feat: add ZFS monitoring (#2209)

- track pool capacity, health, I/O, scrub status, and vdev errors
- report dataset usage and correct ZFS filesystem metrics
- add pool charts, detail views, refresh controls, and health alerts
- persist pool details and include ZFS usage in disk alerts
- support configurable detail intervals and legacy agent compatibility

---------

Co-authored-by: hank <hank@henrygd.me>

agent/agent.go

@@ -48,6 +48,7 @@ type Agent struct {
 	keys                      []gossh.PublicKey                                     // SSH public keys
 	smartManager              *SmartManager                                         // Manages SMART data
 	systemdManager            *systemdManager                                       // Manages systemd services
+	zfsManager                *ZfsManager                                           // Manages ZFS pool and dataset data
 }
 
 // NewAgent creates a new agent with the given data directory for persisting data.
@@ -121,6 +122,19 @@ func NewAgent(dataDir ...string) (agent *Agent, err error) {
 	// initialize handler registry
 	agent.handlerRegistry = NewHandlerRegistry()
 
+	agent.zfsManager = newZfsManager()
+
+	// ZFS_INTERVAL env var to update ZFS detail data at this interval
+	if zfsIntervalEnv, exists := utils.GetEnv("ZFS_INTERVAL"); exists {
+		if duration, err := time.ParseDuration(zfsIntervalEnv); err == nil && duration > 0 {
+			agent.zfsManager.detailInterval = duration
+			agent.systemDetails.ZfsInterval = duration
+			slog.Info("ZFS_INTERVAL", "duration", duration)
+		} else {
+			slog.Warn("Invalid ZFS_INTERVAL", "err", err)
+		}
+	}
+
 	// initialize disk info
 	agent.initializeDiskInfo()
 

agent/disk.go

@@ -537,7 +537,16 @@ func normalizeDeviceName(value string) string {
 func (a *Agent) initializeDiskIoStats(diskIoCounters map[string]disk.IOCountersStat) {
 	a.fsNames = a.fsNames[:0]
 	now := time.Now()
+	// ZFS datasets have no /proc/diskstats entry, so they are excluded from
+	// I/O tracking instead of warning about a missing device (#1541).
+	var zfsMountpoints map[string]bool
+	if a.zfsManager != nil {
+		zfsMountpoints = a.zfsManager.ZfsMountpoints()
+	}
 	for device, stats := range a.fsStats {
+		if zfsMountpoints[stats.Mountpoint] {
+			continue
+		}
 		// skip if not in diskIoCounters
 		d, exists := diskIoCounters[device]
 		if !exists {
@@ -562,20 +571,31 @@ func (a *Agent) updateDiskUsage(systemStats *system.Stats) {
 		!a.lastDiskUsageUpdate.IsZero() &&
 		time.Since(a.lastDiskUsageUpdate) < a.diskUsageCacheDuration
 
+	// ZFS dataset mountpoints use `zfs list` values because statfs(2) reports
+	// dataset-level usage that excludes child datasets (#1541).
+	var zfsUsage map[string]zfsDatasetUsage
+	if a.zfsManager != nil {
+		zfsUsage = a.zfsManager.DatasetUsage()
+	}
+
 	// disk usage
 	for _, stats := range a.fsStats {
 		// Skip non-root filesystems if caching is active
 		if cacheExtraFs && !stats.Root {
 			continue
 		}
-		if d, err := disk.Usage(stats.Mountpoint); err == nil {
-			stats.DiskTotal = utils.BytesToGigabytes(d.Total)
-			stats.DiskUsed = utils.BytesToGigabytes(d.Used)
-			if stats.Root {
-				systemStats.DiskTotal = utils.BytesToGigabytes(d.Total)
-				systemStats.DiskUsed = utils.BytesToGigabytes(d.Used)
-				systemStats.DiskPct = utils.TwoDecimals(d.UsedPercent)
+		var total, used uint64
+		var usedPct float64
+		if u, ok := zfsUsage[stats.Mountpoint]; ok {
+			total = u.used + u.avail
+			used = u.used
+			if total > 0 {
+				usedPct = float64(used) / float64(total) * 100
 			}
+		} else if d, err := disk.Usage(stats.Mountpoint); err == nil {
+			total = d.Total
+			used = d.Used
+			usedPct = d.UsedPercent
 		} else {
 			// reset stats if error (likely unmounted)
 			slog.Error("Error getting disk stats", "name", stats.Mountpoint, "err", err)
@@ -583,6 +603,14 @@ func (a *Agent) updateDiskUsage(systemStats *system.Stats) {
 			stats.DiskUsed = 0
 			stats.TotalRead = 0
 			stats.TotalWrite = 0
+			continue
+		}
+		stats.DiskTotal = utils.BytesToGigabytes(total)
+		stats.DiskUsed = utils.BytesToGigabytes(used)
+		if stats.Root {
+			systemStats.DiskTotal = stats.DiskTotal
+			systemStats.DiskUsed = stats.DiskUsed
+			systemStats.DiskPct = utils.TwoDecimals(usedPct)
 		}
 	}
 

