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feat: add bcache support (cache + backing device types)
Add three new disko types for Linux bcache block-layer caching: - bcache_cache: partition content type for cache devices (SSD/eMMC) - bcache_backing: partition content type for backing devices (HDD/SD) - bcache: top-level aggregator that creates the bcache set, attaches cache to backing, and manages nested content This enables declarative bcache configurations where a fast device (SSD, eMMC) caches a slower device (HDD, SD card), significantly improving I/O performance for the backing device.
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‎example/bcache.nix‎

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{
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disko.devices = {
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disk = {
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cache-disk = {
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type = "disk";
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device = "/dev/my-cache-disk";
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content = {
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type = "gpt";
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partitions = {
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boot = {
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size = "1M";
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type = "EF02"; # for grub MBR
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};
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cache = {
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size = "100%";
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content = {
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type = "bcache_cache";
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set = "main";
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};
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};
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};
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};
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};
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backing-disk = {
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type = "disk";
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device = "/dev/my-backing-disk";
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content = {
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type = "gpt";
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partitions = {
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backing = {
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size = "100%";
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content = {
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type = "bcache_backing";
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set = "main";
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};
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};
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};
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};
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};
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};
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bcache = {
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main = {
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type = "bcache";
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device = "/dev/bcache0";
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cacheMode = "writeback";
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content = {
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type = "filesystem";
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format = "ext4";
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mountpoint = "/";
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};
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};
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};
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};
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}

‎lib/default.nix‎

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@@ -79,6 +79,8 @@ let
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_partitionTypes = {
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inherit (diskoLib.types)
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bcachefs
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bcache_cache
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bcache_backing
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btrfs
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filesystem
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zfs
@@ -105,6 +107,8 @@ let
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_deviceTypes = {
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inherit (diskoLib.types)
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bcachefs
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bcache_cache
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bcache_backing
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table
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gpt
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btrfs
@@ -684,6 +688,7 @@ let
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let
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devices = {
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inherit (cfg.config)
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bcache
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bcachefs_filesystems
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disk
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mdadm
@@ -695,6 +700,11 @@ let
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in
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{
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options = {
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bcache = lib.mkOption {
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type = lib.types.attrsOf diskoLib.types.bcache;
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default = { };
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description = "bcache device (cache + backing)";
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};
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bcachefs_filesystems = lib.mkOption {
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type = lib.types.attrsOf diskoLib.types.bcachefs_filesystem;
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default = { };
@@ -769,6 +779,7 @@ let
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# @todo Do we need to add bcachefs-tools or not?
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destroyDependencies = with pkgs; [
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util-linux
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bcache-tools
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e2fsprogs
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mdadm
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zfs

