fixed sampling ratio, the GigaStor
collects its statistics based on your sampling ratio regardless of available system
resources and traffic to the capture card. If, for example, you have the ratio set
to 1, you are telling the GigaStor to sample every single packet that it sees. This
has a potential negative side effect—especially in very high traffic conditions
—because there could be a significant impact on the GigaStor’s processing
resources (either write-to-disk or read-from-disk), thereby slowing other
processes active at the same time. The potential advantage is that your statistics
will more closely resemble what you see in actual packet analysis, but may not
exactly match it.
There are millions and millions of packets traversing your network. Over a long
enough time frame the statistics are going to be equally valid if you sample
every 10 or 100 or 1000 packets rather than every single packet. Again, statistics
sampling does not prevent you from clicking the Analyze button to view the
actual packets the GigaStor captured with no sampling at all.
This explains why you might see more stations in Top Talkers within Decode
and Analysis than in IP pairs on the GigaStor Control Panel. Usually, the risk of
packet loss significantly outweighs any discrepancy between the statistics in the
GigaStor Control Panel and the actual packets it captured.
Understanding GigaStor indexing
This section describes how the GigaStor captures packets and indexes them for
statistics.
Indexing is an important part of how the GigaStor is able to be as efficient as it
is. A brief synopsis of indexing in the GigaStor is this:
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All captured packets are written to disk. None of the settings in the
GigaStor Control Panel control what is written to disk in any way.
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Indexing is not used for packet capture. It is only for statistics.
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GigaStor Control Panel > Settings > Capture and Analysis options tells the
GigaStor which packets to index for statistics.
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GigaStor Control Panel > Settings > Collect and Show GigaStor Indexing
Information by tells GigaStor how many entries it can use every 15
seconds. After the maximum number of entries for a 15 second period is
reached, new data that was not already being indexed is not indexed for
that 15 second period; however, packets that were already being indexed
continue to be indexed during that 15 second period.
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Every 15 seconds the GigaStor writes all indexed data for 15 second
interval that was just indexed. The indexed data is cleared from memory
and indexing of the next 15 seconds begins.