simple-rrd-merge.py

#!/usr/bin/env python

"""Simple script to merge multiple RRD files together.

Accepts any number of RRD file names as arguments.
Produces an "rrdtool dump" style file on stdout.
The last RRD file should have a slot for every possible record in the
resulting merged RRD.

Run something like:
$ python simple-merge-rrd.py filea.rrd fileb.rrd filec.rrd | \
  rrdtool restore /dev/stdin merged.rrd
"""

import re
import subprocess
import sys


def main():
    rrd_data = {}
    rrds = sys.argv[1:]
    last_rrd = len(rrds) - 1

    for i, rrdname in enumerate(rrds):
        p = subprocess.Popen(
                    ('rrdtool', 'dump', rrdname), stdout=subprocess.PIPE)
        for j, line in enumerate(p.stdout):
            m = re.search(r'<cf>(.*)</cf>', line)
            if m:
                cf = m.group(1)
            m = re.search(r'<pdp_per_row>(.*)</pdp_per_row>', line)
            if m:
                pdp = m.group(1)

            m = re.search(r' / (\d+) --> (.*)', line)
            if m:
                k = cf + pdp
                rrd_data.setdefault(k, {})
                if (m.group(1) not in rrd_data[k]) or (
                        'NaN' in rrd_data[k][m.group(1)]):    # Fill in only when there is no existing data
                    rrd_data[k][m.group(1)] = line
                line = rrd_data[k][m.group(1)]

            if i == last_rrd:
                print line.rstrip()


if __name__ == '__main__':
    main()

Fixed tiny bug: update when 'NaN' in the content of the line, not the dict keyset.