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    8.4G  8.0G  952K 100% /
tmpfs        tmpfs    499M     0  499M   0% /dev/shm
/dev/vda1     ext4    485M   33M  428M   8% /boot

# dumpe2fs /dev/vg_main/lv_root | grep 'Reserved block count'
dumpe2fs 1.41.12 (17-May-2010)
Reserved block count:     111513

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 0 /dev/vg_main/lv_root 
tune2fs 1.41.12 (17-May-2010)
Setting reserved blocks percentage to 0% (0 blocks)
# df -Th
Filesystem    Type    Size  Used Avail Use% Mounted on
/dev/mapper/vg_main-lv_root
              ext4    8.4G  8.0G  437M  95% /
tmpfs        tmpfs    499M     0  499M   0% /dev/shm
/dev/vda1     ext4    485M   33M  428M   8% /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 online

# resize2fs /dev/vg_main/lv_root
resize2fs 1.41.12 (17-May-2010)
Filesystem at /dev/vg_main/lv_root is mounted on /; on-line resizing required
old desc_blocks = 1, new_desc_blocks = 2
Performing an on-line resize of /dev/vg_main/lv_root to 4850688 (4k) blocks.
The filesystem on /dev/vg_main/lv_root is now 4850688 blocks long.

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

tune2fs -m 5 /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  8.0G  9.4G  46% /
tmpfs        tmpfs    499M     0  499M   0% /dev/shm
/dev/vda1     ext4    485M   33M  428M   8% /boot