The ZFS tab shows the ARC, and names every caching tier a pool has or lacks

A Cache section between the pools and the schedules. One card for the ARC:
size against its ceiling with the target and floor, sparklines of size and
hit rate from the sampler's history, what it holds (MRU/MFU/dirty, data vs
metadata, compression, the ghost lists), the prefetcher and dbuf cache, the
memory-pressure counters, and the since-boot breakdown by demand/prefetch
and data/metadata. Then a card per pool for L2ARC, SLOG, special vdev,
dedup table and spares — the devices and their fill where they exist, and
where they don't, plainly none and what that means (the ZIL writing to the
pool's own vdevs, with how much it has written).

Live numbers come from the sampler, which reads arcstats every tick and
publishes zfs.arc.* sensors; the fuller picture is read on request, with
zpool list -Hpv and zfs get run without sudo and cached for thirty seconds.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

internal/metrics/metrics.go

@@ -147,6 +147,7 @@ type Sample struct {
 	FS       []FS       `json:"fs"`
 	UPS      *UPS       `json:"ups"`
 	GPUs     []GPU      `json:"gpus"`
+	ARC      *ARC       `json:"arc"` // the ZFS read cache; nil without the module
 	Uptime   float64    `json:"uptime"`
 
 	fsT, upsT int64 // when the slow sections were really read; sensors report these, not T
@@ -176,6 +177,7 @@ type Collector struct {
 	lastTable *ProcTable
 	upsProbe  func() *UPS // nil when no UPS daemon is installed
 	gpu       gpuState
+	prevARC   arcPrev
 
 	subMu sync.Mutex
 	subs  map[chan struct{}]struct{} // told after every sample; see Subscribe
@@ -360,6 +362,7 @@ func (c *Collector) sample() {
 	}
 	s.UPS, s.upsT = c.lastUPS, c.upsAt.UnixMilli()
 	s.GPUs = c.gpus(now, cpuTemp(s.Temps))
+	s.ARC = c.arc(now)
 
 	if first {
 		return // deltas are meaningless on the priming pass