First you need to increase the size of the disk being presented to the Linux OS. This is most likely done by expanding the virtual disk in KVM/VMWare/Hyper-V or by adjusting your RAID controller / storage system to increase the volume size. You can often do this while Linux is running; without shutting down or restarting. I’ve extended my 100GB disk to 200GB for my example machine.
Once that is done, you may need to get Linux to rescan the disk for the new free space. Check for free space by running cfdisk and see if there is free space listed, use “q” to exit once you’re done.
Linux increase disk size space cfdisk.
If you don’t see free space listed, then initiate a rescan of /dev/sda with echo 1>/sys/class/block/sda/device/rescan. Once done, rerun cfdisk and you should see the free space listed.