Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Decision: HDD defaults

Principle: HDD defaults

Context

braid manages a NAS pool of LUKS-encrypted btrfs RAID1 drives. The typical deployment is bulk storage on large-capacity spinning drives (e.g., 12–16 TB HDDs). Several defaults already assume rotational media:

  • cryptsetup open omits --allow-discards, so TRIM/discard requests from btrfs never reach the underlying device. btrfs also exposes a mount-layer discard knob (discard=async, the kernel default since 6.2 on devices that advertise discard support), but braid’s LUKS layer gates it: without --allow-discards, the mapped device never reports discard support upward, so the kernel default never activates and any explicit discard=async would be silently dropped.
  • noatime mount rationale references HDD spindown prevention.
  • Monthly scrub interval is tuned for spinning disk wear and noise.

Making braid flash-aware would mean adding --allow-discards (with its security tradeoff of leaking block-usage patterns through the encryption layer), flash-specific scrub/balance scheduling, and flash-targeted test coverage. None of this is warranted for the target use case.

Note: braid already handles flash media in its monitoring paths — NVMe SMART parsing (cli/src/parse/smartctl.rs) and transport-type detection (cli/src/tui/probe.rs) work with any drive type. This decision is about operational defaults, not monitoring.

Decision

Defaults are chosen for HDD NAS deployments. Flash media (SSDs, NVMe, USB sticks) may function but are not a validated or optimized target.

Tradeoffs accepted

  • No TRIM passthrough — braid pins discard off at the LUKS layer by omitting --allow-discards and, by consequence, at the btrfs mount layer because no effective discard=async can pass through regardless of kernel default. SSDs used with braid experience increased write amplification and performance degradation over time.
  • No flash-specific testing — flash-related issues in LUKS or mount configuration may go unnoticed.

Alternatives considered

Default-on btrfs compression (compress=zstd:1)

Rejected. braid targets HDD bulk-storage NAS pools where the dominant content is media and archives: video, audio, photos, and other formats already compressed at the application layer. Transparent filesystem compression usually saves little or no space on that mix, while the btrfs heuristic that skips incompressible extents still spends CPU on each write. On low-power NAS hardware, that cost is not free.

Reversal is also partial. Removing compress=... affects future writes only; extents already written compressed stay compressed until the data is rewritten or explicitly defragmented. Making compression the default would bake that conversion cost into pre-v1.0 software for users who later discover their workload does not benefit. For that reason, cli/src/cmd.rs base_mount_options() intentionally omits compression.

Fedora’s compress=zstd:1 precedent is workstation root filesystems on SSDs: binaries, logs, configs, and package payloads. That precedent does not transfer cleanly to HDD bulk-storage NAS workloads. Users with compression-friendly data, such as text, code, or document servers, can opt specific paths into compression today with btrfs property set <path> compression zstd; reference/btrfs-progs/Documentation/btrfs-property.rst documents this modern per-inode interface. This is preferable to legacy chattr +c, which uses ext2-style flags and defaults to zlib. No braid feature gate is needed for this per-path opt-in.

See

  • cli/src/cmd.rs — CryptsetupLuksOpen and CryptsetupLuksOpenKeyFile omit --allow-discards
  • cli/src/cmd.rs — base_mount_options() omits any discard option, relying on the kernel default that is itself gated by the LUKS layer
  • cli/src/cmd.rs — base_mount_options() sets noatime; rationale references HDD spindown
  • Sane defaults — scrub interval tuned for spinning disks