$ sudo fdisk /dev/sdb

Welcome to fdisk (util-linux 2.31.1).
Changes will remain in memory only, until you decide to write them.
Be careful before using the write command.


Command (m for help): p
Disk /dev/sdb: 14.7 GiB, 15811477504 bytes, 30881792 sectors
Units: sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disklabel type: dos
Disk identifier: 0x15ca46a5

Device     Boot Start      End  Sectors  Size Id Type
/dev/sdb1        8192    93802    85611 41.8M  c W95 FAT32 (LBA)
/dev/sdb2       98304 12582911 12484608    6G 83 Linux

Command (m for help): d
Partition number (1,2, default 2): 2

Partition 2 has been deleted.

Command (m for help): n
Partition type
   p   primary (1 primary, 0 extended, 3 free)
   e   extended (container for logical partitions)
Select (default p): p
Partition number (2-4, default 2): 2
First sector (2048-30881791, default 2048): 98304
Last sector, +sectors or +size{K,M,G,T,P} (98304-30881791, default 30881791): 

Created a new partition 2 of type 'Linux' and of size 14.7 GiB.
Partition #2 contains a f2fs signature.

Do you want to remove the signature? [Y]es/[N]o: y

The signature will be removed by a write command.

Command (m for help): w
The partition table has been altered.
Syncing disks.

$ sudo resize.f2fs /dev/sdb2
Info: [/dev/sdb2] Disk Model: STORAGE DEVICE  0828GENE
Info: Segments per section = 1
Info: Sections per zone = 1
Info: sector size = 512
Info: total sectors = 30783488 (15031 MB)
	Can't find a valid F2FS superblock at 0x0
Info: MKFS version
  "Linux version 4.4.0-117-generic (buildd@lgw01-amd64-029) (gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9) ) #141-Ubuntu SMP Tue Mar 13 11:58:07 UTC 2018"
Info: FSCK version
  from "Linux version 4.4.0-117-generic (buildd@lgw01-amd64-029) (gcc version 5.4.0 20160609 (Ubuntu 5.4.0-6ubuntu1~16.04.9) ) #141-Ubuntu SMP Tue Mar 13 11:58:07 UTC 2018"
    to "Linux version 4.14.0-3-amd64 (debian-kernel@lists.debian.org) (gcc version 7.3.0 (Debian 7.3.0-3)) #1 SMP Debian 4.14.17-1 (2018-02-14)"
Info: superblock features = 0 : 
Info: superblock encrypt level = 0, salt = 00000000000000000000000000000000
Info: total FS sectors = 12484608 (6096 MB)
Info: CKPT version = d
Info: Write valid nat_bits in checkpoint
[migrate_block:  58] Migrate data block 3400 -> 8ebec
[migrate_block:  58] Migrate data block 3401 -> 9740b
[migrate_block:  58] Migrate data block 3402 -> fb6b5
.
.
.
[migrate_block:  58] Migrate data block 6bfd -> ff0b0
[migrate_block:  58] Migrate data block 6bfe -> ff0b1
[migrate_block:  58] Migrate data block 6bff -> ff0b2
Info: Write valid nat_bits in checkpoint
Try to do defragement: Done
[migrate_ssa: 240] Info: Done to migrate SSA blocks: sum_blkaddr = 0x2600 -> 0x4e00
[migrate_nat: 353] Info: Done to migrate NAT blocks: nat_blkaddr = 0xa00 -> 0xa00
[migrate_sit: 411] Info: Done to restore new SIT blocks: 0x600
Info: Write valid nat_bits in checkpoint
[rebuild_checkpoint: 541] Info: Done to rebuild checkpoint blocks
[rebuild_superblock: 557] Info: Done to rebuild superblock

Done.