Since this is a root/boot volume we are talking about anything on this page is VERY risky.  Be warned this post may provide you with a strategy or hints but you are completely responsible for your system.

Of course the best route to extend a root volume in an unmounted state is to boot of a Live CD.  In some cases and even more relevant a lot of cloud instances you do not have normal physical or virtualization methods to access server consoles.

I have been trying a couple ideas.  First one is using dracut to resize the root volume on system bootup.  I have had success with this method.  The second method is using pivot_root.  On this method it may be slightly easier on systemd servers but for my paritcular need I have an older Centos 6 flavor without systemd and so far I could not get pivot_root and resize online to work.

Below is notes on method 1 (dracut resize):

Specs:
VirtualBox VM CentOS6.4
Disk layout 15G
- /boot 200M ext4
- / LV 13G ext4
- /u01 LV 2G ext4

GOAL:
1. Resize / file system smaller without boot CD only SSH access. Using dracut bootup.
2. Extend /u01 with the additional space in the VG

	
[root@localhost ~]# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroupSys-LogVolRoot
                       11G  2.1G  8.7G  20% /
tmpfs                 1.9G     0  1.9G   0% /dev/shm
/dev/sda1             194M   29M  155M  16% /boot
/dev/mapper/VolGroupSys-LogVolU01
                      2.1G   68M  2.0G   4% /u01
[root@localhost ~]# pvs
  PV         VG          Fmt  Attr PSize  PFree
  /dev/sda2  VolGroupSys lvm2 a--  14.80g 1.70g

[root@localhost ~]# vgs
  VG          #PV #LV #SN Attr   VSize  VFree
  VolGroupSys   1   2   0 wz--n- 14.80g 1.70g

[root@localhost ~]# lvs
  LV         VG          Attr      LSize  Pool Origin Data%  Move Log Cpy%Sync Convert
  LogVolRoot VolGroupSys -wi-ao--- 11.00g                                             
  LogVolU01  VolGroupSys -wi-ao---  2.11g    
  
# pwd
/usr/share/dracut/modules.d/95rootfs-block

# vi mount-root.sh
[..]
if [ -n "$root" -a -z "${root%%block:*}" ]; then
    ## custom code testing lvresize on root vol
    lvm vgchange -ay --config " global {locking_type=1} "
    rm -f /etc/lvm/lvm.conf
    e2fsck -C 0 -f /dev/VolGroupSys/LogVolRoot
    resize2fs -p -f /dev/VolGroupSys/LogVolRoot 8G
    lvm lvresize -f /dev/VolGroupSys/LogVolRoot -L 8G
    resize2fs -p -f /dev/VolGroupSys/LogVolRoot
    e2fsck -C 0 -f /dev/VolGroupSys/LogVolRoot

    mount -t ${fstype:-auto} -o "$rflags" "${root#block:}" "$NEWROOT" \
[..]
	
# dracut -f --install 'resize2fs e2fsck'

# reboot

Verify after the reboot

# pvs
  PV         VG          Fmt  Attr PSize  PFree
  /dev/sda2  VolGroupSys lvm2 a--  14.80g 4.70g

# vgs
  VG          #PV #LV #SN Attr   VSize  VFree
  VolGroupSys   1   2   0 wz--n- 14.80g 4.70g

# lvs
  LV         VG          Attr      LSize Pool Origin Data%  Move Log Cpy%Sync Convert
  LogVolRoot VolGroupSys -wi-ao--- 8.00g                                             
  LogVolU01  VolGroupSys -wi-ao--- 2.11g                                             

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroupSys-LogVolRoot
                      7.9G  2.1G  5.8G  26% /
tmpfs                 1.9G     0  1.9G   0% /dev/shm
/dev/sda1             194M   30M  155M  16% /boot
/dev/mapper/VolGroupSys-LogVolU01
                      2.1G   68M  2.0G   4% /u01

** remove custom code from mount-root.sh

# dracut -f
# reboot

#################################################################
### EXTEND u01
#################################################################

# lvextend -L+2G /dev/VolGroupSys/LogVolU01 
  Extending logical volume LogVolU01 to 4.11 GiB
  Logical volume LogVolU01 successfully resized

# resize2fs /dev/VolGroupSys/LogVolU01 
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/VolGroupSys/LogVolU01 is mounted on /u01; on-line resizing required
old desc_blocks = 1, new_desc_blocks = 1
Performing an on-line resize of /dev/VolGroupSys/LogVolU01 to 1076224 (4k) blocks.
The filesystem on /dev/VolGroupSys/LogVolU01 is now 1076224 blocks long.

