Mount points, volumes, and physical drives, oh my!

Raymond Chen

Raymond

At the bottom of the storage hierarchy are physical drives. These are units of physical storage, access to which is governed by a single disk controller. Your SSD or hard drive is a physical drive.¹

The next layer up is the volume. A volume is region of storage that is managed by a single file system. The relationship between volumes and physical drives is typically one-to-one, but it doesn’t have to be.

For example, a physical drive might not have any volume associated with it at all. For example, it could be a raw hard drive that hasn’t been partitioned or formatted yet.

You might take your physical drive and create multiple partitions, and then format each partition separately. Each of those formatted partitions is its own volume.

Or you might get really fancy and use a feature like spanned volumes or Storage Spaces to take multiple physical drives and combine them into one giant volume.

Once you have your volumes, you need to make them accessible somehow.²

Mount points are places that volumes are inserted into the namespace and become paths. The most usual place to see them is as a drive letter. For example, your system boot volume is almost certainly mounted as C:.

Volumes don’t have to be mounted as drive letters, though. You can also mount them inside a subdirectory of an existing volume, sort of like grafting one tree onto another. One way to do this is by going to the Disk Management tool, right-clicking a volume, and selecting Change drive letter and paths. From there, you can add a path for a volume, and the contents of the volume will be visible via that path.

Note that a volume can be mounted in multiple places, or it might not be mounted at all.

Next time, we’ll look at how to navigate these concepts in code.

¹ Paradoxically, you can have virtual physical drives, like a RAM drive.

² Theoretically, you could eschew mounting the volume and just access it via its volume GUID.

C:\>type \\?\Volume{8c3513a4-d064-4c99-81fc-66e20810ec3c}\windows\win.ini
; for 16-bit app support
[fonts]
[extensions]
[mci extensions]
[files]
[Mail]
MAPI=1

I mean, theoretically you could do that, but you’d also be a little bit crazy.

8 comments

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  • Maria Spirito
    Maria Spirito

    Both the drive letter and the volume{GUID} names are just symbolic links to the real mount point (device object) in the NT object tree which you can see if you look for them with Sysinternal’s Object Viewer tool.

  • Avatar
    M. W.

    I’ve never used anything fancier than multiple drive letters because things get complex and messy very quickly. Specifically, many programs don’t properly support things like NTFS mount points, so doing something as simple as checking used/free space becomes an exercise in manual bookkeeping. 😕

    > I mean, theoretically you could do that, but you’d also be a little bit crazy.

    As Nick of Science Asylum would say, It’s okay to be a little bit crazy. 😀

    • Avatar
      cheong00

      Just to make things look crazier, since Win2003 you can have DFS volume that has multiple mountpoints (looks like subfolders) mounting on different file servers.

  • Avatar
    Jonathan BarnerMicrosoft employee

    If you want to see which volume guid is which, you can use MountVol:

    > mountvol C: /L
        \\?\Volume{901c71df-e058-4f09-930a-23670e92a165}\

    And yes, you can mount a VHD(x) or a WIM using dism. It’s useful for offline installation of stuff before starting VMs.

    Also, some RAID controllers can present multiple RAID disks as a single “physical disk”.