Traffic history#

The third view on Interfaces, reached from the segmented control in the header. It answers the question the Dashboard chart cannot: was the link saturated at three in the morning?

The Dashboard's chart is the live sampler's ring — 240 samples five seconds apart, about twenty minutes, held in memory. This is the archive that ring is folded into, and it is on the router's disk.

What it keeps#

Window	Resolution	Comes from
Last hour	one point per minute	the minute tier
Last 24 hours	one point per minute	the minute tier
Last 7 days	one point per hour	the hour tier
Last 31 days	one point per hour	the hour tier

Up to 32 interfaces. Beyond that the archive settles rather than thrashing: the interfaces already being recorded keep their rings, and any live interface that could not get a slot is named on the page rather than silently drawn as absent. A slot is only taken from an interface that has stopped being sampled.

The page prints these numbers from what the agent reports, not from a sentence written here, so the retention it promises and the ring it holds cannot drift apart.

What it costs the router#

Nothing. The archive makes no op-mode call of its own: it folds the samples the live sampler has already taken — one show interfaces counters every five seconds, whatever the interface count — and a test asserts that a fold adds zero router calls. Reading a window is arithmetic over what is already in memory.

Reading it honestly#

The view is explicit about which of four situations it is in, because an empty chart means four different things:

Not recording. The archive loop is not running in this agent, so the window is empty for that reason and not because the link was quiet.
Recording since <time>, no points yet. Started less than one step ago — the first minute point appears after a full minute has been observed.
Nothing recorded in this window. It has been recording longer than that, and there is genuinely nothing here.
A break in the line. The agent recorded nothing for part of the window. The chart breaks the line rather than joining across the hole, and says how many breaks there are.

A partial bucket — the minute that is still filling — is the average of the seconds actually observed, not of the whole minute, so the newest point is never diluted by time that has not happened yet.

Surviving a restart#

The archive is written to traffic-history.json.gz under the agent's data directory every five minutes, and again when the agent is asked to stop — and the agent waits for that last write rather than exiting into it. So an orderly restart loses nothing, and a crash or a power cut loses at most the last five minutes of points in both tiers, because the bucket that was open at the time was never written.

If the agent has no data directory the archive still runs in memory and the page says so, because a history that will not survive a restart must not be presented as one that will.