Debugging “gaps in graphs” syndrome

This document discusses various causes of missing Graphite data, AKA gappy graphs AKA holy graphs (or even completely blank graphs). You should verify each issue in the order they are listed.

Problems at the Graphite end of things

Whisper files have the wrong storage schema

If you forgot to copy NAV’s suggested rules for carbon-cache’s storage-schemas.conf configuration file, you will have problems. Same goes if a NAV upgrade included new rules you forgot to add.

The Whisper round robin database format expects data points to come in at the same rate as specified in its first retention archive (the highest resolution archive). The typical default values from storage-schemas.conf will create Whisper files with their highest resolution archive set to 1 minute intervals. Traffic data from NAV is collected in 5 minute intervals, which means only every fifth data point would be populated in this scenario. That’s not nearly often enough to draw a continuous line between data points.

The whisper-info (or can be used to inspect individual .wsp files to see what resolution their first retention archive is configured with (this is the secondsPerPoint value of archive number 0). If this isn’t 300 seconds (5 minutes) for any .wsp file located under the ports/ directory of a device, then you know these have been created with the wrong schema.

There are two ways to resolve this. Both begin with adopting the proper storage schema rules from NAV’s suggested config. Following that, you can either:

  1. Delete all the existing .wsp files that have the wrong schema. You will lose data.
  2. Use the whisper-resize (or program to resize individual .wsp files, by specifying the correct schema (as per. NAV’s suggested schema rules).

UDP packets are being dropped

Because NAV sends data to Carbon using UDP, there is no guaranteed data reception. This could be solved by using TCP, but with a considerable performance penalty. As data collection in NAV is very bursty, it has occurred that the kernel’s UDP receive buffer has overflowed, causing the kernel to drop packets. This leads to gaps in the graphs.

Verify packet drops

On Linux, to verify that packets are being dropped, you can look in the /proc/net/udp file and find the line with the local port the Carbon daemon is listening to (default port 2003, or 07D3 in hex). The number of packets dropped since the daemon started is shown in the last column. To output only this number, use:

$ awk '$2~/07D3/{print $NF}' /proc/net/udp

If this number keeps increasing, you are affected by the packet dropping issue.

Increasing the UDP receive buffer

If packets are being dropped, you can try to increase the kernel’s network receive buffer to avoid this. On Linux this can be done with the following commands:

sysctl net.core.rmem_max                  # See current setting
sysctl -w net.core.rmem_max=16777216      # Set max buffer to 16MB
sysctl -w net.core.rmem_default=16777216  # Set default buffer to 16MB

Experiment with different values until the packet dropping stops. You need to restart the carbon daemon (carbon-cache or carbon-relay, depending on your setup) to make the changes take effect.

Carbon’s cache is saturated

If the carbon-cache daemon (or daemons, if you have configured multiple) is unable to write data to your storage medium at a fast enough rate, its internal cache will be saturated, and it will start to drop incoming metrics. This will typically happen if the volume and rate of incoming metrics is larger than your I/O subsystem can support writing.

Use this NAV/Graphite URL to render a graph that can give some insight into what’s going on inside your carbon-cache:


This graph shows the relationship between incoming data points, and datapoints committed to disk, while superimposing the size of the internal cache on top. You should be able to quickly identify any capacity issues here: The rate if incoming data points is continuously higher than the rate of committed points, and the cache size is ever-increasing (until it at some points hits the max cache size, configured in carbon.conf).

A healthy graph may look something like this (where the rate of incoming and committed points are roughly equal, and increases in cache size are only temporary):


You could add this graph to your NAV dashboard to monitor it continuously.

The only way around this is to scale up your Graphite infrastructure. You can add faster drives (solid state drives aren’t a bad idea), or set up a cluster of multiple Graphite servers. Please consult Graphite’s own documentation (or Google) on how to accomplish this.

As an alternative to this, you could consider whether you need to collect traffic statics from every access port in your network. If you change the category of an access switch from SW to EDGE in SeedDB, NAV will effectively stop collecting traffic counters from its interfaces.

Problems at the NAV end of things


Unless you have been changing the job configurations of ipdevpoll considerably, there are two main jobs that collect statistics using SNMP: 1minstats and 5minstats - the former running on a 1 minute interval, the latter on 5 minute intervals. The 1minstats job primarily collects all system stats and sensor values, whereas the 5minstats job typically collects interface counters.

ipdevpoll stats jobs are failing

If any of ipdevpoll’s metric collection jobs are failing for a device, the metrics collected by that job may either have incomplete or missing data, depending on how/when the jobs are failing. There are several ways to check how the jobs are doing for one or more devices:

  1. Go to the device’s IP Device Info page (e.g. search for the device name in the nav bar at the top of every NAV web page). Check the job status listings (at the lower right of the page). If the 5minstats or 1minstats job are marked as red, the last time this job was run, it failed. If the jobs are marked yellow, they have not been run or completed within the expected time interval.

  2. Grep the logs for errors (for any device):

    grep 'ERROR.*minstats' /var/log/nav/ipdevpoll.log
  3. Open the Watchdog tool from the Toolbox menu. Any ipdevpoll jobs that are repeatedly failing will show as errors here.

Stats jobs failing due to timeout errors

Do the devices that are timing out have any common factors? Same model, vendor, software?

Some devices are particularly slow to respond to SNMP requests at large, while some are slow to respond to particular SNMP requests - in particular, we often see agents that are slow to build large responses to GET-BULK requests, which NAV always uses in a SNMP v2c session.

You can try to alleviate the timeout problems by increasing ipdevpoll’s SNMP request timeout, alternatively in conjunction with reducing the max-repetitions value (which is the maximum number of consecutive values ipdevpoll asks an agent to return in a single SNMP response packet when employing GET-BULK requests).

ipdevpoll stats jobs aren’t running at the correct interval

The Whisper round robin database format expects data points to come in at the same rate as specified in its first retention archive. If ipdevpoll does not send metrics to carbon at the rate expected for the given metric’s schema, gaps may occur in the data.

If the device example-gw is missing data in its port counter metrics, you can attempt to verify that the 5minstats job for example-gw is actually running on the configured 5 minute interval by grepping the logs and checking the timestamps of job completion (when you have already confirmed the jobs themselves aren’t failing):

# grep 'INFO.*5minstats example-gw.*completed' /var/log/nav/ipdevpoll.log
2017-11-10 13:46:21,400 [INFO schedule.netboxjobscheduler] [5minstats] 5minstats for completed in 0:00:06.151333. next run in 0:04:53.848691.
2017-11-10 13:51:21,510 [INFO schedule.netboxjobscheduler] [5minstats] 5minstats for completed in 0:00:06.259981. next run in 0:04:53.740050.
2017-11-10 13:56:21,293 [INFO schedule.netboxjobscheduler] [5minstats] 5minstats for completed in 0:00:06.042444. next run in 0:04:53.957581.
2017-11-10 14:01:21,747 [INFO schedule.netboxjobscheduler] [5minstats] 5minstats for completed in 0:00:06.476202. next run in 0:04:53.523833.

This example shows that the 5minstats is consistently running on 5 minute intervals.

Once you cross a certain threshold, depending on your hardware setup and the number of nodes/ports you are monitoring with NAV, you may find that ipdevpoll is having issues scheduling its jobs in a timely fashion: There’s too much work, and too little resources to complete it on time.

This is when you should start experimenting with ipdevpoll’s multiprocess mode.