Write up about butter transition

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+rss = "Reinstalling and backing up NixOS on Btrfs+tmpfs"
+date = Date(2021, 11, 14)
++++
+@def tags = ["fun", "nixos", "btrfs", "tmpfs", "backup", "luks"]
+
+# Reinstalling and backing up NixOS on Btrfs+tmpfs
+
+In 2018, dad bought me a new laptop to replace the good ole Compaq nx7010
+whose screen unfortunately got infected by some sort of microbe and dieded
+shortly afterwards.  The new one, whilst having a considerably worse build
+quality (like all other late-2010s ones when compared to mid-2000s models),
+had 12.5 times as much storage: a 250 GB M.2 SSD and a 500 GB SATA HDD.
+
+My data hoarding habit has grown exponentially ever since.  Initially,
+I used to back up the data from the SSD to the HDD but after a few years,
+I ran out of space and decided to get some more storage.  Instead of buying
+a portable hard disk like a normal person would, I went for an SATA SSD,
+as it was rather difficult to find a 7200 rpm 2.5-inch[^metric] HDD
+in the market at the time.
+
+I then asked my father for a spare SATA-to-USB case (he switched to using
+a dock a while ago, and like other dads, nothing is ever thrown away)
+and prepared to swap the drives.  As cloning the data would have been too easy,
+I decided to *spice things up* by reinstalling the OS.  Back then I was
+dual-booting Debian and NixOS, but the former had hardly been ever booted
+for months so it was time to let it go:
+
+![Elsa rolling on the floor crying](/assets/let-it-go.png)
+
+In addition, I wanted to jump on the new and shinny[^new] train of Btrfs.
+It has compression, snapshots and subvolumes, what's not to love?  Let's
+replace something I'd been using for nearly a decade with a file system
+I had absolutely zero experience with, what could possibly go wrong, right?
+
+\toc
+
+## Reinstallation
+
+I was going to reinstall NixOS with an ephemeral root,
+which had been covered to death in the following brilliant resources:
+
+* [Erase your darlings: immutable infrastructure for mutable systems][grahamc]
+* [NixOS ❄: tmpfs as root][elis]
+* [Nix community's impermanence modules][impermanence]
+* [Paranoid NixOS Setup][christine]
+
+The only twist here is that I was using Btrfs instead of ZFS or ext4 like in
+other guides.  This choice would influence how to back up in the later section.
+
+### Preparation
+
+First of all, I temporarily copied data to the SATA SSD from the M.2,
+including [my Nix configurations].  Using either `cp` or `rsync` didn't
+seem to have any effect on the performance, and in the mean time I also
+went ahead and grabbed a [NixOS unstable live image] and `dd`'ed it to
+a flash drive.  As I'm tracking unstable, installing from the same version
+would allowed me to skip switching the channel and a lot of downloading.
+
+### Partitioning
+
+After booting up the live image, I opened up a root shell with `sudo -i`.
+As expected, `fdisk` reports the M.2 SSD as `/dev/nvme0n1`.  Paranoid as always,
+I decided to give the EFI system partition a whole gibibyte, swap eight
+to match memory[^memory] and the rest as a single chonky Btrfs partition:
+
+```sh
+parted /dev/nvme0n1 -- mklabel gpt
+parted /dev/nvme0n1 -- mkpart ESP fat32 1MiB 1GiB
+parted /dev/nvme0n1 -- set 1 boot on
+
+parted /dev/nvme0n1 -- mkpart Swap linux-swap 1GiB 9GiB
+mkswap -L Swap /dev/nvme0n1p2
+swapon /dev/nvme0n1p2
+
+parted /dev/nvme0n1 -- mkpart primary 9GiB 100%
+mkfs.btrfs -L Butter /dev/nvme0n1p3
+```
+
+As I typed this, I realized that I should have set up encryption
+for the last partition so I would probably need to reinstall in the near future
+to fix this mistake.  Anyway, with the target system's root mounted as tmpfs,
+I would need to persist `/nix` (obviously), `/etc` (mostly for authentication
+and other secret stuff not included in `configuration.nix` that I was too lazy
+to opt in individually), `/var/log`, `/root` and `/home`:
+
+```sh
+mount /dev/nvme0n1p3 /mnt
+btrfs subvolume create /mnt/nix
+btrfs subvolume create /mnt/etc
+btrfs subvolume create /mnt/log
+btrfs subvolume create /mnt/root
+btrfs subvolume create /mnt/home
+umount /mnt
+```
+
+Most subvolumes can be mounted with `noatime`, except for `/home`
+where I frequently need to sort files by modification time.  All of them
+should have forced compression though:
+
+```sh
+mount -t tmpfs -o mode=755 none /mnt
+mkdir -p /mnt/{boot,nix,etc,var/log,root,home}
+mount /dev/nvme0n1p1 /mnt/boot
+mount -o subvol=nix,compress-force=zstd,noatime /dev/nvme0n1p3 /mnt/nix
+mount -o subvol=etc,compress-force=zstd,noatime /dev/nvme0n1p3 /mnt/etc
+mount -o subvol=log,compress-force=zstd,noatime /dev/nvme0n1p3 /mnt/var/log
+mount -o subvol=root,compress-force=zstd,noatime /dev/nvme0n1p3 /mnt/root
+mount -o subvol=home,compress-force=zstd /dev/nvme0n1p3 /mnt/home
+```
+
+### Configuration
+
+With everything mounted, `nixos-generate-config --root /mnt` could be run
+to generate a basic configuration.  But wait, didn't I say something
+about my dot files?  That's correct, but it's not easy to handcraft
+the `hardware-configuration.nix`.  After making sure all are mounted
+with the right options and `services.fstrim.enable` is `true`, I copied
+other configuration files to `/etc/nixos` and finished this step.
