Best way to resize root partition?

Marcih · Apprentice Joined: 19 Feb 2018 Posts: 213

I'm currently considering installing another operating system alongside my Gentoo install (trying out Hackintosh, to be specific) and my ext4 rootfs partition spans the entire disk:

alamahant · Advocate Joined: 23 Mar 2019 Posts: 3926

Hi,
You can use resize2fs
Boot to a livecd and run

Hu · Administrator Joined: 06 Mar 2007 Posts: 22856

Strictly speaking, you can resize the root partition live, including shrinking it. Shrinking it like this will almost certainly destroy the filesystem. What you want is to resize the root filesystem, then resize the partition containing that filesystem. During the time that the filesystem is completely unmounted, you can have the partition be smaller than the filesystem, but the filesystem is not usable during this time (and will likely fail to mount). The only reason to do it is if you make a mistake, which you can correct if you catch it in time. Concretely, I would do as alamahant suggests (with some expansion):

Reboot to a livecd. To shrink a filesystem, it must not be mounted. The easiest way to unmount the root filesystem is to be running a livecd. This is noteworthy because for some less critical filesystems, you may be able to stop enough services that the filesystem can be unmounted while the system is running.
e2fsck the filesystem, as alamahant said.
Shrink the filesystem to the target size.
Shrink the partition to the target size. If in doubt, you may wish to shrink the filesystem too far, then regrow it later. For example, shrink the filesystem to 9.8G, shrink the partition to 10G, then grow the filesystem to fill the tail. This is not necessary for correct operation, but it is cheap, and may be easier than worrying about whether the tools exactly agree on the interpretation of the sizes you give.
Be sure that the kernel reloads the partition table. It may refuse if the device is busy. If you are using a livecd, the device should not be busy.
e2fsck the filesystem again. This should be clean. If it is not, panic.
If you chose to excessively shrink the filesystem, run resize2fs here to grow it back to fill the available space.
Run e2fsck again, just to be very cautious. This also should be clean.
Mount your filesystem from the livecd, just to verify that it mounts correctly.
Unmount and reboot back into your system

If you were using LVM, all the same principles would apply, but you would use LVM tools instead of partition editing tools for the step where you resize the block device.

halcon · l33t Joined: 15 Dec 2019 Posts: 649

Hu, thanks for this tutorial! I tried it out in practice tonight. The root filesystem, shrinked from 500GB to 60 GB, boots fine; its number of files (reported by e2fsck) didn't change.