Varying table index for disk I/O stats

[0.8.7i from Ubuntu 12.04 package]

I am monitoring disk I/O stats using
http://docs.cacti.net/usertemplate:data:host_mib:diskio

The SNMP query template is at the end of this mail. Here's an example of the
data it's collecting:

$ snmpwalk -On -v2c -c XXXXXX XXXXXX 1.3.6.1.4.1.2021.13.15.1.1 | grep '\.55 '
.1.3.6.1.4.1.2021.13.15.1.1.1.55 = INTEGER: 55
.1.3.6.1.4.1.2021.13.15.1.1.2.55 = STRING: md127
.1.3.6.1.4.1.2021.13.15.1.1.3.55 = Counter32: 2775494144
.1.3.6.1.4.1.2021.13.15.1.1.4.55 = Counter32: 1303653888
.1.3.6.1.4.1.2021.13.15.1.1.5.55 = Counter32: 12228936
.1.3.6.1.4.1.2021.13.15.1.1.6.55 = Counter32: 1514190
.1.3.6.1.4.1.2021.13.15.1.1.9.55 = INTEGER: 0
.1.3.6.1.4.1.2021.13.15.1.1.10.55 = INTEGER: 0
.1.3.6.1.4.1.2021.13.15.1.1.11.55 = INTEGER: 0
.1.3.6.1.4.1.2021.13.15.1.1.12.55 = Counter64: 5680722259456
.1.3.6.1.4.1.2021.13.15.1.1.13.55 = Counter64: 645548748288

This works fine, except for the following issue.

Suppose I decide to monitor stats for device "md127". Cacti internally maps
this to the table row number, say x=55 as shown above.

At some point in the future, perhas due to reboot or device insertion or
removal, the table may change.  For example, md127 may become x=54.

When this happens, Cacti starts graphing the wrong value (or no value at
all). But if I browse Graph management, it does show that it is monitoring
the wrong attribute (e.g. it shows "sdz" instead of "md127")

To correct the problem, I have to go to Devices and click "(Verbose Query)"
next to "SNMP - Get Disk IO", or delete and recreate the graph.  But of
course, it's not until I realise there's a problem that I do this, and a lot
of wrong data may have been recorded.

Anyway, as far as I can tell, Cacti has made a sticky decision that it
should be monitoring row x=55. Is it possible to make it instead store that
it should be monitoring "md127" and map this to SNMP table index on every
query?

Thanks,

Brian Candler.

--------- disk_io.xml ---------
<interface>
        <name>Get Disk IO Information</name>
        <index_order_type>numeric</index_order_type>
        <oid_index>.1.3.6.1.4.1.2021.13.15.1.1.1</oid_index>
        <fields>
                <hrDiskIOIndex>
                        <name>Index</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>input</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.1</oid>
                </hrDiskIOIndex>
                <hrDiskIODescr>
                        <name>Description</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>input</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.2</oid>
                </hrDiskIODescr>
                <hrDiskIONRead>
                        <name>Number of Bytes Read</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>output</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.3</oid>
                </hrDiskIONRead>
                <hrDiskIONWrite>
                        <name>Number of Bytes Written</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>output</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.4</oid>
                </hrDiskIONWrite>
                <hrDiskIOReads>
                        <name>Number of Read accesses</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>output</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.5</oid>
                </hrDiskIOReads>
                <hrDiskIOWrites>
                        <name>Number of Write accesses</name>
                        <method>walk</method>
                        <source>value</source>
                        <direction>output</direction>
                        <oid>.1.3.6.1.4.1.2021.13.15.1.1.6</oid>
                </hrDiskIOWrites>
        </fields>
</interface>


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