[Bug 1759522] Re: windows qemu-img create vpc/vhdx error

[maro]

I also discovered just a few days ago the problem that sparse VHD/VHDX
image files are not being accepted by Windows.

It appears that qemu-img on Windows always tries to create images as
sparse files. Only in some cases (e.g. when operating on a NTFS file
system) will the file actually be a sparse file.

Being a sparse file seems to be no issue when using these file with QEMU
itself. Most file formats that qemu-img is able to create will be
unknown to Windows own tools, but the one exception are VHD/VHDX files.

As long as a file carries the sparse attribute it will be "un-
acceptable" to the Windows tools (e.g. diskpart/diskmgmt). As a crude
work-around I've noticed that a copy of such a file will have "lost" the
sparse attribute, and therefore can be mounted. Likewise any copy of a
file created on a different system will not be a sparse file (and hence
the issue does not arise).

I'm attaching a log file (with a few comments) of some steps that
demonstrate the problem, and how I worked around it. The test was done
with qemu-img v4.1.0, but I'm fairly certain that this issue has been
present since "forever" (and a quick re-test with v4.2.0 confirmed that
it has not "gone away").

So, a simple work-around exists, but I see myself unable to suggest a
patch. I guess the patch should be specific to prevent the creation of
sparse VHD/VHDX files on Windows. But from the superficial reading I did
I could not work out whether the image format information would be
available in 'raw_co_create()' (of: 'block/file-win32.c').

** Attachment added: "sparse-VHD_issue.log"
   
https://bugs.launchpad.net/qemu/+bug/1759522/+attachment/5332081/+files/sparse-VHD_issue.log

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https://bugs.launchpad.net/bugs/1759522

Title:
  windows qemu-img create vpc/vhdx error

Status in QEMU:
  New

Bug description:
  On windows, using qemu-img (version 2.11.90) to create vpc/vhdx
  virtual disk tends to fail. Here's the way to reproduce:

  1. Install qemu-w64-setup-20180321.exe

  2. Use `qemu-img create -f vhdx -o subformat=fixed disk.vhdx 512M` to create 
a vhdx:
     Formatting 'disk.vhdx', fmt=vhdx size=536870912 log_size=1048576 
block_size=0 subformat=fixed

  3. Execute `qemu-img info disk.vhdx` gives the result, (note the `disk size` 
is incorrect):
     image: disk.vhdx
     file format: vhdx
     virtual size: 512M (536870912 bytes)
     disk size: 1.4M
     cluster_size: 8388608

  4. On Windows 10 (V1709), double click disk.vhdx gives an error:
     Make sure the file is in an NTFS volume and isn't in a compressed folder 
or volume.

     Using Disk Management -> Action -> Attach VHD gives an error:
     The requested operation could not be completed due to a virtual disk 
system limitation. Virtual hard disk files must be uncompressed and uneccrypted 
and must not be sparse.

  Comparison with Windows 10 created VHDX:

  1. Using Disk Management -> Action -> Create VHD:
     File name: win.vhdx
     Virtual hard disk size: 512MB
     Virtual hard disk format: VHDX
     Virtual hard disk type: Fixed size

  2. Detach VHDX

  3. Execute `qemu-img info win.vhdx` gives the result:
     image: win.vhdx
     file format: vhdx
     virtual size: 512M (536870912 bytes)
     disk size: 516M
     cluster_size: 33554432

  Comparison with qemu-img under Ubuntu:

  1. Version: qemu-img version 2.5.0 (Debian 1:2.5+dfsg-5ubuntu10.16),
  Copyright (c) 2004-2008 Fabrice Bellard

  2. qemu-img create -f vhdx -o subformat=fixed lin.vhdx 512M
     Formatting 'lin.vhdx', fmt=vhdx size=536870912 log_size=1048576 
block_size=0 subformat=fixed

  3. qemu-img info lin.vhdx
     image: lin.vhdx
     file format: vhdx
     virtual size: 512M (536870912 bytes)
     disk size: 520M
     cluster_size: 8388608

  4. Load lin.vhdx under Windows 10 is ok

  The same thing happens on `vpc` format with or without
  `oformat=fixed`, it seems that windows version of qemu-img has some
  incorrect operation? My guess is that windows version of qemu-img
  doesn't handle the description field of vpc/vhdx, which leads to an
  incorrect `disk size` field.

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