[Qemu-devel] [Bug 1490611] Re: Using qemu >=2.2.1 to convert raw->VHD (fixed) adds extra padding to the result file, which Microsoft Azure rejects as invalid

[Kevin Wolf]

In short: VHD is a mess and Microsoft isn't compatible with itself.
That's the root cause of all the trouble we have with it.

When the qemu VHD driver was written, it was designed to be compatible
with Virtual PC, which used the disk geometry as the definite source for
the image size. In order to achieve exactly the same results on qemu, we
had to calculate the image size the same way, otherwise VMs would see a
larger disk when run in qemu compared to Virtual PC. qemu-img always
creates images so that real size and geometry match, which is the
rounding you are seeing. The commit that you bisected to fixed that
geometry and real size were inconsistent on fixed size disks.

Now HyperV treats images differently. It still stores a geometry, but it
doesn't actually use it to determine the size of a disk. Images created
by qemu-img are generally okay because geometry and real size match, but
when opening a HyperV image with qemu, the rounding we had to apply for
Virtual PC is suddenly wrong. This has given us some trouble before. Now
the requirement of Azure, which basically means we can't round any more
even though we have to do it for Virtual PC compatibility, completes the
mess.

While we considered hacks for opening images correctly, like checking
the creator application in the image, we can't do that automatically
when creating an image. I'm afraid the best we can do here is to add an
explicit subformat option that the user needs to specify manually.

Jeff, any opinion on this? And should we finally implement the
vpc_open() hack to distinguish by creator_app?

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https://bugs.launchpad.net/bugs/1490611

Title:
  Using qemu >=2.2.1 to convert raw->VHD (fixed) adds extra padding to
  the result file, which Microsoft Azure rejects as invalid

Status in QEMU:
  Fix Released

Bug description:
  Starting with a raw disk image, using "qemu-img convert" to convert
  from raw to VHD results in the output VHD file's virtual size being
  aligned to the nearest 516096 bytes (16 heads x 63 sectors per head x
  512 bytes per sector), instead of preserving the input file's size as
  the output VHD's virtual disk size.

  Microsoft Azure requires that disk images (VHDs) submitted for upload
  have virtual sizes aligned to a megabyte boundary. (Ex. 4096MB,
  4097MB, 4098MB, etc. are OK, 4096.5MB is rejected with an error.) This
  is reflected in Microsoft's documentation: https://azure.microsoft.com
  /en-us/documentation/articles/virtual-machines-linux-create-upload-
  vhd-generic/

  This is reproducible with the following set of commands (including the
  Azure command line tools from https://github.com/Azure/azure-xplat-
  cli). For the following example, I used qemu version 2.2.1:

  $ dd if=/dev/zero of=source-disk.img bs=1M count=4096

  $ stat source-disk.img 
    File: ‘source-disk.img’
    Size: 4294967296      Blocks: 798656     IO Block: 4096   regular file
  Device: fc01h/64513d    Inode: 13247963    Links: 1
  Access: (0644/-rw-r--r--)  Uid: ( 1000/  smkent)   Gid: ( 1000/  smkent)
  Access: 2015-08-18 09:48:02.613988480 -0700
  Modify: 2015-08-18 09:48:02.825985646 -0700
  Change: 2015-08-18 09:48:02.825985646 -0700
   Birth: -

  $ qemu-img convert -f raw -o subformat=fixed -O vpc source-disk.img
  dest-disk.vhd

  $ stat dest-disk.vhd 
    File: ‘dest-disk.vhd’
    Size: 4296499712      Blocks: 535216     IO Block: 4096   regular file
  Device: fc01h/64513d    Inode: 13247964    Links: 1
  Access: (0644/-rw-r--r--)  Uid: ( 1000/  smkent)   Gid: ( 1000/  smkent)
  Access: 2015-08-18 09:50:22.252077624 -0700
  Modify: 2015-08-18 09:49:24.424868868 -0700
  Change: 2015-08-18 09:49:24.424868868 -0700
   Birth: -

  $ azure vm image create testimage1 dest-disk.vhd -o linux -l "West US"
  info:    Executing command vm image create
  + Retrieving storage accounts                                                 
 
  info:    VHD size : 4097 MB
  info:    Uploading 4195800.5 KB
  Requested:100.0% Completed:100.0% Running:   0 Time: 1m 0s Speed:  6744 KB/s 
  info:    https://[redacted].blob.core.windows.net/vm-images/dest-disk.vhd was 
uploaded successfully
  error:   The VHD 
https://[redacted].blob.core.windows.net/vm-images/dest-disk.vhd has an 
unsupported virtual size of 4296499200 bytes.  The size must be a whole number 
(in MBs).
  info:    Error information has been recorded to /home/smkent/.azure/azure.err
  error:   vm image create command failed

  I also ran the above commands using qemu 2.4.0, which resulted in the
  same error as the conversion behavior is the same.

  However, qemu 2.1.1 and earlier (including qemu 2.0.0 installed by
  Ubuntu 14.04) does not pad the virtual disk size during conversion.
  Using qemu-img convert from qemu versions <=2.1.1 results in a VHD
  that is exactly the size of the raw input file plus 512 bytes (for the
  VHD footer). Those qemu versions do not attempt to realign the disk.
  As a result, Azure accepts VHD files created using those versions of
  qemu-img convert for upload.

  Is there a reason why newer qemu realigns the converted VHD file? It
  would be useful if an option were added to disable this feature, as
  current versions of qemu cannot be used to create VHD files for Azure
  using Microsoft's official instructions.

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