agent/disk_zfs_test.go

@@ -0,0 +1,109 @@
+//go:build testing
+
+package agent
+
+import (
+	"testing"
+
+	"github.com/henrygd/beszel/agent/zfs"
+	"github.com/henrygd/beszel/internal/entities/system"
+	"github.com/shirou/gopsutil/v4/disk"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// TestUpdateDiskUsageZfsMountpoint verifies that a filesystem whose mountpoint
+// is a ZFS dataset reports `zfs list` usage (which includes child datasets)
+// instead of the dataset-scoped statfs values (#1541).
+func TestUpdateDiskUsageZfsMountpoint(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{
+			{Name: "tank", Used: 12000000000000, Avail: 11999000000000, Mountpoint: "/tank"},
+		}, nil
+	}
+	agent := &Agent{
+		fsStats: map[string]*system.FsStats{
+			"tank": {Root: false, Mountpoint: "/tank"},
+		},
+		zfsManager: zm,
+	}
+
+	var stats system.Stats
+	agent.updateDiskUsage(&stats)
+
+	fs := agent.fsStats["tank"]
+	require.NotNil(t, fs)
+	assert.Equal(t, 22350.81, fs.DiskTotal) // (used + avail) in GiB
+	assert.Equal(t, 11175.87, fs.DiskUsed)
+	// Non-root filesystems do not populate system-level stats.
+	assert.Equal(t, float64(0), stats.DiskTotal)
+}
+
+// TestUpdateDiskUsageZfsRootPopulatesSystemStats verifies the root disk values
+// are derived from ZFS usage when the root mountpoint is a ZFS dataset.
+func TestUpdateDiskUsageZfsRootPopulatesSystemStats(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{
+			{Name: "rpool/ROOT/pve-1", Used: 900000000000, Avail: 300000000000, Mountpoint: "/"},
+		}, nil
+	}
+	agent := &Agent{
+		fsStats: map[string]*system.FsStats{
+			"rpool/ROOT/pve-1": {Root: true, Mountpoint: "/"},
+		},
+		zfsManager: zm,
+	}
+
+	var stats system.Stats
+	agent.updateDiskUsage(&stats)
+
+	assert.Equal(t, 1117.59, agent.fsStats["rpool/ROOT/pve-1"].DiskTotal)
+	assert.Equal(t, 838.19, agent.fsStats["rpool/ROOT/pve-1"].DiskUsed)
+	assert.Equal(t, 75.0, stats.DiskPct)
+	assert.Equal(t, 1117.59, stats.DiskTotal)
+	assert.Equal(t, 838.19, stats.DiskUsed)
+}
+
+// TestUpdateDiskUsageWithoutZfsManager falls back to statfs when no manager is
+// present (e.g. tests constructing bare Agent values).
+func TestUpdateDiskUsageWithoutZfsManager(t *testing.T) {
+	agent := &Agent{
+		fsStats: map[string]*system.FsStats{
+			"root": {Root: true, Mountpoint: "/"},
+		},
+	}
+
+	var stats system.Stats
+	agent.updateDiskUsage(&stats)
+
+	assert.True(t, agent.fsStats["root"].DiskTotal > 0, "root usage should come from statfs")
+	assert.True(t, stats.DiskTotal > 0)
+}
+
+// TestInitializeDiskIoStatsSkipsZfsMountpoints verifies ZFS filesystems are
+// excluded from diskstats I/O tracking instead of warning about a missing device.
+func TestInitializeDiskIoStatsSkipsZfsMountpoints(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{{Name: "tank", Mountpoint: "/tank"}}, nil
+	}
+	agent := &Agent{
+		fsStats: map[string]*system.FsStats{
+			"tank": {Root: false, Mountpoint: "/tank"},
+			"sda1": {Root: false, Mountpoint: "/mnt/data"},
+		},
+		zfsManager: zm,
+		diskPrev:   make(map[uint16]map[string]prevDisk),
+	}
+
+	agent.initializeDiskIoStats(map[string]disk.IOCountersStat{
+		"sda1": {Name: "sda1", ReadBytes: 100, WriteBytes: 100},
+	})
+
+	assert.Equal(t, []string{"sda1"}, agent.fsNames)
+	assert.Equal(t, uint64(100), agent.fsStats["sda1"].TotalRead)
+	// ZFS entry is present but untouched by diskstats initialization.
+	assert.Equal(t, uint64(0), agent.fsStats["tank"].TotalRead)
+}

agent/handlers.go

@@ -51,6 +51,7 @@ func NewHandlerRegistry() *HandlerRegistry {
 	registry.Register(common.GetContainerInfo, &GetContainerInfoHandler{})
 	registry.Register(common.GetSmartData, &GetSmartDataHandler{})
 	registry.Register(common.GetSystemdInfo, &GetSystemdInfoHandler{})
+	registry.Register(common.GetZfsData, &GetZfsDataHandler{})
 
 	return registry
 }
@@ -178,6 +179,23 @@ func (h *GetSmartDataHandler) Handle(hctx *HandlerContext) error {
 	}, hctx.RequestID)
 }
 
