[CentOS-devel] The status of our Vagrant images for Hyper-V

Laurentiu Pancescu

lpancescu at gmail.com
Thu Apr 6 12:03:55 UTC 2017

Hi there,

I created the first experimental Vagrant images for Hyper-V on March 14, 
after the CBS upgrade.  They seem to be functional, but they generate a 
lot of disk I/O during "vagrant up", according to Thomas and Michael.  I 
think we end up with 40GB files as disk images - this is the maximum 
size of the virtual hard drives we create, but most of that is empty.  I 
compared the contents of our box file to the one from kozo/centos-7, 
which is produced natively using Packer and Hyper-V: our image contains 
a real 40GB disk image, while the kozo image only has a 2.1GB image.

Image Factory converts the KVM .qcow2 image to a .vhd by calling 
"qemu-img convert -O vpc <source> <destination>".  This produces a 40GB 
sparse file taking just 1.1GB of space (both with the fixed and dynamic 
subformats).  The problem occurs while packing the sparse image into the 
.box file: Python's tarfile module only has read-only support for sparse 
files (a GNU tar extension) - even in Python 3, so the archive ends up 
with a regular 40GB file.

Creating 40GB sparse files seems to be specific to qemu-img: "VBoxManage 
convertmedium disk box.vmdk box.vhd" produces a 1.1GB regular file 
(VirtualBox' convert tool doesn't seem to support QCOW2, so I had to 
first convert the QCOW2 image to VMDK).

We should probably either:
- call GNU tar from Image Factory to preserve the sparseness of our 
image files (and hope that Windows handles them properly)
- use VBoxManage to convert to .vhd (VirtualBox is not in our repos, but 
Oracle offers a .rpm for EL7 systems).

In the mean time, we will probably release v1703 without Hyper-V 
support, since CentOS Linux 6.9 was already released.

Best regards,

[vagrant at localhost ~]$ tar -tvzf c7.box  # that's ours
drwxr-xr-x root/root         0 2017-03-14 11:23 Virtual Machines/
-rw-r--r-- root/root     39100 2017-03-14 11:23 Virtual Machines/vm.XML
-rw-r--r-- root/root       114 2017-03-14 11:23 Vagrantfile
drwxr-xr-x root/root         0 2017-03-14 11:22 Virtual Hard Disks/
-rwxr-xr-x root/root 42949672960 2017-03-14 11:23 Virtual Hard 
-rw-r--r-- root/root          22 2017-03-14 11:23 metadata.json

[vagrant at localhost ~]$ unzip -l hyperv.box  # kozo/centos-7
Archive:  hyperv.box.zip
   Length      Date    Time    Name
---------  ---------- -----   ----
     36898  02-25-2017 13:05   Virtual 
     61440  02-25-2017 13:05   Virtual 
        64  02-25-2017 12:47   metadata.json
2185232384  02-25-2017 13:05   Virtual Hard Disks/CentOS7.vhdx
---------                     -------
2185330786                     4 files

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