Online Resize Filesystem

1. Ensure the system is in a stable state, stop all services

   systemctl stop httpd
   systemctl stop nfs-server
   # and so on....
   

2. Unmount all unused filesystems

   umount -a
   

   If you see any on-disk filesystems still mounted, then something is still running that shouldn't be. Check what it is using fuser:

   # if necessary:
   yum install psmisc
   # then:
   fuser -vm 
   systemctl stop 
   umount -a
   # repeat as required...
   

3. Make the temporary root

   mkdir /tmp/tmproot
   mount -t tmpfs none /tmp/tmproot
   mkdir /tmp/tmproot/{proc,sys,dev,run,usr,var,tmp,oldroot}
   cp -ax /{bin,etc,mnt,sbin,lib,lib64} /tmp/tmproot/
   cp -ax /usr/{bin,sbin,lib,lib64} /tmp/tmproot/usr/
   cp -ax /var/{account,empty,lib,local,lock,nis,opt,preserve,run,spool,tmp,yp} /tmp/tmproot/var/
   

   This creates a very minimal root system, which breaks (among other things) manpage viewing (no /usr/share), user-level customizations (no /root or /home) and so forth. This is intentional, as it constitutes encouragement not to stay in such a jury-rigged root system any longer than necessary.

   At this point you should also ensure that all the necessary software is installed, as it will also assuredly break the package manager. Glance through all the steps, and make sure you have the necessary executables.

4. Pivot into the root

   mount --make-rprivate / # necessary for pivot_root to work
   pivot_root /tmp/tmproot /tmp/tmproot/oldroot
   for i in dev proc sys run; do mount --move /oldroot/$i /$i; done
   

   systemd causes mounts to allow subtree sharing by default (as with mount --make-shared), and this causes pivot_root to fail. Hence, we turn this off globally with mount --make-rprivate /. System and temporary filesystems are moved wholesale into the new root. This is necessary to make it work at all; the sockets for communication with systemd, among other things, live in /run, and so there's no way to make running processes close it.

5. Ensure remote access survived the changeover

   systemctl restart sshd
   systemctl status sshd
   

   After restarting sshd, ensure that you can get in, by opening another terminal and connecting to the machine again via ssh. If you can't, fix the problem before moving on.

   Once you've verified you can connect in again, exit the shell you're currently using and reconnect. This allows the remaining forked sshd to exit and ensures the new one isn't holding /oldroot.

6. Close everything still using the old root

   fuser -vm /oldroot
   

   This will print a list of processes still holding onto the old root directory. On my system, it looked like this:

                USER        PID ACCESS COMMAND
   /oldroot:    root     kernel mount /oldroot
                root          1 ...e. systemd
                root        549 ...e. systemd-journal
                root        563 ...e. lvmetad
                root        581 f..e. systemd-udevd
                root        700 F..e. auditd
                root        723 ...e. NetworkManager
                root        727 ...e. irqbalance
                root        730 F..e. tuned
                root        736 ...e. smartd
                root        737 F..e. rsyslogd
                root        741 ...e. abrtd
                chrony      742 ...e. chronyd
                root        743 ...e. abrt-watch-log
                libstoragemgmt    745 ...e. lsmd
                root        746 ...e. systemd-logind
                dbus        747 ...e. dbus-daemon
                root        753 ..ce. atd
                root        754 ...e. crond
                root        770 ...e. agetty
                polkitd     782 ...e. polkitd
                root       1682 F.ce. master
                postfix    1714 ..ce. qmgr
                postfix   12658 ..ce. pickup
   

   You need to deal with each one of these processes before you can unmount /oldroot. The brute-force approach is simply kill -9 $PID $PID2 $PID3 .... for each, but this can break things. To do it more softly:

   If the root drive you want to resize is an LVM drive, you may also need to restart some other running services, even if they do not show up in the list created by fuser -vm /oldroot. If you find you are unable to resize an LVM drive under Step 7, try systemctl restart systemd-udevd.

   Some processes can't be dealt with via simple systemctl restart. For me these included auditd (which doesn't like to be killed via systemctl, and so just wanted a kill -15). These can be dealt with individually.

   The last process you'll find, usually, is systemd itself. For this, run systemctl daemon-reexec.

   Once you're done, the table should look like this:

                USER        PID ACCESS COMMAND
   /oldroot:    root     kernel mount /oldroot
   

7. Unmount the old root

   umount /oldroot
   

   At this point, you can carry out whatever manipulations you require. If its not succeed and you have a console view (KVM) try killall -9 -u root.
   
   

8. Resize the Filesystem

   resize2fs /dev/sda2 40G
   

    

9. Allign the partition table

   fdisk /dev/sda
   

   The last step is deleting the partition, but remember the start sector, and re-create with new size + 1GB for safety, and if the fdisk asks remove any metadata or signature tell NO.
   Optionally create here a new partition with all available free space.
   Sync by pressing (w) enter then reboot.

Credit and Reference: https://unix.stackexchange.com/questions/226872/how-to-shrink-root-filesystem-without-booting-a-livecd/227318#227318