+////////////////////////////////////////////////////////////////////////////
+////////////////////////////////////////////////////////////////////////////
+
+// GetZfsDataHandler handles ZFS detail data requests
+type GetZfsDataHandler struct{}
+
+func (h *GetZfsDataHandler) Handle(hctx *HandlerContext) error {
+	if hctx.Agent.zfsManager == nil {
+		return hctx.SendResponse(nil, hctx.RequestID)
+	}
+	var req common.ZfsDataRequest
+	if err := cbor.Unmarshal(hctx.Request.Data, &req); err != nil {
+		return err
+	}
+	return hctx.SendResponse(hctx.Agent.zfsManager.GetDetail(req.Force), hctx.RequestID)
+}
+
 ////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////
 ////////////////////////////////////////////////////////////////////////////

agent/handlers_test.go

@@ -4,8 +4,10 @@ package agent
 
 import (
 	"testing"
+	"time"
 
 	"github.com/fxamacker/cbor/v2"
+	"github.com/henrygd/beszel/agent/zfs"
 	"github.com/henrygd/beszel/internal/common"
 	"github.com/henrygd/beszel/internal/entities/smart"
 	"github.com/stretchr/testify/assert"
@@ -30,6 +32,32 @@ func TestNewAgentResponseSmartData(t *testing.T) {
 	assert.True(t, response.SmartComplete)
 }
 
+func TestGetZfsDataHandlerForceRefresh(t *testing.T) {
+	poolCalls := 0
+	zm := &ZfsManager{detailInterval: time.Hour}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		poolCalls++
+		return []zfs.PoolStat{{Name: "tank", Alloc: uint64(poolCalls)}}, nil
+	}
+	zm.poolStatusesFn = func() ([]zfs.PoolStatus, error) { return nil, nil }
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, nil }
+	zm.GetDetail(false)
+
+	requestData, err := cbor.Marshal(common.ZfsDataRequest{Force: true})
+	assert.NoError(t, err)
+	ctx := &HandlerContext{
+		Agent: &Agent{zfsManager: zm},
+		Request: &common.HubRequest[cbor.RawMessage]{
+			Action: common.GetZfsData,
+			Data:   requestData,
+		},
+		SendResponse: func(any, *uint32) error { return nil },
+	}
+
+	assert.NoError(t, (&GetZfsDataHandler{}).Handle(ctx))
+	assert.Equal(t, 2, poolCalls)
+}
+
 func (m *MockHandler) Handle(ctx *HandlerContext) error {
 	if m.handleFunc != nil {
 		return m.handleFunc(ctx)

agent/system.go

@@ -219,6 +219,9 @@ func (a *Agent) getSystemStats(cacheTimeMs uint16) system.Stats {
 	// disk i/o (cache-aware per interval)
 	a.updateDiskIo(cacheTimeMs, &systemStats)
 
+	// zfs pool stats
+	a.zfsManager.Update(&systemStats)
+
 	// network stats (per cache interval)
 	a.updateNetworkStats(cacheTimeMs, &systemStats)
 

agent/test-data/zfs/zfs_list.txt

@@ -0,0 +1,9 @@
+tank	12000000000000	11999000000000	/tank
+tank/apps	1000000000000	11999000000000	/tank/apps
+tank/backup	2000000000000	11999000000000	/tank/backup
+tank/media	1000000000000	11999000000000	/tank/my media
+rpool	900000000000	300000000000	-
+rpool/ROOT	1000000000	300000000000	-
+rpool/ROOT/pve-1	890000000000	300000000000	/
+rpool/data	9000000000	300000000000	-
+rpool/data/subvol-100-disk-0	400000000000	300000000000	/subvol-100-disk-0

agent/test-data/zfs/zpool_list.txt

@@ -0,0 +1,2 @@
+tank	23999000000000	12000000000000	11999000000000	ONLINE
+rpool	1200000000000	900000000000	300000000000	DEGRADED

agent/test-data/zfs/zpool_status.txt

@@ -0,0 +1,29 @@
+  pool: tank
+ state: ONLINE
+  scan: scrub repaired 0B in 00:05:12 with 0 errors on Sun Jun  1 02:00:12 2025
+config:
+
+	NAME        STATE     READ WRITE CKSUM
+	tank        ONLINE       0     0     0
+	  mirror-0  ONLINE       0     0     0
+	    sda     ONLINE       0     0     0
+	    sdb     ONLINE       0     0     0
+
+errors: No known data errors
+
+  pool: rpool
+ state: DEGRADED
+ status: One or more devices could not be used because the label is missing or
+	invalid.  Sufficient replicas exist for the pool to continue functioning in a
+	degraded state.
+  scan: scrub in progress since Sun Jun  8 01:00:00 2025
+	10.00% done, 01:30:00 to go, 0.00/s
+config:
+
+	NAME        STATE     READ WRITE CKSUM
+	rpool       DEGRADED     0     0     0
+	  mirror-0  DEGRADED     0     0     0
+	    sda     ONLINE       0     0     0
+	    sdb     FAULTED      1     2     3
+
+errors: 1 data errors, use '-v' for a list

