Extend a root LVM partition online

There is also a quick remedy for the emergency situation when your root partition runs out of disk space. There is a feature specific to ext3 and ext4 that can help the goal of resolving the full disk situation. Unless explicitly changed during filesystem creation, both by default reserve five percent (5%) of a volume capacity to the superuser (root).

 # df -Th
 Filesystem    Type    Size  Used Avail Use% Mounted on
 /dev/mapper/vg_main-lv_root
               ext4    .4G  .0G  952K % /
 tmpfs        tmpfs    499M       499M   % /dev/shm
 /dev/vda1     ext4    485M   33M  428M   % /boot

 # dumpe2fs /dev/vg_main/lv_root | grep 'Reserved block count'
 dumpe2fs 1.41. (-May-)
 Reserved block count:     

It turned out 111513 of 4KB blocks were reserved for the superuser, which was exactly five percent of the volume capacity.

How to enable it?

 # tune2fs -m  /dev/vg_main/lv_root
 tune2fs 1.41. (-May-)
 Setting reserved blocks percentage to % ( blocks)
 # df -Th
 Filesystem    Type    Size  Used Avail Use% Mounted on
 /dev/mapper/vg_main-lv_root
               ext4    .4G  .0G  437M  % /
 tmpfs        tmpfs    499M       499M   % /dev/shm
 /dev/vda1     ext4    485M   33M  428M   % /boot

Now that we have some free space on the root partition to work on we can extend the LVM partition:

Create a new partition of appropriate size using fdisk

 fdisk /dev/sdb1

This is a key sequence on the keyboard to create a new LVM type (8e) partition:

n, p, 1, enter (accept default first sector), enter (accept default last sector), t, 8e, w

Create a new Physical Volume

 # pvcreate /dev/sdb1
   Writing physical volume data to disk "/dev/sdb1"
   Physical volume "/dev/sdb1" successfully created

Extend a Volume Group

 # vgextend vg_main /dev/sdb1
   Volume group "vg_main" successfully extended

Extend your LVM

- extend the size of your LVM by the amount of free space on PV

 # lvextend /dev/vg_main/lv_root /dev/sdb1
   Extending logical volume lv_root to 18.50 GiB
   Logical volume lv_root successfully resized

- or with a given size

 lvextend -L +10G /dev/vg_main/lv_root

Finally resize the file system on-line

 # resize2fs /dev/vg_main/lv_root
 resize2fs 1.41. (-May-)
 Filesystem at /dev/vg_main/lv_root is mounted on /; on-line resizing required
 old desc_blocks = , new_desc_blocks =
 Performing an on-line resize of /dev/vg_main/lv_root to  (4k) blocks.
 The filesystem on /dev/vg_main/lv_root is now  blocks long.

- or use xfs_growfs for xfs file system

 # xfs_growfs /dev/vg_main/lv_root

Now we can set the reserved blocks back to the default percentage - 5%

 # tune2fs -m  /dev/mapper/vg_main-lv_root
 Results:

 # df -Th
 Filesystem    Type    Size  Used Avail Use% Mounted on
 /dev/mapper/vg_main-lv_root
               ext4     19G  .0G  .4G  % /
 tmpfs        tmpfs    499M       499M   % /dev/shm
 /dev/vda1     ext4    485M   33M  428M   % /boot