I've seen, a number of times, users asking about how to resize a FFS
partition on NetBSD, and getting no reply in return. 

The gpt(8) utility, which I'm notoriously fan of, will easily allow to
resize disks and partition, but what about resizing the actual
filesystem, in order to match the new disk layout?

Other BSDs and Solaris provide a `growfs' command to address such need,
as far as the UFS/FFS filesystem is concerned.

NetBSD instead comes with resize_ffs(8):
<https://man.netbsd.org/resize_ffs.8>

So, let's see how to resize a FFS partition on NetBSD (GPT disk). 

■ Shrinking
I have a  64G USB flash device with a GPT parition layout.
The disk contains a 56G CGD-encrypted virtual device, which in turn
hosts a single FFS partition.

$ gpt show sd1
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34       4062         Unused
       4096  120954880      1  GPT part - NetBSD Cryptographic Disk
  120958976       4063         Unused
  120963039         32         Sec GPT table
  120963071          1         Sec GPT header

1. Resize partition '1' - which here is the CGD disk - to half of its
size, in this case 28G, using a 4M alignment:

$ gpt resize -i 1 -a 4m -s 28g sd1
/dev/rsd1: Partition 1 resized: 4096 58728448

$ gpt show sd1
      start       size  index  contents
          0          1         PMBR
          1          1         Pri GPT header
          2         32         Pri GPT table
         34       4062         Unused
       4096   58728448      1  GPT part - NetBSD Cryptographic Disk
   58732544   62230495         Unused
  120963039         32         Sec GPT table
  120963071          1         Sec GPT header

2. Flush all configured wedges on sd1 and force autodiscoverign them
over: 

$ dkctl sd1 makewedges
successfully scanned /dev/rsd1.

$ dmesg | grep sd1 | cut -d']' -f2
dk3 at sd1: "flash64", 120954880 blocks at 4096, type: cgd

( 2.1 configure the CGD disk ) 

$ cgdconfig cgd1 /dev/dk3
$ cgdconfig -l
cgd0: dk2 
cgd1: dk3 
cgd2: not in use
cgd3: not in use

3. Check the size in sectors of the partition to shrink, using `disklabel': 

$ disklabel -iI cgd1
Enter '?' for help
partition>P
4 partitions:
*        size    offset     fstype [fsize bsize cpg/sgs]
 a: 120954880         0     4.2BSD      0     0     0  # (Cyl.      0 -  59059)
 d: 120954880         0     unused      0     0        # (Cyl.      0 -  59059)
partition>Q

4. Shrink the partition to half off its size in sectors:
('p' prints a progress bar)

$ resize_ffs -s 60477440 -pv /dev/cgd1a
It's required to manually run fsck on file system before you can resize it

 Did you run fsck on your disk (Yes/No) ? Yes
Shrinking fs from 30238720 blocks to 15119360 blocks

5. mount the partition to verify the news size:

$ mount /dev/cgd1a  /mnt
$ df- hg
/dev/cgd1a           28          0         26   0% /mnt

■ Growing
Now, let's bring back the partition to its original size.

1. Move the partition boundaries to their former state:

$ gpt resize -i1 -a 4m -s 56g sd1
/dev/rsd1: Partition 1 resized: 4096 117448704

2. Grow the fileystem inside the enlarged partition.

$ resize_ffs -pv /dev/cgd1a
Growing fs from 15119360 blocks to 30238720 blocks.

3. check the new size:

$ mount /dev/cgd1a  /mnt
$ df- hg
/dev/cgd1a           56          0         54   0% /mnt

■ Expanding disks 
If you want to grow a partition in a virtual disk beyond the original
disk's size you'll also need:

$ gpt resizedisk -a 4m -s *g <dev>

Which shall move the GPT backup copy to the new end of the disk, update
all size fields for the new media size, and adjust the pointer to the
secondary GPT header.

If you accidentally enlarge a partion past the disk's end, you can
recover the secondary GPT header with: 

$ gpt recover <dev>
gpt: <dev>: recovered secondary GPT table from primary