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ADR-0015: NVMe-Only Storage Policy

Status: Accepted • Date: 2026-02-18

1. Context & Problem

Silvasonic is designed to run autonomously for years on unattended edge devices (Raspberry Pi 5). The storage medium directly impacts data integrity, write endurance, and throughput — all of which are critical for continuous audio recording.

SD Cards, while convenient for initial provisioning, are fundamentally unsuitable for production workloads:

  • Low write endurance: Consumer SD cards degrade rapidly under sustained write loads (audio buffers, database WAL, swap).
  • Throughput limits: SD card I/O is insufficient for concurrent dual-stream recording (Raw + Processed) and database operations.
  • Silent corruption: SD cards are prone to filesystem corruption after unexpected power loss — the single most common failure mode for field stations.

2. Decision

We chose: A strict "NVMe Only" storage policy for all production deployments.

Reasoning:

  1. NVMe is mandatory. All production Silvasonic stations MUST use an M.2 NVMe SSD (Gen 2 or Gen 3) connected via a PCIe HAT on the Raspberry Pi 5.
  2. SD Cards are prohibited in production. An SD card may be used temporarily during initial provisioning (bootloader update, OS flashing onto NVMe). Once provisioned, the SD card MUST be physically removed. The system boots and runs entirely from the NVMe SSD.
  3. Unified storage namespace. The OS, swap partition, database (TimescaleDB), and audio recording buffers all reside on the NVMe drive.
  4. USB-attached storage (legacy). External SSDs via USB 3.0 are tolerated for experimental Raspberry Pi 4 setups, but are not officially supported for production due to bus contention and lower reliability compared to PCIe NVMe.

3. Options Considered

  • SD Card as primary storage: Rejected. Fundamentally incompatible with the write endurance and throughput requirements of continuous bioacoustic recording. Silent corruption risk is unacceptable for a system designed to run autonomously for years.
  • SD Card for OS + NVMe for data: Rejected. Split-storage introduces complexity (mount management, boot reliability) and the SD card remains a single point of failure for the OS partition.
  • NVMe only (chosen): Single high-speed, high-endurance storage device for everything. Simple, reliable, and aligned with the "Resilience over Features" design principle.

4. Consequences

  • Positive:
    • Maximum write endurance — NVMe SSDs are rated for thousands of TBW (Terabytes Written), orders of magnitude above SD cards.
    • High throughput for concurrent dual-stream recording and database operations.
    • No split-storage complexity — single filesystem, single backup target, single failure domain.
    • Eliminates the most common field failure mode (SD card corruption after power loss).
  • Negative:
    • Higher per-unit cost compared to SD-card-only setups.
    • Requires a PCIe HAT (additional hardware component).
    • Initial provisioning requires a temporary SD card or USB boot media to flash the NVMe.