[CentOS] Question re: CentOS-6.0, KVM, and /dev/sr0

Wed Aug 31 17:47:18 UTC 2011

On Tue, August 30, 2011 18:57, psprojectplanning at gmail.com
wrote:
> On 29/08/2011 15:46, James B. Byrne wrote:
>> I am experimenting with KVM and I wish to create a
>> virtual machine image in a logical volume.  I can
>> create the new lv without problem but when I go to
>> format its file system then I get these
>> warnings:
>>
>> Warning: WARNING: the kernel failed to re-read the
>> partition table on /dev/sda (Device or resource busy).
>> As a result, it may not reflect all of your changes
>> until after reboot.
.  .  .
>> The favour of a direct copy of any reply to the mailing
>> list is requested as I am a digest subscriber.
>>

> You do not need to reboot every time you adjust a Logical
> Volume. Do you also need to format the file system for a
> KVM guest's Logical Volume?

I formatted the new lv as ext4.
>
> I'm currently juggling servers to try and get a free
> machine to test KVM on Centos 6, but i have recently
> found, with another RHEL clone I'm testing, that if
> you do not set up the LogVol with virsh or i suppose
> virt-manager you will have issues getting the guest
> machines to run.

I am using virt-manager to set up the vms

>
> If you look at chapter 26.1.4.1 & 26.1.4.2  of the Red
> Hat Visualization Guide, for RHEL6, it explains how to
> use fdisk to create an partition for the Logical Volume,
> set it to a Linux LVM type and create the
> storage pool for the KVM guests (page 217 & 218).

I am using that guide and I thank you for the specific
reference. Nonetheless, I had the same problems when I
used fdisk.

>
> On my current RHEL clone test system, to create the
> VolGroup / Storage pool i used the virsh commands on
> pages 222 & 223 of the Red Hat
> Visualization Guide (which were similar to the following):
>
> # virsh pool-define-as guest_images_lvm logical - -
> /dev/cciss/c0d0p3
> libvirt_lvm /dev/libvirt_lvm
> # virsh pool-build VolGroupGuests
> # virsh pool-start guest_images_lvm
> # virsh pool-autostart guest_images_lvm
> # virsh pool-list --all
>
>      Name                 State      Autostart
>      -----------------------------------------
>      guest_images_lvm     active     yes
>
> To create the actual logical volume for the virtual
> machine I used the
> following command:
> # virsh --connect qemu:///system vol-create-as
> guest_images_lvm volume1 20G
>
> I don't remember formatting a file system prior to
> installing the KVM guest, but new i am new to KVM
> and I'm experimenting as well.
>
> jk

I believe that the main problem I experienced was due to a
change in the behaviour of virt-manager from 5.6 to 6.0. 
A change that I consider a defect and have reported as Bug
734529.

Essentially, the parted error messages are meaningless
insofar as the new lv is indeed properly formatted and
found and mounted as is shown in the output of parted -l

> Model: Linux device-mapper (linear) (dm)
> Disk /dev/mapper/vg_inet02-lv_guest01: 129GB
> Sector size (logical/physical): 512B/512B
> Partition Table: loop
>
> Number  Start  End    Size   File system  Flags
>   1      0.00B  129GB  129GB  ext4
>

I have no idea what is causing the errors to be reported
by parted but it evidently has no impact on the result.

However, the behaviour of virtual machine manager has
changed so that it no longer permits the operator to
specific an alternate location and image file name, unless
that file already exists.  What happens is that if one
chooses to navigate to an alternate location, say
/var/vms/lv_guest_01, in the file browser; and if that
location has no content, then the file browser enters an
indefinite wait state which can only be ended by
navigating to somewhere else in the file system that has
content.

In 5.6, one could navigate to an empty directory and then
supply a new file name which would be used to hold the new
image.  In 6.0 one must first create that file name in the
desired location and only then can the virtual machine
manager use it to save the new image because only then can
it be selected in the file browser.

Otherwise, one has to enter the host's storage options and
add storage volumes there.  This appears at first blush to
give equivalent functionality to the old behaviour but it
is far from being obvious to the user.

It was the combination of the parted errors and
encountering the unexpected behaviour of the virtual
machine manager that had me confused.  I inferred that the
second issue was consequential to the first when in fact
the first had no effect and neither had anything to do
with the other.

-- 
***          E-Mail is NOT a SECURE channel          ***
James B. Byrne                mailto:ByrneJB at Harte-Lyne.ca
Harte & Lyne Limited          http://www.harte-lyne.ca
9 Brockley Drive              vox: +1 905 561 1241
Hamilton, Ontario             fax: +1 905 561 0757
Canada  L8E 3C3