+
+### Installation
+
+NixOS installation is as simple as running `nixos-install`.  But my job was
+not done after setting the root password and rebooting into the new system.
+It was working, but not functional.  There was nothing meaningful for me
+to do on it, so I had to log in (as root), `passwd`'ed the user and copied
+the home folder back from the temporary drive.
+
+After freeing the new SATA SSD, I also filled it with butter.  Yes,
+all the way, no GPT, no MBR, just Btrfs, whose subvolumes were used
+in place of partitions:
+
+```sh
+mkfs.btrfs -f -L Fly /dev/sdb
+mkdir -p /mnt
+mount /dev/sdb /mnt
+btrfs subvolume create /mnt/movies
+```
+
+At that time the only disposable data I had were my movies collection.
+The HDD also contained other data but they were rebalanced at `/home`
+(on the M.2).  After swapping the SATA SSD inside the laptop, I logged in
+as the normal user and get the exact same environment before the reinstallation.
+
+### Profits
+
+Thanks to subvolumes and compression, the free spaces were no longer fragmented
+and I think I gained like 100 GB (not counting the old Debian's root).  Backup
+would also be less painful with Btrfs snapshots (instead of plain `rsync`
+like I used to) as shown as follows.
+
+## Backup
+
+With all data migrated, the HDD could be used for backing up.  First,
+some legacy data I no longer access were moved there, then I started to back up
+my `/home` partition:
+
+### Initialization
+
+Having learned my lesson, I did not forget to set up [LUKS] this time:
+
+```sh
+cryptsetup luksFormat /dev/sdb
+cryptsetup luksOpen /dev/sdb backup
+```
+
+To make use of snapshots, the backup drive gotta be Btrfs as well.
+The compression level was turned up to 14 this time (default was 3):
+
+```sh
+mkfs.btrfs -L Backup /dev/mapper/backup
+mkdir /backup
+mount -o noatime,compress-force=zstd:14 /dev/mapper/backup /backup
+```
+
+Following [Btrfs Wiki], I made the first `/home` snapshot and sent it
+to the backup drive:
+
+```sh
+btrfs subvolume create /backup/home
+today=$(date --iso-8601)
+btrfs subvolume snapshot -r /home /home/$today
+sync
+btrfs send /home/$today | btrfs receive /backup/home
+```
+
+### Repetition
+
+For next backups, I also mounted the drive and created a snapshot:
+
+```sh
+cryptsetup luksOpen /dev/sdb backup
+mount -o noatime,compress-force=zstd:14 /dev/mapper/backup /backup
+today=$(date --iso-8601)
+btrfs subvolume snapshot -r /home /home/$today
+sync
+```
+
+Say the latest snapshot was on the `$previous` day, I only needed to send
+the difference between the old and new backup.  Afterwards, it is safe
+to delete the local `$previous` snapshot to save some space.
+
+```sh
+btrfs send -p /home/$previous /home/$today | btrfs receive /backup/home
+btrfs subvolume delete /home/$previous
+```
+
+Is this more complicated than good ole `rsync`?  Yes.  Is it safer?  Also yes,
+thanks to copy-on-write.  Would I bother using one of the tools suggested
+in the wiki?  Probably not, I've already documented everything in this article
+in case I forget anything.
+
+[^metric]: 63.5 mm for those outside of the land of guns and burgers
+[^new]: OK, maybe not new, but certainly shinny
+[^memory]: Slightly larger since some of the memory is dedicated to graphics
+
+[grahamc]: https://grahamc.com/blog/erase-your-darlings
+[elis]: https://elis.nu/blog/2020/05/nixos-tmpfs-as-root
+[impermanence]: https://github.com/nix-community/impermanence
+[christine]: https://christine.website/blog/paranoid-nixos-2021-07-18
+[my Nix configurations]: https://git.sr.ht/~cnx/dotfiles/tree/master/item/nix
+[NixOS unstable live image]: https://channels.nixos.org/nixos-unstable
+[LUKS]: https://gitlab.com/cryptsetup/cryptsetup
+[Btrfs Wiki]: https://btrfs.wiki.kernel.org/index.php/Incremental_Backup