RE: How does the hrStorageTable handle very large disks (> 20tb, for instance).

Hi Dave,
Thanks for the long response!

Let me touch back on it.  When I said counter I wasn't thinking- it is indeed an integer32.  And you are correct in that AllocatedUnit * storageSize can accommodate a larger disk size.

You mentioned something that I have a question about (which also leads to my potential misunderstanding of the situation).  My knowledge of AllocatedUnits is the block size of the disk.  I said that it can't handle a very large disk because of the AllocatedUnits being locked to block size and not a custom scale factor.  Is this wrong?  Does AllocatedUnits not HAVE to be block size?

Another note: when I said it wrapped the counter I meant it wrapped the Integer (You can pretty much replace counter with integer everywhere I said it).  This does indeed happen on our devices.

I looked into the 'dskTable' a bit.  I upgraded a dev box from 5.5 to the latest release for testing purposes.  I attempted to use the 'includeAllDisks' flag in order to have all of our disks available on-the-fly.  This unfortunately didn't work.  In the cases where our disks are large, maybe the best route for us would be for them to add a:
'disk DISKNAME' 
To our Snmpd.conf file?

Thanks and sorry for confusing some of my facts earlier.



-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Dave Shield
Sent: Thursday, March 01, 2012 3:55 AM
To: Day, Robert
Cc: [email protected]
Subject: Re: How does the hrStorageTable handle very large disks (> 20tb, for instance).

On 29 February 2012 16:59, Day, Robert <[email protected]> wrote:
> I've been dealing with this problem for some time.  We have some large 
> disks here that cannot be measured accurately in the hrStorageTable 
> due to a 32 bit integer being too small.

That's not strictly true.

The hrStorageTable uses two column objects to represent the size of a disk partition (or other storage element) - hrStorageSize and
hrStorageAllocationUnits.   Both of these are 32-bit values, but in
combination can handle elements up to 2^64   (16 exabytes)

If hrStorageAllocationUnits is fixed at 1 (i.e. individual bytes), or 1024 (i.e. measured in K), then this will limit hrStorageSize and similar to 2^32 = 2 Gb or 2^42 = 2 Tb respectively.
   But the hrStorageTable can handle larger disks quite happily, and more recent versions of the Net-SNMP agent are starting to take advantage of this.


>  I understand that when the value is too large,  it raps the counter.

That is true of counter-based objects - but that's not really relevant
here.   The only counters in the Host Resources MIB are two error
counters (hrStorageAllocationFailures and hrDeviceErrors).
   The size metrics are simple Integer values - probably the most sensible behaviour on overflow would be to latch at the maximum
value.   (See the description of Gauge objects in SMI)

>   Does net-snmp have any mib available similar to the hrStorageTable 
> that shows disk logical disk information with 64 bit counters?

a)  Forget about counters - they're not relevant here

b)  The hrStorageTable can already handle larger disks - see above

c)  The Net-SNMP specific 'dskTable' has entries which *are* limited
       to 2Tb   (i.e. 32-bit values, measured in Kb)
    namely dskTotal, dskAvail and dskUsed.

    However these are accompanied by equivalent "paired" objects
     dskTotal{High,Low}, dskAvail{High,Low} and dskUsed{High,Low}
    which provide 64-bit representations (again measured in Kb),
    and so will handle disks up to 2 Zettabytes.
         That should be sufficient for the forseeable future!

Dave

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