agent/zfs/zfs.go

@@ -0,0 +1,160 @@
+// Package zfs provides functions to read ZFS statistics.
+package zfs
+
+import (
+	"bufio"
+	"bytes"
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+	"time"
+)
+
+var commandTimeout = 10 * time.Second
+
+var commandOutput = func(name string, args ...string) ([]byte, error) {
+	ctx, cancel := context.WithTimeout(context.Background(), commandTimeout)
+	defer cancel()
+	cmd := exec.CommandContext(ctx, name, args...)
+	cmd.Env = append(os.Environ(), "LC_ALL=C", "LANG=C")
+	out, err := cmd.Output()
+	if ctx.Err() != nil {
+		return nil, fmt.Errorf("%s timed out after %s: %w", name, commandTimeout, ctx.Err())
+	}
+	return out, err
+}
+
+// ErrNoZfs is returned when the ZFS utilities or kernel interfaces are unavailable.
+var ErrNoZfs = errors.New("zfs utilities unavailable")
+
+// PoolStat is a snapshot of a ZFS pool's capacity and health.
+type PoolStat struct {
+	Name   string
+	Size   uint64 // total capacity in bytes
+	Alloc  uint64 // allocated bytes
+	Free   uint64 // free bytes
+	Health string // ONLINE, DEGRADED, FAULTED, ...
+}
+
+// PoolKernelStat is the inexpensive pool telemetry exposed by the ZFS kernel.
+// NRead and NWrite are cumulative byte counters since the pool was imported.
+type PoolKernelStat struct {
+	Name   string
+	Health string
+	NRead  uint64
+	NWrite uint64
+}
+
+// PoolIoStats holds calculated per-second I/O rates for a pool.
+type PoolIoStats struct {
+	NRead  uint64
+	NWrite uint64
+}
+
+// Dataset is a single ZFS dataset with usage information.
+type Dataset struct {
+	Name       string
+	Used       uint64
+	Avail      uint64
+	Mountpoint string
+}
+
+// PoolStats returns capacity and health for all pools on the system using
+// `zpool list`. Frequent health and I/O sampling uses PoolKernelStats instead.
+func PoolStats() ([]PoolStat, error) {
+	out, err := commandOutput("zpool", "list", "-Hp", "-o", "name,size,alloc,free,health")
+	if err != nil {
+		var exitErr *exec.ExitError
+		if errors.As(err, &exitErr) && strings.Contains(string(exitErr.Stderr), "no pools available") {
+			return nil, nil
+		}
+		return nil, fmt.Errorf("zpool list: %w", err)
+	}
+	return parseZpoolListOutput(out)
+}
+
+// Datasets returns all datasets on the system with usage and mountpoint
+// information using `zfs list` (recursive by default).
+func Datasets() ([]Dataset, error) {
+	out, err := commandOutput("zfs", "list", "-Hp", "-o", "name,used,avail,mountpoint")
+	if err != nil {
+		return nil, fmt.Errorf("zfs list: %w", err)
+	}
+	return parseZfsListOutput(out)
+}
+
+// parseZpoolListOutput parses `zpool list -Hp -o name,size,alloc,free,health` output.
+// Columns are tab-separated; numeric columns are raw bytes.
+func parseZpoolListOutput(out []byte) ([]PoolStat, error) {
+	var pools []PoolStat
+	scanner := bufio.NewScanner(bytes.NewReader(out))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+		if line == "no pools available" && len(pools) == 0 {
+			return nil, nil
+		}
+		fields := strings.Split(line, "\t")
+		if len(fields) < 5 {
+			return nil, fmt.Errorf("unexpected zpool list line: %q", line)
+		}
+		size, err := strconv.ParseUint(fields[1], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing size for pool %q: %w", fields[0], err)
+		}
+		alloc, err := strconv.ParseUint(fields[2], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing alloc for pool %q: %w", fields[0], err)
+		}
+		free, err := strconv.ParseUint(fields[3], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing free for pool %q: %w", fields[0], err)
+		}
+		pools = append(pools, PoolStat{
+			Name:   fields[0],
+			Size:   size,
+			Alloc:  alloc,
+			Free:   free,
+			Health: fields[4],
+		})
+	}
+	return pools, scanner.Err()
+}
+
+// parseZfsListOutput parses `zfs list -Hp -o name,used,avail,mountpoint` output.
+// The mountpoint column may contain spaces, so it is split on tabs only.
+func parseZfsListOutput(out []byte) ([]Dataset, error) {
+	var datasets []Dataset
+	scanner := bufio.NewScanner(bytes.NewReader(out))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		if line == "" {
+			continue
+		}
+		fields := strings.SplitN(line, "\t", 4)
+		if len(fields) < 4 {
+			return nil, fmt.Errorf("unexpected zfs list line: %q", line)
+		}
+		used, err := strconv.ParseUint(fields[1], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing used for dataset %q: %w", fields[0], err)
+		}
+		avail, err := strconv.ParseUint(fields[2], 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("parsing avail for dataset %q: %w", fields[0], err)
+		}
+		datasets = append(datasets, Dataset{
+			Name:       fields[0],
+			Used:       used,
+			Avail:      avail,
+			Mountpoint: fields[3],
+		})
+	}
+	return datasets, scanner.Err()
+}

agent/zfs/zfs_freebsd.go

@@ -3,9 +3,17 @@
 package zfs
 
 import (
+	"errors"
+
 	"golang.org/x/sys/unix"
 )
 
 func ARCSize() (uint64, error) {
 	return unix.SysctlUint64("kstat.zfs.misc.arcstats.size")
 }
+
+// FreeBSD does not expose Linux's per-pool procfs kstats. Capacity, health,
+// and detail collection still work through the cached utilities.
+func PoolKernelStats() ([]PoolKernelStat, error) {
+	return nil, errors.ErrUnsupported
+}

agent/zfs/zfs_linux.go

@@ -5,14 +5,18 @@ package zfs
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
 )
 
