Discard unused blocks on a mounted filesystems. Useful for reducing the image size on a VM. Must be supported by the device. Check:

$ lsblk --discard/-D
NAME   DISC-ALN DISC-GRAN DISC-MAX DISC-ZERO
vda           0      512B      64G         0
├─vda1        0      512B      64G         0
├─vda2        0      512B      64G         0
└─vda3        0      512B      64G         0

to check discarding capabilities of each block device.

DISC-ALN: Disk alignment DISC-GRAN: Disk granularity, smallest chunk which can be discarded DISC-MAX: Largest amount which can be discarded in a single request

If you're using packer with qemu/kvm, use newer devices q35 over pc (outdated intel), which might not support discard.

fstrim /path/to/mount