# vgs
  VG          #PV #LV #SN Attr   VSize  VFree
  VolGroupSys   1   2   0 wz--n- 14.80g 2.70g

# lvs
  LV         VG          Attr      LSize Pool Origin Data%  Move Log Cpy%Sync Convert
  LogVolRoot VolGroupSys -wi-ao--- 8.00g                                             
  LogVolU01  VolGroupSys -wi-ao--- 4.11g                                             

# df -h
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/VolGroupSys-LogVolRoot
                      7.9G  2.1G  5.8G  26% /
tmpfs                 1.9G     0  1.9G   0% /dev/shm
/dev/sda1             194M   29M  155M  16% /boot
/dev/mapper/VolGroupSys-LogVolU01
                      4.1G   69M  3.8G   2% /u01

#################################################################					  
### APPENDIX: 
#################################################################

** Use /var/messages/boot.log to check dracut failures on our custom code
[bash]
# more boot.log 
e2fsck 1.41.12 (17-May-2010)
Pass 1: Checking inodes, blocks, and sizes
_CentOS-6.4-x86_: |===                                             -  4.8%   
Pass 2: Checking directory structure                                           
Pass 3: Checking directory connectivity                                        
Pass 4: Checking reference counts
Pass 5: Checking group summary information
_CentOS-6.4-x86_: 76680/835584 files (0.1% non-contiguous), 578555/3328000 blocks
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/VolGroupSys/LogVolRoot to 2621440 (4k) blocks.
Begin pass 3 (max = 102)
Scanning inode table          XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
The filesystem on /dev/VolGroupSys/LogVolRoot is now 2621440 blocks long.

  Read-only locking type set. Write locks are prohibited.
  Can't get lock for VolGroupSys
resize2fs 1.41.12 (17-May-2010)
Resizing the filesystem on /dev/VolGroupSys/LogVolRoot to 3328000 (4k) blocks.
Begin pass 1 (max = 22)
Extending the inode table     XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
The filesystem on /dev/VolGroupSys/LogVolRoot is now 3328000 blocks long.
[..]

** Howto check if lvm and resize2fs is in intramfs image

# lsinitrd /boot/initramfs-2.6.32-358.el6.x86_64.img | grep lvm
-rw-r--r--   1 root     root          657 Feb 22  2013 etc/udev/rules.d/64-lvm.rules
-r--r--r--   1 root     root         1286 Feb 22  2013 etc/udev/rules.d/11-dm-lvm.rules
drwxr-xr-x   2 root     root            0 May 21 13:27 etc/lvm
-rw-r--r--   1 root     root        37554 May 21 13:27 etc/lvm/lvm.conf
-rwxr-xr-x   1 root     root         2243 Feb 22  2013 sbin/lvm_scan
-r-xr-xr-x   1 root     root      1013336 May 21 13:27 sbin/lvm
[..]
-rwxr-xr-x   1 root     root          525 Feb 22  2013 cmdline/30parse-lvm.sh

# lsinitrd /boot/initramfs-2.6.32-358.el6.x86_64.img | grep resize2fs

#################################################################
#### Some links ...
#################################################################

Kernel panic on boot following "dracut Warning: LVM rootvg/rootlv not found": https://access.redhat.com/solutions/1282013
Rename LVM Volume Group Holding Root File System Volume: https://oraganism.wordpress.com/2013/03/09/rename-lvm-vg-for-root-fs-lv/
How to debug Dracut problems: https://fedoraproject.org/wiki/How_to_debug_Dracut_problems
Inject ephemeral disk into root disk: https://github.com/eucalyptus/eucalyptus/wiki/Inject-ephemeral-disk-into-root-disk
Convert an Existing System to Use Thin LVs: https://dustymabe.com/2013/09/07/convert-an-existing-system-to-use-thin-lvs/
Partition Resize fails on LVM: https://github.com/flegmatik/linux-rootfs-resize/issues/8
How to write a pre-mount startup script?: https://unix.stackexchange.com/questions/87814/how-to-write-a-pre-mount-startup-script