+var procZfsPath = "/proc/spl/kstat/zfs"
+
 func ARCSize() (uint64, error) {
-	file, err := os.Open("/proc/spl/kstat/zfs/arcstats")
+	file, err := os.Open(filepath.Join(procZfsPath, "arcstats"))
 	if err != nil {
 		return 0, err
 	}
@@ -29,6 +33,167 @@ func ARCSize() (uint64, error) {
 			return strconv.ParseUint(fields[2], 10, 64)
 		}
 	}
+	if err := scanner.Err(); err != nil {
+		return 0, err
+	}
 
 	return 0, fmt.Errorf("size field not found in arcstats")
 }
+
+// PoolKernelStats reads pool state and cumulative I/O counters directly from
+// procfs. These kstats are the same interfaces used by node_exporter's Linux
+// ZFS collector and avoid keeping a `zpool iostat` subprocess alive.
+func PoolKernelStats() ([]PoolKernelStat, error) {
+	poolDirs := make(map[string]struct{})
+	for _, filename := range []string{"state", "io", "objset-*"} {
+		paths, err := filepath.Glob(filepath.Join(procZfsPath, "*", filename))
+		if err != nil {
+			return nil, err
+		}
+		for _, path := range paths {
+			poolDirs[filepath.Dir(path)] = struct{}{}
+		}
+	}
+	if len(poolDirs) == 0 {
+		return nil, ErrNoZfs
+	}
+	pools := make([]PoolKernelStat, 0, len(poolDirs))
+	for poolDir := range poolDirs {
+		nread, nwrite, err := readPoolCounters(poolDir)
+		if err != nil {
+			if errors.Is(err, os.ErrNotExist) {
+				continue // pool may have been exported after the glob
+			}
+			return nil, err
+		}
+		state, err := os.ReadFile(filepath.Join(poolDir, "state"))
+		if err != nil && !errors.Is(err, os.ErrNotExist) {
+			return nil, err
+		}
+		pools = append(pools, PoolKernelStat{
+			Name: filepath.Base(poolDir), Health: strings.ToUpper(strings.TrimSpace(string(state))),
+			NRead: nread, NWrite: nwrite,
+		})
+	}
+	if len(pools) == 0 {
+		return nil, ErrNoZfs
+	}
+	return pools, nil
+}
+
+// readPoolCounters supports both ZFS kernel interfaces. OpenZFS through 2.3
+// exposes aggregate vdev counters in "io". When that file is unavailable, sum
+// the logical I/O counters exposed for each dataset in the pool.
+func readPoolCounters(poolDir string) (uint64, uint64, error) {
+	nread, nwrite, err := readPoolIO(filepath.Join(poolDir, "io"))
+	if err == nil || !errors.Is(err, os.ErrNotExist) {
+		return nread, nwrite, err
+	}
+	return readPoolObjsets(poolDir)
+}
+
+func readPoolIO(path string) (uint64, uint64, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return 0, 0, err
+	}
+	defer file.Close()
+
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) < 2 || fields[0] != "nread" {
+			continue
+		}
+		if !scanner.Scan() {
+			break
+		}
+		values := strings.Fields(scanner.Text())
+		if len(values) < 2 {
+			break
+		}
+		nread, err := strconv.ParseUint(values[0], 10, 64)
+		if err != nil {
+			return 0, 0, fmt.Errorf("parsing nread in %s: %w", path, err)
+		}
+		nwrite, err := strconv.ParseUint(values[1], 10, 64)
+		if err != nil {
+			return 0, 0, fmt.Errorf("parsing nwritten in %s: %w", path, err)
+		}
+		return nread, nwrite, nil
+	}
+	if err := scanner.Err(); err != nil {
+		return 0, 0, err
+	}
+	return 0, 0, fmt.Errorf("I/O counters not found in %s", path)
+}
+
+func readPoolObjsets(poolDir string) (uint64, uint64, error) {
+	paths, err := filepath.Glob(filepath.Join(poolDir, "objset-*"))
+	if err != nil {
+		return 0, 0, err
+	}
+	if len(paths) == 0 {
+		return 0, 0, fmt.Errorf("dataset I/O counters not found in %s", poolDir)
+	}
+
+	var totalRead, totalWrite uint64
+	objsetsRead := 0
+	for _, path := range paths {
+		nread, nwrite, err := readObjsetIO(path)
+		if errors.Is(err, os.ErrNotExist) {
+			continue // dataset may have been destroyed after the glob
+		}
+		if err != nil {
+			return 0, 0, err
+		}
+		totalRead += nread
+		totalWrite += nwrite
+		objsetsRead++
+	}
+	if objsetsRead == 0 {
+		return 0, 0, fmt.Errorf("dataset I/O counters not found in %s", poolDir)
+	}
+	return totalRead, totalWrite, nil
+}
+
+func readObjsetIO(path string) (uint64, uint64, error) {
+	file, err := os.Open(path)
+	if err != nil {
+		return 0, 0, err
+	}
+	defer file.Close()
+
+	var nread, nwrite uint64
+	var foundRead, foundWrite bool
+	scanner := bufio.NewScanner(file)
+	for scanner.Scan() {
+		fields := strings.Fields(scanner.Text())
+		if len(fields) < 3 {
+			continue
+		}
+		var target *uint64
+		switch fields[0] {
+		case "nread":
+			target = &nread
+			foundRead = true
+		case "nwritten":
+			target = &nwrite
+			foundWrite = true
+		default:
+			continue
+		}
+		value, err := strconv.ParseUint(fields[2], 10, 64)
+		if err != nil {
+			return 0, 0, fmt.Errorf("parsing %s in %s: %w", fields[0], path, err)
+		}
+		*target = value
+	}
+	if err := scanner.Err(); err != nil {
+		return 0, 0, err
+	}
+	if !foundRead || !foundWrite {
+		return 0, 0, fmt.Errorf("incomplete I/O counters in %s", path)
+	}
+	return nread, nwrite, nil
+}

