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Shrink root filesystem online

Shrinking a root filesystem usually requires unmounting it, which is impossible while the operating system is running from it. The standard solution is to boot from a LiveCD/USB or using an initramfs hook. However, for remote servers (VPS, Cloud Instances) or headless devices without easy physical access, this isn't always an option.

This guide demonstrates a technique to "pivot" the root filesystem into a temporary RAM disk (tmpfs), allowing you to unmount the physical root device and resize it safely on a live system.

High Risk Operation

This is an advanced and dangerous procedure. If you make a mistake, you may lose access to the system or corrupt the filesystem.

  • Backup your data before proceeding.
  • Ensure you have out-of-band access (KVM, Serial Console, or a Hosting Provider's Recovery Console) in case SSH connectivity is lost.

Overview

The strategy involves:

flowchart TD
    A["Normal Boot"] -->|"Mount tmpfs"| B["Create /tmp/tmproot"]
    B -->|"Copy Binaries"| C["Populate RAM Root"]
    C -->|"pivot_root"| D["Active Root: RAM"]
    D -->|"Restart Services"| E["Unlock Old Root"]
    E -->|"Unmount /oldroot"| F["Resize Filesystem/Partition"]
    F -->|"Reboot"| G["Normal Boot (Resized)"]

  1. Creating a minimal root filesystem in RAM (tmpfs).

  2. Copying essential binaries (mount, ls, ssh, resize2fs, etc.) to this RAM disk.

  3. Using pivot_root to swap the running root to the RAM disk.

  4. Restarting SSHD from the RAM disk to maintain connection.

  5. Unmounting the original root disk.

  6. Performing the resize operations.

  7. Pivoting back to the original root (optional, or just reboot).

Prepare the System

Stop as many services as possible to minimize open file handles.

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systemctl stop apache2 mariadb nginx docker

Install necessary tools if missing (e.g. psmisc for fuser):

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apt-get install psmisc

Create Temporary Root

Create a mount point and mount a tmpfs (RAM filesystem) there. This will be our new temporary "OS".

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mkdir /tmp/tmproot
mount -t tmpfs none /tmp/tmproot

Create the directory structure: 

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mkdir /tmp/tmproot/{proc,sys,dev,run,usr,var,tmp,oldroot,bin,sbin,lib,lib64,mnt,etc,root}

Copy essential binaries and libraries. It is safer to copy "too much" than too little.

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# Basic directories
cp -ax /bin /tmp/tmproot/
cp -ax /sbin /tmp/tmproot/
cp -ax /lib /tmp/tmproot/
cp -ax /lib64 /tmp/tmproot/
cp -ax /etc /tmp/tmproot/
cp -ax /root /tmp/tmproot/

# /usr binaries (often needed for sshd, python, etc.)
cp -ax /usr/bin /tmp/tmproot/usr/
cp -ax /usr/sbin /tmp/tmproot/usr/
cp -ax /usr/lib /tmp/tmproot/usr/
cp -ax /usr/lib64 /tmp/tmproot/usr/

# Var data (needed for runtime sockets, logs)
cp -ax /var/run /tmp/tmproot/var/
cp -ax /var/log /tmp/tmproot/var/
cp -ax /var/lib /tmp/tmproot/var/

Tip

If cp complains about specific sockets or recursive loops, you can often ignore those errors for this purpose, provided the essential files are copied.

Pivot Root

This is the critical moment. We will switch the system's root directory from disk to our RAM copy.

  1. Prepare mounts: pivot_root requires the mounts to be private (not shared). 

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    mount --make-rprivate /

  2. Pivot: 

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    pivot_root /tmp/tmproot /tmp/tmproot/oldroot

  3. Move Kernel Mounts: We need to move /proc, /sys, /dev, and /run to the new root so the system continues to function. 

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    for i in dev proc sys run; do mount --move /oldroot/$i /$i; done

Restore Access

After pivoting, your current SSH session might still be technically working, but it's tied to the old root. To properly unmount the old disk later, you need to restart services so they run from the new RAM root.

  1. Restart SSH: 

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    systemctl restart sshd

    Important

    Verify access immediately! Open a new terminal window and try to SSH into the server. Do NOT close your existing session until you confirm you can log in again.

  2. Reconnect: Log out of your current session and log back in. This ensures your shell is running from the new RAM root.

Free Up the Old Drive

Now check what processes are still holding the old filesystem (/oldroot).

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fuser -vm /oldroot

You will see a list of PIDs. You need to kill or restart them.

  • Systemd services: Try restarting them first systemctl restart <service>.
  • Stubborn processes: Kill them. Since we are in a temporary RAM OS, it's okay to be aggressive if services aren't essential for the text console.

Example, kill everything accessing /oldroot: 

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fuser -k -v -m /oldroot

Note: Be careful not to kill your own SSH session or the SSH daemon itself!

Verify nothing is using /oldroot: 

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fuser -vm /oldroot
It should return empty.

Unmount and Resize

Unmount the physical drive.

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umount /oldroot

Now the drive is unmounted, you can proceed with standard resizing operations.

  1. Check Filesystem: 

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    e2fsck -f /dev/sda1

  2. Resize Filesystem: (Shrink to 20G for example) 

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    resize2fs /dev/sda1 20G

  3. Resize Partition: Use fdisk, sfdisk, or parted to resize the partition on the disk to match the new filesystem size (or slightly larger). Example using fdisk to delete and recreate larger/smaller

    Important

    Be EXTREMELY careful to use the exact same start sector!

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    fdisk /dev/sda

Pivot Back (Optional) or Reboot

The cleanest way to return to normal is usually to simple reboot the system. Since we modified the physical disk, a reboot ensures the kernel re-reads the partition table cleanly.

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reboot

If you really want to pivot back without rebooting: 1. Mount the modified physical root to /oldroot. 2. pivot_root /oldroot /oldroot/tmp/tmproot. 3. Move /dev, /proc, etc. back. 4. Restart services.

However, rebooting is highly recommended after partition table changes.

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