# Fenris

Fenris observes an NVMe drive’s real-world use and translates that history into an understandable endurance outlook.

## Language

**Observation history**:
The persisted record of drive activity gathered while Fenris monitoring is enabled, retained across restarts and reboots.
_Avoid_: Calibration data, temporary history

**Observed usage habit**:
The pattern of active, idle, and powered-off hours represented by the observation history, with recent sustained behavior carrying more relevance than distant behavior.
_Avoid_: Current usage, benchmark workload

**Usage-adjusted theoretical lifespan**:
The theoretical time until the drive’s write endurance is exhausted if its observed usage habit continues; it is an endurance projection, not a predicted hardware-failure date.
_Avoid_: Future life, actual lifespan, failure date

**Projection confidence**:
The degree to which the observation history is sufficiently long, complete, and stable to support the usage-adjusted theoretical lifespan.
_Avoid_: Accuracy percentage, certainty

**Monitoring period**:
A span during which Fenris monitoring is enabled; powered-off time remains part of the usage habit, while deliberately disabled time does not.
_Avoid_: Daemon uptime, calibration window

**Observation store**:
The single SQLite database at `/var/lib/fenris/observations.db` that persists the observation history, monitoring periods, hour observations, day aggregates, and endurance baseline.
_Avoid_: Data directory, history.jsonl, the database (generic)

**Hour observation**:
One row per UTC hour in the observation store, recording that hour's usage-habit split into active, idle, powered-off, and unknown seconds, plus write/read deltas, thermal evidence, and coverage.
_Avoid_: Hourly record, hourly.jsonl entry

**Day aggregate**:
One row per UTC day derived from hour observations; the grain at which usage-habit evidence is judged.
_Avoid_: Daily summary, daily stats

**Controller segment**:
A span of observation history within which the drive's controller identity is unchanged and counters are monotonic; write deltas are never computed across a segment boundary.
_Avoid_: Counter reset handling, drive swap detection

**Endurance baseline**:
The write-endurance value a projection consumes: a verified rated-TBW override stored with provenance when one exists, otherwise a coarse implied baseline derived from vendor wear and labeled as such.
_Avoid_: TBW value, failure threshold, max writes

**Sustained regime**:
The most recent stretch of the observation history over which the observed usage habit has been stable; the interval whose write rate the usage-adjusted theoretical lifespan consumes.
_Avoid_: Current window, detection period

**Habit change**:
A sustained divergence between recent and earlier daily write rates that starts a new sustained regime.
_Avoid_: Spike, anomaly

**Scenario range**:
The spread of lifespan projections computed from the 7-, 28-, and 90-day horizons of the observation history, shown in place of a statistical interval.
_Avoid_: Confidence interval, error bar

**Coverage**:
The share of wall-clock seconds inside monitoring periods whose usage-habit classification is known rather than unknown.
_Avoid_: Uptime, sample count