Getting live network bandwidth numbers on Linux

Today I got curious about a simple question: was my iSCSI target machine actually running at its full potential read speed?

The machine exports individual disks to its clients, so measuring single disk performance wouldn't give me the answer. Summing up IO across all the disks would have given me a number, but so would just getting the network bandwidth utilization; if the machine was saturating its gigabit link, it was clearly running as fast as it could.

There doesn't seem to be a program that will directly show this information (at least not on Red Hat Enterprise 5), but you can get the total byte counts for an interface from ifconfig, which means that with a small script I had what I wanted. (Then I rewrote it to read the stats directly from /proc/net/dev instead of running ifconfig and groping through the output.)

Since it may be useful for other people, here's what I'm calling netvolmon:

#!/bin/sh
# usage: netvolmon DEV [INTERVAL]
DEV=$1
IVAL=5
if [ "$#" -eq 2 ]; then
    IVAL=$2
fi

getrxtx() {
    grep "$1:" /proc/net/dev | sed 's/^.*://' |
        awk '{print $1, $9}'
}

rxtx=$(getrxtx $DEV)
while sleep $IVAL; do
    nrxtx=$(getrxtx $DEV)
    (echo $IVAL $rxtx $nrxtx) |
    awk '{rxd = ($4 - $2) / (1024*1024*$1);
          txd = ($5 - $3) / (1024*1024*$1);
          printf "%6.2f MB/s RX %6.2f MB/s TX\n",
                 rxd, txd}'
    rxtx="$nrxtx"
done

Unfortunately this illustrates one reason why shell scripting is so pervasive: it is such a convenient way of banging rocks together in a hurry. Once I hit on the trick of using awk for all the arithmetic, it probably took me longer to fiddle with the output formatting than to write the rest of the script.

(And I have to give bash a big raspberry for making array variables useless for precisely the situation where they would be most useful, namely picking individual elements out of the output of a command that prints multiple pieces of information.)