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Storage and memory

Disk usage​

df -h                  # free space per filesystem, human readable
df /root -H            # a specific mount, powers of 1000

du -sh /var            # total size of a directory
du -shc /var/*         # size of each entry plus a grand total
du -h --max-depth=1 /var

Block devices​

lsblk                  # list block devices (e.g. to find an SD card)

LVM​

/etc/fstab shows how the root filesystem is mounted, which reveals whether LVM is in use:

  • A line starting with UUID=... or /dev/sdaX is a plain partition.
  • A line under /dev/mapper/... indicates LVM.
cat /etc/fstab
lvs -o +devices /dev/mapper/ubuntu--vg-ubuntu--lv

Root partition only uses part of the volume​

A fresh Ubuntu LVM install often allocates only part of the disk to the root logical volume. This is deliberate — the unallocated space can be used to create another logical volume, or to grow an existing one on the fly.

Extend the root filesystem (the --resizefs option grows the filesystem after the volume, in one step):

# Add 5 GB
sudo lvextend --resizefs -L +5G ubuntu-vg/ubuntu-lv

# Use all remaining free space
sudo lvextend --resizefs -l +100%FREE ubuntu-vg/ubuntu-lv

Without --resizefs, the logical volume grows but the filesystem inside it does not — you then have to run the filesystem's own grow command, for example resize2fs /dev/mapper/ubuntu--vg-ubuntu--lv (ext4) or xfs_growfs / (XFS).

Memory​

free -h                # memory and swap usage
htop                   # interactive process/memory viewer

# Percentage of memory free
free | awk '/Mem/ { printf("free: %.2f %%\n", $4/$2 * 100.0) }'

# Show a specific process tree
top -c -p $(pgrep -d',' -f systemd)

Swap​

sudo swapon -s                 # show current swap
sudo swapoff -a                # turn all swap off

# Create a 4 GB swap file
sudo fallocate -l 4G /swapfile
sudo chmod 600 /swapfile       # root-only, required
sudo mkswap /swapfile
sudo swapon /swapfile

# Make it permanent
echo '/swapfile none swap sw 0 0' | sudo tee -a /etc/fstab

# Swappiness (lower favours RAM over swap)
sysctl vm.swappiness
sudo sysctl vm.swappiness=25
# Persist in /etc/sysctl.conf, then:
sudo sysctl -p

To resize an existing swap file, disable and remove it first, then recreate:

sudo swapoff /swapfile
sudo rm /swapfile
# 16 GB via dd: bs * count = 1M * 16384
sudo dd if=/dev/zero of=/swapfile bs=1M count=16384
sudo chmod 600 /swapfile
sudo mkswap /swapfile
sudo swapon /swapfile

0600 permission on the swap file is mandatory — mkswap/swapon refuse a world-readable one.