the Zen of Communication.

Last time (here) I introduced an OID tree for the descriptions of Storage.  I hope you copied and pasted your results into a new window, because we’ll need them.

If not, here’s mine:

c:\SNMP>snmputil walk 127.0.0.1 public .1.3.6.1.2.1.25.2.3.1.3
Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.1
Value    = String A:\

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.2
Value    = String C:\ Label:  Serial Number 2053422

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.3
Value    = String D:\ Label:Data  Serial Number c9d83a42

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.4
Value    = String E:\

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.5
Value    = String F:\

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.6
Value    = String G:\ Label:FileDump  Serial Number 0dc359f2

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.7
Value    = String Virtual Memory

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageDescr.8
Value    = String Physical Memory

End of MIB subtree.

c:\SNMP>

So these are all the OIBs we can pull from the branch .1.3.6.1.2.1.25.2.3.1.3, which is the “Host Resource Storage Description” or hrStorageDescr for short. 

In MRTG’s case, we can’t set up a chart to poll .1.3.6.1.2.1.25.2.3.1.3, because it has subvalues.  So how do we get there?  Well, first – keep track of what number goes to what drive.  .2 is my C: drive, .3 is my D: drive, and .5 is my F: drive.  .4 and .5 have no descriptions because they’re CD-ROM drives.  I want to stress:  Your values may be different!  Your C: drive may be .3, .4, or any other number depending on your system configuration.

Now just for kicks, try entering this (one line):

c:\SNMP>snmputil get 127.0.0.1 public .1.3.6.1.2.1.25.2.3.1.5.2

You’ll notice the subtle change – we’re not ..2.3.1.3.2 anymore, we just switched to ..2.3.1.5.2 .  We stayed with .2 at the end because that’s the value for the C: drive (on my system – yours may be different!).  But the .5 is now a different OID tree.  Want to find out what?  Type (again, one line):

c:\SNMP>snmputil walk 127.0.0.1 public .1.3.6.1.2.1.25.2.3.1.5

You should get a listing of all the total storage sizes for all volumes on your system, including Virtual and Physical memory spaces.  But it’s not going to look sensible, because it’s giving us the number of blocks on each volume – not Kilobytes, Megabytes, or Gigabytes.  [For a complete explanation, read this post from the MRTG Mailing List.]

Remember what the value was for .2, or whatever your C:\ drive was.

So we have the number of blocks, but that doesn’t tell us much.  We want to know in megs or gigs what the total storage space is.  Next we need to find out what the block sizes are.  Lucky for us, there’s an SNMP OID for that.

c:\SNMP>snmputil walk 127.0.0.1 public .1.3.6.1.2.1.25.2.3.1.4

This will ‘walk’ the OID tree of block sizes for each volume on our system.  Lets say we just wanted to find the C: drive block size.  We’d use:

c:\SNMP>snmputil get 127.0.0.1 public .1.3.6.1.2.1.25.2.3.1.4.2

to which my system responded:

Variable = host.hrStorage.hrStorageTable.hrStorageEntry.hrStorageAllocationUnits.2
Value = Integer32 4096

Which tells me that my C: drive, has a block size of 4096 bytes.  So, for our own math here – My C:\ drive has a total number of 8958237 blocks.  We multiply that by 4096 to get 36,692,938,752 bytes.  To reduce that to Gigabytes, we divide by 1024, three times.

36,692,938,752 / 1024 = 35,832,948 Kilobytes.
35,832,948 Kb / 1024 = 34,993 Megabytes
34,993 Mb / 1024 = 34.17 Gigabytes.

Which, if I’ve done my math correctly, is exactly what I should see when you pull up the properties on my C: drive.  And sure enough -

So that’s a very basic introduction-by-example to an OID tree, and it’s uses.  I’ll write another example about calculating the percentage of used space on a drive with MRTG, which will use more OIDs. 

For more information on MRTG, SNMP, and MIBs, see this post.

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