agent/zfs/zfs_linux_test.go

@@ -0,0 +1,90 @@
+//go:build testing && linux
+
+package zfs
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestPoolKernelStats(t *testing.T) {
+	root := t.TempDir()
+	oldPath := procZfsPath
+	procZfsPath = root
+	t.Cleanup(func() { procZfsPath = oldPath })
+
+	poolDir := filepath.Join(root, "tank")
+	require.NoError(t, os.MkdirAll(poolDir, 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(poolDir, "io"), []byte(
+		"11 3 0x00 1 80 0 0\n"+
+			"nread nwritten reads writes wtime wlentime wupdate rtime rlentime rupdate wcnt rcnt\n"+
+			"1884160 6450688 22 978 0 0 0 0 0 0 0 0\n",
+	), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(poolDir, "state"), []byte("DEGRADED\n"), 0o644))
+
+	stats, err := PoolKernelStats()
+	require.NoError(t, err)
+	require.Len(t, stats, 1)
+	assert.Equal(t, PoolKernelStat{
+		Name: "tank", Health: "DEGRADED", NRead: 1884160, NWrite: 6450688,
+	}, stats[0])
+}
+
+func TestPoolKernelStatsOpenZfs24(t *testing.T) {
+	root := t.TempDir()
+	oldPath := procZfsPath
+	procZfsPath = root
+	t.Cleanup(func() { procZfsPath = oldPath })
+
+	poolDir := filepath.Join(root, "tank")
+	require.NoError(t, os.MkdirAll(poolDir, 0o755))
+	require.NoError(t, os.WriteFile(filepath.Join(poolDir, "state"), []byte("ONLINE\n"), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(poolDir, "objset-0x1"), []byte(
+		"34 1 0x01 28 7872 0 0\n"+
+			"name type data\n"+
+			"dataset_name 7 tank\n"+
+			"nwritten 4 2000\n"+
+			"nread 4 1000\n",
+	), 0o644))
+	require.NoError(t, os.WriteFile(filepath.Join(poolDir, "objset-0x2"), []byte(
+		"34 1 0x01 28 7872 0 0\n"+
+			"name type data\n"+
+			"dataset_name 7 tank/videos\n"+
+			"nwritten 4 400\n"+
+			"nread 4 300\n",
+	), 0o644))
+
+	stats, err := PoolKernelStats()
+	require.NoError(t, err)
+	require.Len(t, stats, 1)
+	assert.Equal(t, PoolKernelStat{
+		Name: "tank", Health: "ONLINE", NRead: 1300, NWrite: 2400,
+	}, stats[0])
+}
+
+func TestPoolKernelStatsNoZfs(t *testing.T) {
+	oldPath := procZfsPath
+	procZfsPath = t.TempDir()
+	t.Cleanup(func() { procZfsPath = oldPath })
+
+	_, err := PoolKernelStats()
+	assert.ErrorIs(t, err, ErrNoZfs)
+}
+
+func TestReadPoolIORejectsMalformedCounters(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "io")
+	require.NoError(t, os.WriteFile(path, []byte("nread nwritten\nnope 10\n"), 0o644))
+	_, _, err := readPoolIO(path)
+	require.Error(t, err)
+}
+
+func TestReadObjsetIORequiresAllCounters(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "objset-0x1")
+	require.NoError(t, os.WriteFile(path, []byte("nread 4 10\n"), 0o644))
+	_, _, err := readObjsetIO(path)
+	require.Error(t, err)
+}

