Graphite SNMP Counter transforms

I'm using graphite version 0.9.10 to read ifHCInOctets and ifHCOutOctets, which I'm polling with the collectd snmp and graphite_write plugins; I'm using collectd version 5.1.0.

Data arrives into graphite with no problems; however, I want to graph as bits-per-second. To test whether my stats are correct, I started a CD iso download and watched the download rate… it varied between 1.0Mbps and 2.0Mbps.

Common-sense tells you that you need to multiply Octets counters by 8 to get bits; however, it seems that I need to divide by 8 to make graphite display correctly.

When I multiply by a 0.125 scale factor, scale(scaleToSeconds(nonNegativeDerivative(<SERIES>), 60),0.125), the formula correctly converts to bits-per-second and I see numbers between 1Mpbs and 2Mbps…

When I multiply by a 8.0 scale factor, scale(scaleToSeconds(nonNegativeDerivative(<SERIES>), 60),8), the results are obviously wrong… the graph peaks at 120Mbps. I know that's wrong, because this is a residential cable modem with a 5M cap.

Question: If I'm sending octets to graphite, why is scale(<foo>, 8) producing incorrect results?

/opt/collectd/etc/collectd.conf

LoadPlugin syslog 
LoadPlugin cpu
LoadPlugin interface
LoadPlugin load
LoadPlugin memory
LoadPlugin network
LoadPlugin snmp
LoadPlugin write_graphite

<Plugin snmp>
   <Data "std_traffic">
       Type "if_octets"
       Table true
       Instance "IF-MIB::ifName"
       Values "IF-MIB::ifHCInOctets" "IF-MIB::ifHCOutOctets"
   </Data>

   <Host "fw.pennington.net">
       Address "172.16.1.1"
       Version 2
       Community "public"
       Collect "std_traffic"
       Interval 60
   </Host>
</Plugin>

<Plugin write_graphite>
  <Carbon>
    Host "localhost"
    Port "2003"
    Prefix ""
    Postfix ""
    StoreRates false
    AlwaysAppendDS false
    EscapeCharacter "_"
  </Carbon>
</Plugin>

/opt/graphite/conf/storage-schema.conf:

[carbon]
pattern = ^carbon\.
retentions = 60s:90d

[default]
pattern = .*
retentions = 60s:1w, 5m:1y

/opt/graphite/conf/carbon.conf:

[cache]

USER = carbon

MAX_CACHE_SIZE = inf

MAX_UPDATES_PER_SECOND = 500

MAX_CREATES_PER_MINUTE = 50

LINE_RECEIVER_INTERFACE = 0.0.0.0
LINE_RECEIVER_PORT = 2003

ENABLE_UDP_LISTENER = False
UDP_RECEIVER_INTERFACE = 0.0.0.0
UDP_RECEIVER_PORT = 2003

PICKLE_RECEIVER_INTERFACE = 0.0.0.0
PICKLE_RECEIVER_PORT = 2004

USE_INSECURE_UNPICKLER = False

CACHE_QUERY_INTERFACE = 0.0.0.0
CACHE_QUERY_PORT = 7002

USE_FLOW_CONTROL = True

LOG_UPDATES = False

WHISPER_AUTOFLUSH = False

[relay]
LINE_RECEIVER_INTERFACE = 0.0.0.0
LINE_RECEIVER_PORT = 2013
PICKLE_RECEIVER_INTERFACE = 0.0.0.0
PICKLE_RECEIVER_PORT = 2014

RELAY_METHOD = rules

REPLICATION_FACTOR = 1

DESTINATIONS = 127.0.0.1:2004

MAX_DATAPOINTS_PER_MESSAGE = 500
MAX_QUEUE_SIZE = 10000

USE_FLOW_CONTROL = True

[aggregator]
LINE_RECEIVER_INTERFACE = 0.0.0.0
LINE_RECEIVER_PORT = 2023

PICKLE_RECEIVER_INTERFACE = 0.0.0.0
PICKLE_RECEIVER_PORT = 2024

DESTINATIONS = 127.0.0.1:2004

REPLICATION_FACTOR = 1

MAX_QUEUE_SIZE = 10000

USE_FLOW_CONTROL = True

MAX_DATAPOINTS_PER_MESSAGE = 500

MAX_AGGREGATION_INTERVALS = 5

Output from whisper-fetch.py

root@tsunami:/opt/graphite/conf# python /usr/local/bin/whisper-fetch.py --pretty /opt/graphite/storage/whisper/fw_pennington_net/snmp/if_octets-Ethernet0_0/rx.wsp
Mon Sep 10 02:53:00 2012        110454375894.000000
...
Tue Sep 11 02:50:00 2012        110532796093.000000
Tue Sep 11 02:51:00 2012        110532819931.000000 <------------ Correct
Tue Sep 11 02:52:00 2012        None
root@tsunami:/opt/graphite/conf#

Output from show interface eth0/0

mpenning-fw# sh int eth0/0
Interface Ethernet0/0 "", is up, line protocol is up
  Hardware is 88E6095, BW 100 Mbps, DLY 100 usec
        Auto-Duplex(Full-duplex), Auto-Speed(100 Mbps)
        Description: TIME WARNER 5Mbps circuit
        Available but not configured via nameif
        MAC address 0019.0726.4a39, MTU not set
        IP address unassigned
        157040376 packets input, 110532814004 bytes, 0 no buffer
                                 ^^^^^^^^^^^^^^^^^^
        Received 68921847 broadcasts, 0 runts, 0 giants
        0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
        0 L2 decode drops
        8589974681 switch ingress policy drops
        57851429 packets output, 8036229250 bytes, 0 underruns
        0 output errors, 0 collisions, 0 interface resets
        0 babbles, 0 late collisions, 0 deferred
        0 lost carrier, 0 no carrier
        0 rate limit drops
        0 switch egress policy drops
mpenning-fw#