Is it possible to on-line shrink a EXT4 volume with LVM?: https://serverfault.com/questions/528075/is-it-possible-to-on-line-shrink-a-ext4-volume-with-lvm/528076
resize a Linux root partition while it's still mounted: http://www.ivarch.com/blogs/oss/2007/01/resize-a-live-root-fs-a-howto.shtml
How to shrink root filesystem without booting a livecd: https://unix.stackexchange.com/questions/226872/how-to-shrink-root-filesystem-without-booting-a-livecd?utm_medium=organic&utm_source=google_rich_qa&utm_campaign=google_rich_qa

[/bash]

If you removed a disk from a server and forgot to cleanly remove the configuration you may experience something similar.

Below shows what happens with lvremove:

# lvremove --force --verbose /dev/vg02/u01
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
    There are 1 physical volumes missing.
    Archiving volume group "vg02" metadata (seqno 2).
    Releasing logical volume "u01"
  Aborting vg_write: No metadata areas to write to!

Some people seem to have success with vgreduce commands but I ended up making a fake device and extending the VG in order to cleanup the LV.

# lvs
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  LV   VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  opt  vg01 -wi-ao---- 995.00g                                                    
  u01  vg02 -wi-----p- 248.00g                                                    

# dd if=/dev/zero of=/tmp/tmp.raw bs=1M count=100
100+0 records in
100+0 records out
104857600 bytes (105 MB) copied, 0.0763176 s, 1.4 GB/s

# losetup -f
/dev/loop0

# losetup /dev/loop0 /tmp/tmp.raw 

# vgextend vg02 /dev/loop0
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  Physical volume "/dev/loop0" successfully created.
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  Volume group "vg02" successfully extended

# vgremove vg02 --force
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  Logical volume "u01" successfully removed
  Volume group "vg02" successfully removed

# pvremove /dev/loop0
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  Labels on physical volume "/dev/loop0" successfully wiped.

# losetup -d /dev/loop0  
# rm /tmp/tmp.raw 

# lvs
  LV   VG   Attr       LSize   Pool Origin Data%  Meta%  Move Log Cpy%Sync Convert
  opt  vg01 -wi-ao---- 995.00g                                                    
# vgs
  VG   #PV #LV #SN Attr   VSize   VFree
  vg01   1   1   0 wz--n- 999.99g 4.99g
# pvs
  WARNING: Device for PV ore6TE-2ues-rc5q-mkCN-hEHE-C4Av-paaxEH not found or rejected by a filter.
  PV         VG   Fmt  Attr PSize   PFree
  /dev/sdb   vg01 lvm2 a--  999.99g 4.99g

Boot a Linux live CD. I used Fedora which I know has LVM enabled in the kernel at boot time.

# vgchange -a y
  5 logical volume(s) in volume group "locallvm" now active

# mkdir /mnt/a

# mount /dev/locallvm/root /mnt/a

** Figure out where your /boot physical partition lives. This one was sda2 but it could easily be /dev/sda1 also.

# mount /dev/sda2 /mnt/a/boot

# ls /mnt/a/boot/
abi-3.0.0-12-server         memtest86+.bin
config-3.0.0-12-server      memtest86+_multiboot.bin
grub                        System.map-3.0.0-12-server
initrd.img-3.0.0-12-server  vmcoreinfo-3.0.0-12-server
lost+found                  vmlinuz-3.0.0-12-server

# mount -o bind /sys /mnt/a/sys
** Note:  if –o dev and –o proc don’t work, use –o bind for the next two
# mount -o dev /dev /mnt/a/dev
# mount -o proc /proc /mnt/a/proc

# chroot /mnt/a /bin/bash

Run update-grub
** If /run/lock does not exist, create the directory or else update-grub will fail since /var/lock is linked to /run/lock.

Use ls /boot to find the most current kernel and then run mkinitramfs on it.

mkinitramfs -k -o /boot/initrd.img-3.2.0-29-generic -v 3.2.0-29-generic

This next step is important and you need to do it before you reboot. If necessary change /etc/fstab references to point to new VG name.

Type exit. Choose reboot system from menu. Eject the CD so we don’t boot off CD by default.