agent/zfs/zfs_status.go

@@ -0,0 +1,150 @@
+package zfs
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"regexp"
+	"strconv"
+	"strings"
+)
+
+// PoolStatus holds parsed `zpool status` information for one pool.
+type PoolStatus struct {
+	Name  string
+	State string // ONLINE, DEGRADED, FAULTED, ...
+	Scrub ScrubStatus
+	Vdevs []VdevStatus
+}
+
+// ScrubStatus holds the scrub (or resilver) status parsed from the scan line.
+type ScrubStatus struct {
+	State    string // NONE, SCANNING, FINISHED, CANCELED
+	Progress string // e.g. "10.00%" while scanning
+	Errors   uint64
+}
+
+// VdevStatus is a single vdev row (mirror, raidz, or leaf disk).
+type VdevStatus struct {
+	Name         string
+	State        string
+	ReadErrs     uint64
+	WriteErrs    uint64
+	ChecksumErrs uint64
+}
+
+var (
+	progressRe = regexp.MustCompile(`(\d+\.\d+)%\s+done`)
+	errorsRe   = regexp.MustCompile(`with\s+(\d+)\s+errors`)
+)
+
+// PoolStatuses runs `zpool status` and parses per-pool state, scrub, and vdev
+// information. The human-readable format has been stable across OpenZFS
+// releases; rows are matched by their tabular shape rather than position.
+func PoolStatuses() ([]PoolStatus, error) {
+	out, err := commandOutput("zpool", "status")
+	if err != nil {
+		return nil, fmt.Errorf("zpool status: %w", err)
+	}
+	return parseZpoolStatusOutput(out)
+}
+
+// parseZpoolStatusOutput parses the output of `zpool status`.
+func parseZpoolStatusOutput(out []byte) ([]PoolStatus, error) {
+	var pools []PoolStatus
+	var current *PoolStatus
+	inConfig := false
+	scanContinuation := false // next non-blank line continues the scan line (progress)
+
+	scanner := bufio.NewScanner(bytes.NewReader(out))
+	for scanner.Scan() {
+		line := scanner.Text()
+		trimmed := strings.TrimSpace(line)
+
+		switch {
+		case strings.HasPrefix(trimmed, "pool:"):
+			pools = append(pools, PoolStatus{Name: strings.TrimSpace(strings.TrimPrefix(trimmed, "pool:"))})
+			current = &pools[len(pools)-1]
+			inConfig = false
+			scanContinuation = false
+		case current == nil:
+			continue
+		case strings.HasPrefix(trimmed, "state:"):
+			current.State = strings.TrimSpace(strings.TrimPrefix(trimmed, "state:"))
+		case strings.HasPrefix(trimmed, "scan:"):
+			current.Scrub = parseScanLine(trimmed)
+			// zpool status prints the progress percentage on the line after scan.
+			scanContinuation = true
+		case trimmed == "config:":
+			inConfig = true
+		case scanContinuation:
+			// The line after scan: may be an indented progress continuation.
+			if m := progressRe.FindStringSubmatch(trimmed); m != nil {
+				current.Scrub.Progress = m[1] + "%"
+			}
+			scanContinuation = false
+		case inConfig && (line == "" || strings.HasPrefix(line, " ") || strings.HasPrefix(line, "\t")):
+			// Table rows are indented; blank lines separate sections. The
+			// column header and the pool's own row are skipped.
+			if trimmed != "" && !strings.HasPrefix(trimmed, "NAME") {
+				if vdev, ok := parseVdevLine(trimmed, current.Name); ok {
+					current.Vdevs = append(current.Vdevs, vdev)
+				}
+			}
+		case inConfig:
+			// unindented line (errors:, status:, next pool:) ends the table
+			inConfig = false
+		}
+	}
+	return pools, scanner.Err()
+}
+
+// parseScanLine maps a `scan:` line to a ScrubStatus.
+func parseScanLine(line string) ScrubStatus {
+	var scrub ScrubStatus
+	switch {
+	case strings.Contains(line, "in progress"):
+		scrub.State = "SCANNING"
+	case strings.Contains(line, "canceled"):
+		scrub.State = "CANCELED"
+	case strings.Contains(line, "repaired"), strings.Contains(line, "resilvered"):
+		scrub.State = "FINISHED"
+	default:
+		scrub.State = "NONE"
+	}
+	if m := progressRe.FindStringSubmatch(line); m != nil {
+		scrub.Progress = m[1] + "%"
+	}
+	if m := errorsRe.FindStringSubmatch(line); m != nil {
+		if n, err := strconv.ParseUint(m[1], 10, 64); err == nil {
+			scrub.Errors = n
+		}
+	}
+	return scrub
+}
+
+// parseVdevLine parses one row of the config table. Rows have the shape
+// "NAME STATE READ WRITE CKSUM [extra...]". The first data row is the pool
+// itself and is skipped since it duplicates pool-level info.
+func parseVdevLine(line, poolName string) (VdevStatus, bool) {
+	fields := strings.Fields(line)
+	if len(fields) < 5 {
+		return VdevStatus{}, false
+	}
+	if fields[0] == poolName {
+		return VdevStatus{}, false
+	}
+	read, err1 := strconv.ParseUint(fields[2], 10, 64)
+	write, err2 := strconv.ParseUint(fields[3], 10, 64)
+	cksum, err3 := strconv.ParseUint(fields[4], 10, 64)
+	if err1 != nil || err2 != nil || err3 != nil {
+		return VdevStatus{}, false
+	}
+	return VdevStatus{
+		Name:         fields[0],
+		State:        fields[1],
+		ReadErrs:     read,
+		WriteErrs:    write,
+		ChecksumErrs: cksum,
+	}, true
+}

