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dfm physical summary - the numbers just don't add up

This feature of Operations Manager has vexed me for a long while so its open season for the smart people out there to help me out

I'm going to keep this example simple and concentrate on the spare disk capacity reported on one system however I get equally baffling numbers for data and parity capacity count:

From the Operations Manager server run 'dfm physical summary <hostname>'

I get the following output relating to spare disks:

Spare Disks:    2
Spare Space:    1174841327616

If I look at the sysconfig -r output from the Filers console (I'm using 600GB FC drives here):

Spare disks

RAID Disk       Device  HA  SHELF BAY CHAN Pool Type  RPM  Used (MB/blks)    Phys (MB/blks)
---------       ------  ------------- ---- ---- ---- ----- --------------    --------------
Spare disks for block or zoned checksum traditional volumes or aggregates
spare           0a.61   0a    3   13  FC:A   -  FCAL 15000 560000/1146880000 560208/1147307688
spare           0a.77   0a    4   13  FC:A   -  FCAL 15000 560000/1146880000 560208/1147307688

The Operations Manager is reported in KB and the ONTAP output in MB.  Assuming 1024 is used for MB to KB conversion then the two figures on interest are:

Raw: 560208*1024*2 (Physical size x MB to KB x number of disks) = 1147305984

Useable (right sized): 560000*1024*2 (Used size x MB to KB x number of disks) = 1146880000

Neither of these two results match the spare space output reported by Operations Manager.

Any know how OM is deriving its answer and why is it different from ONTAP?

Forgot to add some details:

OM version 3.8.1

ONTAP version 7.3.3

Re: dfm physical summary - the numbers just don't add up

AFAIK, The spare disk space reported as part of 'dfm physical summary' is the total spare space available in Bytes and not in KB. Also its in sync with what ONTAP shows, lemme explain as to how does it calculate by taking your example:
DFM Spare space reporting = (totalsize in MB obtained from ontap) * 1024 * 1024.
From your eg:

RAID  Disk       Device  HA  SHELF BAY CHAN Pool Type  RPM  Used  (MB/blks)     Phys (MB/blks)
---------       ------  -------------  ---- ---- ----  ----- --------------    --------------
Spare disks  for block or zoned  checksum traditional volumes or aggregates
spare            0a.61    0a    3   13  FC:A   -  FCAL 15000 560000/1146880000 560208/1147307688
spare            0a.77   0a    4   13  FC:A   -   FCAL 15000 560000/1146880000 560208/1147307688

Since there are 2 spare disks the total available spare physical space = 560208 + 560208 = 1120416

Applying this in our DFM calculation:

DFM Spare space reporting = (totalsize in MB obtained from ontap) *  1024 * 1024.

                                                   =   (1120416) * 1024 * 1024

                                                  =   1174841327616

which is same as reported in:

Spare Disks:    2
Spare  Space:    1174841327616

>>Any know how OM is  deriving its answer and why is it different from ONTAP?

     As explained above. Its same as ONTAP.

Guess you didn't know it was reported in Bytes and hence you calculated in KBs...   

-Sanjyoth