Red Hat virt-install and Partitions


I have recently been using virt-install on CentOS 5 to install some virtual machines (I previously posted a summary of the experience). One problem I had is the inability to do an install of Fedora to /dev/hda – it insisted on partitioning the disk and installing it on a partition. I have filed Red Hat Bugzilla #249837 about this problem.

# n=1
# while true ; do dd if=/dev/vg0/orig of=/dev/vg0/dest bs=512 skip=$n count=1000 ; echo $n ; file -sL /dev/vg0/dest ; n=$(($n+1)) ; done

After doing a CentOS or Fedora install I then have to fix things so that my desired option (of using a single unpartitioned virtual disk) is used. To do this I first needed to determine how many sectors at the start of the filesystem were used for the partition table. I used the above shell commands to test skipping different numbers of sectors and using file -sL to determine whether the result was recognised as an Ext2/3 filesystem. Below is part of the output:

/dev/vg0/dest: data
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.018812 seconds, 27.2 MB/s
/dev/vg0/dest: Linux rev 1.0 ext3 filesystem data (large files)
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.115528 seconds, 4.4 MB/s
/dev/vg0/dest: data
1000+0 records in
1000+0 records out
512000 bytes (512 kB) copied, 0.018623 seconds, 27.5 MB/s

As you can see 63 sectors are used for the partition table and other boot blocks at the start of the disk. So I used the below command to fix it:
dd if=/dev/vg0/orig of=/dev/vg0/dest bs=512 skip=63

The next problem after this was booting. The pygrub program that is used by the CentOS Xen installation to boot DomU’s only works with partitioned block devices. This however is quite easy to solve, I merely had to copy the kernel and initrd from the Xen image to the Dom0 filesystem and use the files for booting. Of course every time I upgrade the kernel I will need to copy the files, but that’s not such a great inconvenience. I usually end up running multiple DomU’s with the same distribution so I can copy the kernel and initrd once and use it for all instances.

1 thought on “Red Hat virt-install and Partitions”

  1. Dave says:

    Hi Russel,

    kpartx may make your life a little easier, it creates device maps from partition tables. The package is part of RHEL/CentOS.

    try ‘kpartx -a /dev/vg0/orig’, then (if you’re using LVM in the guest) ‘vgscan’, and ‘vgchange -ay’.


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