I have recently seen a number of issues where customers called in to report a significant difference between the “Size on disk” for the root of a volume, and the reported amount of “Used space” in the volume properties. While considering this, I noticed that my own C: drive had a mysterious 20GB difference.
Below is the story of how I found the answer.
Before we begin, there are two methods used for calculating disk usage…
Method 1 – Volume Bitmap Analysis
The % used and %free indication shown below with the pie chart is based on volume bitmap analysis. The hidden $Bitmap:$Data:”” stream is read from the volume via the FSCTL_GET_VOLUME_BITMAP IO control code, and the ones and zeroes are counted. A binary “1” represents an allocated cluster, while a “0” indicates a free cluster. The total number of set bits indicates the total number of allocated clusters on the volume. This is the fastest and most reliable method of calculating allocated and free space.
Navigating to the “C:\” folder, we select all files/folders, then right click, and then select properties. A dialog will appear as the shell enumerates all files that it can find in the root and subfolders. The “Size” and “Size on disk” fields will eventually display the total size of all primary data streams for all files that are found by Explorer. In my case, this method only finds 80.25 GB of 102 GB (leaving a 21.75 GB difference).
Why such a big difference?
The short answer is – “If you can’t see it with Explorer, it doesn’t get added to the total size.”
How to find the hidden disk usage…
First, check to see if your files are all actually visible with the current user account and Explorer settings. Uncheck the “Hide protected operating system files” and select “Show hidden files and folders”. When this is completed, look at the size on disk again.
Does it match the pie chart within a few percent? If not, continue to Step 2.
Run CHKDSK and save the output. The summary information at the end is what we want to see.
Based on CHKDSK output, we can calculate the total metadata usage by adding up the following…
Space used by 22677 indexes.
Space used by $Badclus file.
Space used by $MFT.
Space used by $Loffile.
In this example, metadata usage accounts for only 1.4 GB. If the metadata number were a high percentage of the usage, then we need to take closer a look at how the volume is being used instead of simply looking for hidden files. High metadata usage will be the subject for part 2 of this blog.
We can see from the CHKDSK output that the total space occupied by our user files is 106,095,080 KB (101.18 GB). This is a large percentage of the total usage, so we should be looking at the user files to see why we can’t see these files with Explorer.
Do you have permissions to see all files/folders the volume?
a. If this is a system drive, look through the “\Documents and Settings”, or “\users” folder and see if you can browse all of the profile folders. If not, you may need contact the owner(s) to check their folder size and see what they have saved in their user folder. If they are like me, they will have 5 years worth of memory dumps, MP3’s, VHD’s, PDF’s, etc.
b. Try “CACLS.EXE x:\ /T”, or just browse through the folders on the drive looking for an “Access Denied” error. Once this happens, give yourself permissions to that part of the subtree (if permitted by your administrative privileges) or have someone who has access to that folder enumerate the contents for you. Check the number of files and the size on disk for all files in that folder and add it to the total.
c. Continue this process until you have a total of all files in all folders.
d. Does the total make sense? If not, then continue to the next step.
In the case of my mysterious 20 GB difference, I found an old user profile from a previous installation. Sure enough, I received an access denied error when browsing to it. To access the folder, I acquired ownership. This user was in the habit of collecting memory dumps and virtual machines in a subfolder on his desktop. I recovered quite a bit of free space by using the delete key. I rescanned the volume, but to my disappointment, there was still a significant difference.
Are there alternate streams? Try using STREAMS.EXE from (http://technet.microsoft.com/en-us/sysinternals/default.aspx). Streams will recurse subdirectories and report space usage by alternate named streams. Pipe this to a text file and analyze it with a text editor or spreadsheet program.
Hard links can be problematic when calculating the total usage via the file scan method. Hard links are not extensively used, but this is worth mentioning. A hard link is an extra index entry that points to an existing file. These are created via the CreateHardLink function. Although the file actually exists in one location, each hard link has its own size field. Because of this, a single file can be added twice to the running total. Another thing to know is that hard links are not synchronized, so it is possible that only one link will show the actual file size (see the example below).
Unfortunately, there are few options available for detecting hard link paradoxes, but it is something to consider when the file scan shows more usage than the bitmap. Since we have the opposite situation here, hard links are not a significant factor.
Is Volume Shadow Copy Service maintaining diff area files for snapshots? Use VSSADMIN LIST SHADOWSTORAGE to find out. Add shadow storage to the running total.
VSSVC pre-allocates space for making volume snapshots. In order to support this feature, diff area files are kept in the “\System Volume Information” folder. This pre-allocated space is used to maintain point-in-time information for the “Previous Versions” feature and for the “System Restore” application. If you are the type of user who prefers to minimize the impact of such features, then you can resize your shadow storage with VSSADMIN so it has less impact on disk usage. I prefer to leave these features at their default settings (and just make a note of how much disk space it is using).
If your numbers still don’t make sense, then it’s time to check for actively hidden files / folders. There are many different rootkit scanners that can help you identify the presence of actively hidden files & folders. Try using a scanner like Rootkit Revealer. If you suspect that your machine has been compromised by a rootkit, refer to http://www.microsoft.com/uk/business/security/security-malware.mspx.
Seeing that I did not have a significant amount of mysterious usage, I was satisfied that there was no malicious logic hiding my files. After taking into account all of the factors, there was only a 1.57GB difference remaining (which was accounted for by the contents of a user’s recycle bin).
Whenever you see a difference between the “Used space”, and “Size on disk”, always ask yourself “What am I not seeing?”
Dennis Middleton “The NTFS Doctor”