[fedora-virt] How to use virtio-scsi on F17 host and F17 guest

[Gianluca Cecchi]

On Wed, Jun 13, 2012 at 10:22 AM, Kevin Wolf wrote:
> Never use qemu-img to modify an image that is in use by a VM, it might
> break the image. You need to use the qemu monitor of that VM to modify
> this image, which in your case means that you should probably use the
> proper libvirt interface, e.g. with virsh.
>
> Kevin

ops .. I didn't keep this into consideration, as I only used qemu-img
resize with powered off vm until now...
So the disk was indeed brutally corrupted at next reboot... never mind.
I reinstalled f17 guest from scratch.
Now we are here:

f17 vm is running

on host:
$ sudo qemu-img info /var/lib/libvirt/images/f17.img
image: /var/lib/libvirt/images/f17.img
file format: qcow2
virtual size: 8.0G (8589934592 bytes)
disk size: 5.7G
cluster_size: 65536

[g.cecchi at ope46 ~]$ sudo virsh blockresize f17
/var/lib/libvirt/images/f17.img 9G
Block device '/var/lib/libvirt/images/f17.img' is resized

In guest the / filesystem is on lvm while /boot is on first partition:
[root at f17 ~]# fdisk -l /dev/sda

Disk /dev/sda: 8589 MB, 8589934592 bytes
64 heads, 32 sectors/track, 8192 cylinders, total 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001e726

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     1026047      512000   83  Linux
/dev/sda2         1026048    16777215     7875584   8e  Linux LVM

[root at f17 ~]# echo 1 > /sys/block/sda/device/rescan

from dmesg output:
[  263.003897] sd 2:0:0:0: [sda] 18874368 512-byte logical blocks:
(9.66 GB/9.00 GiB)
[  263.004344] sda: detected capacity change from 8589934592 to 9663676416

[root at f17 ~]# fdisk -l /dev/sda

Disk /dev/sda: 9663 MB, 9663676416 bytes
64 heads, 32 sectors/track, 9216 cylinders, total 18874368 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0001e726

   Device Boot      Start         End      Blocks   Id  System
/dev/sda1   *        2048     1026047      512000   83  Linux
/dev/sda2         1026048    16777215     7875584   8e  Linux LVM

I run fdisk and delete / create lvm partition bigger (as I usually do
with plain partitions for filesystems on them); then

[root at f17 ~]# partprobe /dev/sda
Error: Partition(s) 2 on /dev/sda have been written, but we have been
unable to inform the kernel of the change, probably because it/they
are in use.  As a result, the old partition(s) will remain in use.
You should reboot now before making further changes.

[root at f17 ~]# kpartx /dev/sda
sda1 : 0 1024000 /dev/sda 2048
sda2 : 0 17848320 /dev/sda 1026048

[root at f17 ~]# pvscan
  PV /dev/sda2   VG vg_f17   lvm2 [7.50 GiB / 0    free]
  Total: 1 [7.50 GiB] / in use: 1 [7.50 GiB] / in no VG: 0 [0   ]

[root at f17 ~]# pvresize /dev/sda2
  Physical volume "/dev/sda2" changed
  1 physical volume(s) resized / 0 physical volume(s) not resized

but nothing changes actually...

[root at f17 ~]# pvs
  PV         VG     Fmt  Attr PSize PFree
  /dev/sda2  vg_f17 lvm2 a--  7.50g    0

Is this a limitation of lvm layer?
I noticed this kind of limitation also on VMware guests (with lsi
logic virtual controller) where apparently I'm unable to inform lvm of
the PV changed size....
I know that I can add another partition at the end of disk instead of
resizing the existing one... but I'd prefer this latter...
Thanks,
Gianluca