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