So I have decided to add a swap partition.
It worked amazingly (and very easily), there was even no need to reboot – at all.
I still did restart, just to make sure the system is bootable – and all was perfectly fine.

My initial setup is very simple: a single /dev/sda1 partition on the /dev/sda disk, fully used by btrfs.
Different important paths/mountpoints are btrfs subvolumes, using flat hierarchy.
For this example, let us assume that /dev/sda (and /dev/sda1) is 25GB large, and that I want to add a 2GB swap /dev/sda2 after /dev/sda1.

Brief explanation before we start:

1. shrink btrfs filesystem by more than 2GB;
2. shrink btrfs partition by 2GB;
3. create new 2GB partition for the swap;
4. resize btrfs filesystem to full size of its new-size partition;
5. initialize swap and turn it on.

Here are the very easy steps! Just make sure you do not make mistakes anywhere

1. If your btrfs volume with ID 5 (top level) is a separate mountpoint: mount it now, e.g. sudo mount /toplevel.
2. Take note of your current partition label and UUID: sudo blkid.
3. Resize btrfs filesystem down (shrink) with a good margin; for example, if I want to add a 2 GB swap, then I can sudo btrfs fi resize -3g /toplevel – here, I’m shrinking btrfs filesystem by about a gigabyte more than necessary. The process is very quick if you have free space, so you can even use a larger margin – say, sudo btrfs fi resize -5g /toplevel.
4. sudo parted, then print to make sure what is the number of your btrfs partition, then resizepart 1 (where 1 is the partition number), and answer a few questions: yes, new_size_here (in our example: 23.0GB), yes. You can also create a swap partition from parted, then quit parted with q and Enter.
5. sudo partprobe to let the OS know that partitions have changed.
6. I have used cfdisk to create a 2GB swap partition: it has a very simple ncurses UI, and is very intuitive. After creating swap partition, do run sudo partprobe again.
7. Resize btrfs filesystem back up to take all of the partition: sudo btrfs fi resize max /toplevel.
8. Simply to be sure, run a scrub: sudo btrfs scrub start -B -r /toplevel.
9. Initialize swap; you can specify uuid and/or label which you may already have in your fstab: mkswap --label=swap --uuid=your1234-your-uuid-1234-youruuid1234 /dev/sda2.
10. sudo blkid to make sure your /dev/sda1 UUID stayed the same (or to get swap uuid/label if you haven’t specified any).
11. Optionally, add the swap line to your /etc/fstab. Then turn on swap with swapon -a.

That’s it! Amazing, isn’t it? On-the-fly filesystem and partition resizing!

Step-by-step:

1. Identify current swap UUID: grep swap /etc/fstab ; you should see a line starting with UUID=b4e6e… – note that UUID; let us assume our UUID is b4e6e5d8-8854-4a3e-8edb-f423ded31e2a
2. Identify swap partition device. If you are using Debian or Ubuntu, device might have been shown to you when running command #1 as a comment of the form
   

   # /dev/sdX4 none swap sw 0 0

   Just to be sure, run cat /proc/swaps (or sudo swapon -s), to be shown something like this:

   Filename Type Size Used Priority
/dev/sdX4 partition 2104504 0 -1

   You can also use partition managers to identify swap device, e.g. sudo fdisk -l. Let us assume our partition device is sdX4.

3. If that swap partition is in use, you need to disable it first: sudo swapoff /dev/sdX4
4. Now that you have both the UUID and the device, run
   sudo mkswap -L new_swap_label -U b4e6e5d8-8854-4a3e-8edb-f423ded31e2a /dev/sdX4
   Do not forget replacing sample values above (label, UUID and device) with your actual values.
5. Enable swap: sudo swapon /dev/sdX4

Now, if you wish so, you may proceed to edit /etc/fstab and replace UUID=…. with LABEL=your_new_swap_label, leaving all other spaces and values in that line untouched.

1. Back up important data from partition just in case
2. Find out size of ‘$Bad’ attribute in $Badclus using ntfsinfo -i 8 partition (partition is for example /dev/sda1). It will be the “Allocated size” value in the “Dumping attribute $DATA (0x80)” (there will be two 0x80 attributes. Only one has an “Allocated size” line). Let us write down (remember) this size as ntfs_size.
3. Use ntfstruncate partition 8 0x80 ‘$Bad’ 0 to set $Bad’s attribute length to zero.
4. Use ntfstruncate partition 8 0x80 ‘$Bad’ ntfs_size to set $Bad’s attribute length back to proper value ntfs_size which was recorded in step 2.
5. Boot into Windows and run chkdsk -f diskname. It will find errors and should fix them.

However, Debian’s ntfsprogs package does not have the ntfstruncate binary.

Here’s how you can easily build one yourself (you may need a few extra packages with build tools for that):

1. apt-get source ntfsprogs
2. cd linux-ntfs-2.0.0
3. ./configure && make extra
4. cd ntfsprogs && ./ntfstruncate

You can also install ntfstruncate system-wide, but I have no idea if that will interfere with the already-installed ntfsprogs package, so I just used this binary directly from linux-ntfs-2.0.0/ntfsprogs.

Bad clusters happen not only when cloning an NTFS partition between 2 physical hard disk drives. Once, after a brief blackout (with a turned on PC, and not on a UPS), the SMART status of my new HDD started showing 1 ‘Pending sector’. Correspondingly, all the SMART tests (performed with a smartctl utility) reported read errors. To help my HDD examine and fix the problem itself, I performed a write operation to that single unreadable sector, which restored it (and reset the ‘Pending sector’ counter back to zero). It was possible, because not the physical sector itself, but the hidden CRC field had errors after power failure; modern hard drives can detect and fix this kind of errors upon write operations (read operations only mark the sector as ‘pending’, because data read is not consistent with the stored CRC).

However, NTFS partition still had 4096 bytes in ‘bad clusters’. A little research revealed the presence of internal NTFS bad clusters counter, which can be reset following the procedures above.