‎lib/types/bcache.nix‎

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{
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config,
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options,
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lib,
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diskoLib,
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rootMountPoint,
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...
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}:
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{
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options = {
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name = lib.mkOption {
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type = lib.types.str;
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default = config._module.args.name;
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description = "Name of the bcache set.";
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example = "main";
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};
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type = lib.mkOption {
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type = lib.types.enum [ "bcache" ];
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internal = true;
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description = "Type";
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};
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device = lib.mkOption {
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type = lib.types.str;
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default = "/dev/bcache0";
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description = ''
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The bcache block device path.
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For single-backing setups this is always `/dev/bcache0`.
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For multi-backing setups, set this explicitly.
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'';
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};
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cacheMode = lib.mkOption {
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type = lib.types.enum [
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"writethrough"
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"writeback"
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"writearound"
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"none"
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];
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default = "writethrough";
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description = "Cache mode for the bcache device.";
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};
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extraCacheArgs = lib.mkOption {
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type = lib.types.listOf lib.types.str;
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default = [ ];
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description = "Extra arguments passed to `make-bcache -C` for the cache device.";
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};
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extraBackingArgs = lib.mkOption {
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type = lib.types.listOf lib.types.str;
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default = [ ];
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description = "Extra arguments passed to `make-bcache -B` for the backing device.";
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};
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content = diskoLib.deviceType {
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parent = config;
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device = config.device;
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};
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_meta = lib.mkOption {
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internal = true;
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readOnly = true;
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type = diskoLib.jsonType;
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default = lib.optionalAttrs (config.content != null) (
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config.content._meta [
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"bcache"
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config.name
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]
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);
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description = "Metadata";
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};
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_create = diskoLib.mkCreateOption {
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inherit config options;
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default =
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let
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devBasename = builtins.baseNameOf config.device;
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in
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''
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# Read cache and backing device paths from temp directory
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if ! test -s "$disko_devices_dir/bcache_cache_${lib.escapeShellArg config.name}"; then
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printf "\033[31mERROR:\033[0m No cache device found for bcache set \"${config.name}\"!\n" >&2
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exit 1
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fi
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if ! test -s "$disko_devices_dir/bcache_backing_${lib.escapeShellArg config.name}"; then
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printf "\033[31mERROR:\033[0m No backing device found for bcache set \"${config.name}\"!\n" >&2
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exit 1
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fi
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cache_dev=$(head -n1 "$disko_devices_dir/bcache_cache_${lib.escapeShellArg config.name}")
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backing_dev=$(head -n1 "$disko_devices_dir/bcache_backing_${lib.escapeShellArg config.name}")
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modprobe bcache
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# Resolve symlink to kernel device name for sysfs matching
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backing_basename="$(basename "$(readlink -f "$backing_dev")")"
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# Idempotency: if the bcache device already exists with our backing device, skip creation
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if [ -b "${config.device}" ] && \
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[ -e "/sys/block/${devBasename}/bcache/backing_dev_name" ] && \
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[ "$(cat "/sys/block/${devBasename}/bcache/backing_dev_name")" = "$backing_basename" ]; then
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: # bcache device already active with correct backing — nothing to do
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else
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# Teardown stale bcache device if it exists (destroy-format-mount scenario)
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if [ -e "/sys/block/${devBasename}/bcache/stop" ]; then
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echo 1 > "/sys/block/${devBasename}/bcache/stop" 2>/dev/null || true
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fi
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# Unregister any active cache sets
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find /sys/fs/bcache -maxdepth 1 -mindepth 1 -type d -exec sh -c '
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for cset; do
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if [ -e "$cset/unregister" ]; then
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echo 1 > "$cset/unregister" 2>/dev/null || true
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fi
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done
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' _ {} + 2>/dev/null || true
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udevadm settle --timeout=10
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# Wait for the bcache device to fully disappear
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for i in $(seq 1 30); do
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[ -b "${config.device}" ] || break
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sleep 1
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done
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# Create bcache set — --force handles pre-existing superblocks
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make-bcache \
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-C "$cache_dev" \
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-B "$backing_dev" \
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--force \
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${lib.optionalString (config.cacheMode == "writeback") "--writeback"} \
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${lib.concatStringsSep " " (map lib.escapeShellArg config.extraCacheArgs)} \
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${lib.concatStringsSep " " (map lib.escapeShellArg config.extraBackingArgs)}
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# Wait for the bcache block device to appear
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for i in $(seq 1 60); do
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[ -b "${config.device}" ] && break
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sleep 1
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done
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fi
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if [ ! -b "${config.device}" ]; then
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printf "\033[31mERROR:\033[0m bcache device ${config.device} did not appear\n" >&2
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exit 1
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fi
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138+
# Set cache mode
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echo "${config.cacheMode}" > "/sys/block/${devBasename}/bcache/cache_mode"
140+
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${lib.optionalString (config.content != null) config.content._create}
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'';
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};
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_mount = diskoLib.mkMountOption {
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inherit config options;
146+
default = lib.optionalAttrs (config.content != null) config.content._mount;
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};
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_unmount = diskoLib.mkUnmountOption {
149+
inherit config options;
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default =
151+
let
152+
devBasename = builtins.baseNameOf config.device;
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content = lib.optionalAttrs (config.content != null) config.content._unmount;
154+
in
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{
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fs = content.fs or { };
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dev = ''
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${content.dev or ""}
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# Stop the bcache device to release cache and backing devices
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if [ -e "/sys/block/${devBasename}/bcache/detach" ]; then
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echo 1 > "/sys/block/${devBasename}/bcache/detach" 2>/dev/null || true
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fi
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if [ -e "/sys/block/${devBasename}/bcache/stop" ]; then
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echo 1 > "/sys/block/${devBasename}/bcache/stop" 2>/dev/null || true
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fi
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# Unregister cache sets
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find /sys/fs/bcache -maxdepth 1 -mindepth 1 -type d -exec sh -c '
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for cset; do
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if [ -e "$cset/unregister" ]; then
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echo 1 > "$cset/unregister" 2>/dev/null || true
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fi
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done
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' _ {} + 2>/dev/null || true
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udevadm settle --timeout=10
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'';
176+
};
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};
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_config = lib.mkOption {
179+
internal = true;
180+
readOnly = true;
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default = [
182+
{
183+
boot.bcache.enable = true;
184+
}
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]
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++ lib.optional (config.content != null) config.content._config;
187+
description = "NixOS configuration";
188+
};
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_pkgs = lib.mkOption {
190+
internal = true;
191+
readOnly = true;
192+
type = lib.types.functionTo (lib.types.listOf lib.types.package);
193+
default =
194+
pkgs:
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[
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pkgs.bcache-tools
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pkgs.kmod
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pkgs.util-linux
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]
200+
++ lib.optionals (config.content != null) (config.content._pkgs pkgs);
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description = "Packages";
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};
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};
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}

‎lib/types/bcache_backing.nix‎

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{
2+
config,
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options,
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lib,
5+
diskoLib,
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parent,
7+
device,
8+
...
9+
}:
10+
{
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options = {
12+
type = lib.mkOption {
13+
type = lib.types.enum [ "bcache_backing" ];
14+
internal = true;
15+
description = "Type";
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};
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device = lib.mkOption {
18+
type = lib.types.str;
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default = device;
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description = "Device to use as bcache backing device.";
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};
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set = lib.mkOption {
23+
type = lib.types.str;
24+
description = "Name of the bcache set this backing device belongs to.";
25+
example = "main";
26+
};
27+
_parent = lib.mkOption {
28+
internal = true;
29+
default = parent;
30+
};
31+
_meta = lib.mkOption {
32+
internal = true;
33+
readOnly = true;
34+
type = lib.types.functionTo diskoLib.jsonType;
35+
default = dev: {
36+
deviceDependencies.bcache.${config.set} = [ dev ];
37+
};
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description = "Metadata";
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};
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_create = diskoLib.mkCreateOption {
41+
inherit config options;
42+
default = ''
43+
echo "${config.device}" >> "$disko_devices_dir/bcache_backing_${lib.escapeShellArg config.set}"
44+
'';
45+
};
46+
_mount = diskoLib.mkMountOption {
47+
inherit config options;
48+
default = { };
49+
};
50+
_unmount = diskoLib.mkUnmountOption {
51+
inherit config options;
52+
default = { };
53+
};
54+
_config = lib.mkOption {
55+
internal = true;
56+
readOnly = true;
57+
default = [ ];
58+
description = "NixOS configuration";
59+
};
60+
_pkgs = lib.mkOption {
61+
internal = true;
62+
readOnly = true;
63+
type = lib.types.functionTo (lib.types.listOf lib.types.package);
64+
default = pkgs: [ pkgs.bcache-tools ];
65+
description = "Packages";
66+
};
67+
};
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}

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