Disk Space Inaccuracy

Content Authoring
1

We discovered a Win2008R2 server that is reporting a 5% discrepancy in size values for one of its drives, depending on the detection used.
The first property below (Raw Size of Disks) is reporting the same values as seen from the operating system. The second property below (Spaces of Drives) is reporting a total space value for one of the drives (Drive V, NTFS, Healthy (Primary Partition)) to be 5% less of what the operating system sees. The OS sees drive V’s total space to be 3329.87 GB. The native relevance, second property below (Spaces of Drives), sees the V drive’s total space to be 3190 GB. What gives?

Raw Size of Disks:
((it as integer) of string values of selects "Size from Win32_DiskDrive WHERE size!='' " of wmi)

Spaces of Drives:
(name of it & " - Size: " & ((total space of it /(1024*1024*1024)) as string & " GB") & ", Used:(" & (((total space of it - free space of it)*100/(total space of it)) as string) & "%25) " & (((total space of it - free space of it) /(1024*1024*1024)) as string & " GB") & ", Free:(" & (((free space of it)*100/(total space of it)) as string) & "%25) "& ((free space of it /(1024*1024*1024)) as string & " GB")) of drives whose(exists name of it AND exists free space of it AND exists type of it AND exists total space of it AND type of it = "DRIVE_FIXED" AND total space of it != 0 AND total space of it > 1024*1024*1024)

2

Suspect that one of them is using binary GB = 1024*1024*1024 and the other using decimal GB = 1000*1000*1000

3

The raw value is 3575553891840. One would have to divide that thrice by approx. 1039 to get close to the incorrect value of 3190.

4

I believe that the first WMI query is looking at Physical Disks, while the ‘drives’ inspector is looking at Logical Partitions. See whether these line up as you expect on your system.

For examples, I can see in DiskPart that I have two physical drives and the first of those drives has four logical partitions:

C:> diskpart.exe
Microsoft DiskPart version 10.0.18362.1

Copyright (C) Microsoft Corporation.
On computer: LP1-US-51779528

DISKPART> list disk

  Disk ###  Status         Size     Free     Dyn  Gpt
  --------  -------------  -------  -------  ---  ---
  Disk 0    Online          476 GB  1024 KB        *
  Disk 1    Online          931 GB      0 B

DISKPART> sel disk 0

Disk 0 is now the selected disk.

DISKPART> list par

  Partition ###  Type              Size     Offset
  -------------  ----------------  -------  -------
  Partition 1    System             500 MB  1024 KB
  Partition 2    Reserved           128 MB   501 MB
  Partition 3    Primary            473 GB   629 MB
  Partition 4    Recovery          2833 MB   474 GB

The WMI query retrieves info for two Physical Drives:

q: selects "size, DeviceID from Win32_DiskDrive" of wmi
A: DeviceID=\\.\PHYSICALDRIVE0
A: Size=512105932800
A: DeviceID=\\.\PHYSICALDRIVE1
A: Size=1000202273280
T: 12.495 ms
I: plural wmi select

While the native ‘drives’ inspector returns data for logical partitions -

q: (name of it, total space of it) of drives
A: C:, 508478615552
A: D:, 1000197849088
T: 0.189 ms
I: plural ( string, integer )

On my machine, the “C:” partition is on PhysicalDrive0, while “D:” is on PhysicalDrive1. The ‘drives’ inspector returns only the information for the logical partitions that are mounted in Windows - and does not include the “System”, “Reserved”, or “Recovery” partitions that are actually on the disk.

5

Is hardware vs partition size coming into play here?
Ha - @JasonWalker beat me to it!

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1 Like
6

Thank you both for your assistance. I’m awaiting a confirmation from the server administrator.

7

In linux it has 5% discrepancy between what df and bigfix reports. It depends how the df and bigfix calculate the disk allocation. I think df uses the block size of 1k or less or more. The bigfix uses the diff block size while calculating the disk utilization.