How do I get from a volume to the physical disk that holds it?

Raymond Chen

Raymond

Last time, we saw how to get from a file path to the volume that holds it. The next step is to get from the volume to the physical disk.

The lazy way is to ask for the device number:

STORAGE_DEVICE_NUMBER number;
DeviceIoControl(handle,
    IOCTL_STORAGE_GET_DEVICE_NUMBER,
    nullptr, 0, // no input
    &number, sizeof(number), // output goes here
    &bytesWritten,
    nullptr);
DWORD physicalDriveNumber = number.DeviceNumber;

This is lazy for multiple reasons:

  • It fails to account for the case where the volume spans multiple physical drives.
  • In my experience, if the volume is a CD-ROM drive with no disk in the drive, the call reports that the physical drive number is 0, which is almost certainly incorrect.

In practice, it seems that if the volume spans multiple physical drives, the IOCTL_STORAGE_GET_DEVICE_NUMBER fails (with ERROR_INVALID_FUNCTION, it seems, which is the Win32 manifestation of the NT status code STATUS_INVALID_DEVICE_REQUEST), so at least you don’t get wrong answers. You just get no answer.

The less lazy (and more likely to be correct) way is to ask the volume for its disk extents. This one is a bit annoying because it returns a variable-sized structure, so you need to ask twice. The first time tells you how big a structure you need, and the second time actually gets the structure.

Since nearly all volumes have only one extent, we can optimize slightly for that case by passing an initial buffer big enough to hold a single extent. If that works, then there’s no need to try a second time.

VOLUME_DISK_EXTENTS* extents = nullptr;

// Anticipate common case where there is only one extent.
VOLUME_DISK_EXTENTS singleExtent;

// But also have a place to manage allocated data.
std::unique_ptr<BYTE[]> lifetime;

DWORD bytesWritten;
if (DeviceIoControl(handle, IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS,
        nullptr, 0,
        &singleExtent, sizeof(singleExtent),
        &bytesWritten,
        nullptr)) {
  // Worked on the first try. Use the preallocated buffer.
  extents = &singleExtent;
} else {
  VOLUME_DISK_EXTENTS* lastQuery = &singleExtent;
  while (GetLastError() == ERROR_MORE_DATA) {
    assert(RTL_CONTAINS_FIELD(lastQuery, bytesWritten, NumberOfDiskExtents));
    DWORD extentCount = lastQuery->NumberOfDiskExtents;
    DWORD allocatedSize = FIELD_OFFSET(VOLUME_DISK_EXTENTS, Extents[extentCount]);
    lifetime.reset(new BYTE[allocatedSize]);
    lastQuery = (VOLUME_DISK_EXTENTS*)lifetime.get();
    if (DeviceIoControl(handle, IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS,
          nullptr, 0,
          lastQuery, allocatedSize,
          &bytesWritten,
          nullptr)) {
      extents = lastQuery;
      break;
    }
  }
}

if (extents) {
  // process the extents
}

The extents tell you which physical drives the volume draws its storage from, and which bytes on those physical drives are devoted to the volume. But for this exercise, we just want the physical drives.

Once you have the physical drive numbers, you can convert them to physical drive handles by building a path of the form \\.\PhysicalDrive# where the # is the decimal expansion of the drive number.

wchar_t physicalDrivePath[80];
wsprintf_s(physicalDrivePath, L"\\\\.\\PhysicalDrive%d", physicalDriveNumber);
driveHandle = CreateFile(physicalVolumePath,
        0, FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
        nullptr, OPEN_EXISTING, 0, nullptr);

Okay, great, now you have a physical drive handle.

Next time, we’ll see that there’s a shortcut available for all this.

Bonus chatter: If you are interested only in the first physical drive of a multi-drive volume, you can do it much more simply, because the ioctl will fill in as much of the buffer as it can. Passing a buffer that can hold one physical drive will give you the first physical drive. (Mind you, the drives don’t appear to be in any particular, order, so really, you’re just grabbing one at random.)

wil::unique_hfile GetFirstPhysicalDiskHandleForVolume(HANDLE volume)
{
  VOLUME_DISK_EXTENTS extents;
  if (!DeviceIoControl(volume, IOCTL_VOLUME_GET_VOLUME_DISK_EXTENTS,
        nullptr, 0,
        &extents, sizeof(extents),
        &bytesWritten,
        nullptr) && GetLastError() != ERROR_MORE_DATA) {
    THROW_LAST_ERROR();
  }

  wchar_t physicalDrivePath[80];
  swprintf_s(physicalDrivePath, L"\\\\.\\PhysicalDrive%u",
             extents.Extents[0].DiskNumber);

  wil::unique_hfile result{ CreateFile(physicalDrivePath, 0,
                      FILE_SHARE_READ | FILE_SHARE_WRITE | FILE_SHARE_DELETE,
                      nullptr, OPEN_EXISTING, FILE_FLAG_BACKUP_SEMANTICS, nullptr) };
  THROW_LAST_ERROR_IF(!result);
  return result;
}

Bonus bonus chatter: It seems that the I/O subsystem can’t decide whether the number is a physical device number, a physical disk number, or a a physical drive number.

7 comments

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    • Avatar
      紅樓鍮

      I think the ioctl returns a variable-sized structure that comprises fixed members in the front and a variable length array following. Vectors only store pure arrays.

  • Avatar
    紅樓鍮

    I actually wish system calls/WinAPI functions that fill user-provided buffers were designed to be called in an “interactive” way, returning the array length/number of bytes needed plus some sort of “continuation object” that can be invoked with a suitably allocated buffer to get the final answer.

    • Avatar
      Jeremy Richards

      I think that would actually be harder to program against. Now you have to deal with the case where the 2nd half of the result doesn’t match the first half if the actual answer changed in between the two queries (e.g. in this case a volume was expanded across more physical drives, or the specific physical drives changed between the calls — admittedly unusual for this example, but would be fairly common for some of the other variable length calls in Win32).

      • Avatar
        紅樓鍮

        That would be a problem for any two system calls made seperately in time, including for example detecting the existence of a directory and then creating a file under that directory, in between which the directory could have been deleted. I defer the implementation of composable atomic system calls to the operating system’s authors.

  • Avatar
    Zenju Z

    Is the FILE_FLAG_BACKUP_SEMANTICS required in CreateFile to open “\\.\PhysicalDriveX”? In the first example “driveHandle = CreateFile” is not used, but in the second one (wil::unique_hfile result{ CreateFile) it is. In my tests it seems FILE_FLAG_BACKUP_SEMANTICS is not required: Just incidental, or better add it anyways?

  • Avatar
    David Walker

    Building on the previous entries in this series, I thought you were starting from a file path — and that means the file has to exist, right?

    If the file exists, then I doubt that it’s on a “CD-ROM drive with no disk in the drive”. But then I realize that you were starting with a volume… and if you actually got the volume from a file path, then there would be a disk in the drive… not counting cases where someone ejects the drive at the right instant. Tricky!