agent/zfs/zfs_test.go

@@ -0,0 +1,143 @@
+//go:build testing
+
+package zfs
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func fixturePath(name string) string {
+	return filepath.Join("..", "test-data", "zfs", name)
+}
+
+func TestParseZpoolListOutput(t *testing.T) {
+	data, err := os.ReadFile(fixturePath("zpool_list.txt"))
+	require.NoError(t, err)
+
+	pools, err := parseZpoolListOutput(data)
+	require.NoError(t, err)
+	require.Len(t, pools, 2)
+
+	assert.Equal(t, PoolStat{Name: "tank", Size: 23999000000000, Alloc: 12000000000000, Free: 11999000000000, Health: "ONLINE"}, pools[0])
+	assert.Equal(t, PoolStat{Name: "rpool", Size: 1200000000000, Alloc: 900000000000, Free: 300000000000, Health: "DEGRADED"}, pools[1])
+}
+
+func TestParseZpoolListOutputIgnoresEmptyLines(t *testing.T) {
+	pools, err := parseZpoolListOutput([]byte("tank\t100\t50\t50\tONLINE\n\n"))
+	require.NoError(t, err)
+	require.Len(t, pools, 1)
+	assert.Equal(t, "tank", pools[0].Name)
+}
+
+func TestParseZpoolListOutputNoPools(t *testing.T) {
+	pools, err := parseZpoolListOutput([]byte("no pools available\n"))
+	require.NoError(t, err)
+	assert.Empty(t, pools)
+}
+
+func TestParseZpoolListOutputRejectsMalformedLine(t *testing.T) {
+	_, err := parseZpoolListOutput([]byte("tank\t100\t50\n"))
+	require.Error(t, err)
+
+	_, err = parseZpoolListOutput([]byte("tank\tnotanumber\t50\t50\tONLINE\n"))
+	require.Error(t, err)
+}
+
+func TestParseZfsListOutput(t *testing.T) {
+	data, err := os.ReadFile(fixturePath("zfs_list.txt"))
+	require.NoError(t, err)
+
+	datasets, err := parseZfsListOutput(data)
+	require.NoError(t, err)
+	require.Len(t, datasets, 9)
+
+	// Mountpoint with a space must be kept intact (tab-split only).
+	assert.Equal(t, "/tank/my media", datasets[3].Mountpoint)
+	// Unmounted datasets/zvols report "-".
+	assert.Equal(t, "-", datasets[4].Mountpoint)
+	assert.Equal(t, uint64(12000000000000), datasets[0].Used)
+	assert.Equal(t, uint64(11999000000000), datasets[0].Avail)
+}
+
+func TestParseZpoolStatusOutput(t *testing.T) {
+	data, err := os.ReadFile(fixturePath("zpool_status.txt"))
+	require.NoError(t, err)
+
+	pools, err := parseZpoolStatusOutput(data)
+	require.NoError(t, err)
+	require.Len(t, pools, 2)
+
+	tank := pools[0]
+	assert.Equal(t, "tank", tank.Name)
+	assert.Equal(t, "ONLINE", tank.State)
+	assert.Equal(t, "FINISHED", tank.Scrub.State)
+	assert.Equal(t, "", tank.Scrub.Progress)
+	assert.Equal(t, uint64(0), tank.Scrub.Errors)
+	// Pool row itself is skipped; mirror + 2 disks remain.
+	require.Len(t, tank.Vdevs, 3)
+	assert.Equal(t, "mirror-0", tank.Vdevs[0].Name)
+	assert.Equal(t, "sda", tank.Vdevs[1].Name)
+	assert.Equal(t, "sdb", tank.Vdevs[2].Name)
+
+	rpool := pools[1]
+	assert.Equal(t, "rpool", rpool.Name)
+	assert.Equal(t, "DEGRADED", rpool.State)
+	assert.Equal(t, "SCANNING", rpool.Scrub.State)
+	assert.Equal(t, "10.00%", rpool.Scrub.Progress)
+	require.Len(t, rpool.Vdevs, 3)
+	assert.Equal(t, "FAULTED", rpool.Vdevs[2].State)
+	assert.Equal(t, uint64(1), rpool.Vdevs[2].ReadErrs)
+	assert.Equal(t, uint64(2), rpool.Vdevs[2].WriteErrs)
+	assert.Equal(t, uint64(3), rpool.Vdevs[2].ChecksumErrs)
+}
+
+func TestParseScanLine(t *testing.T) {
+	assert.Equal(t, "FINISHED", parseScanLine("scan: scrub repaired 0B in 00:05:12 with 0 errors on Sun Jun  1 02:00:12 2025").State)
+	assert.Equal(t, uint64(3), parseScanLine("scan: scrub repaired 10G in 01:00:00 with 3 errors on Sun Jun  1 02:00:12 2025").Errors)
+	assert.Equal(t, "SCANNING", parseScanLine("scan: scrub in progress since Sun Jun  8 01:00:00 2025").State)
+	assert.Equal(t, "CANCELED", parseScanLine("scan: scrub canceled on Sun Jun  1 02:00:12 2025").State)
+	assert.Equal(t, "FINISHED", parseScanLine("scan: resilvered 1.23G in 00:01:00 with 0 errors on Sun Jun  1 02:00:12 2025").State)
+	assert.Equal(t, "NONE", parseScanLine("scan: none requested").State)
+}
+
+func TestCommandOutputForcesLocaleAndTimesOut(t *testing.T) {
+	t.Setenv("BESZEL_ZFS_COMMAND_HELPER", "1")
+	out, err := commandOutput(os.Args[0], "-test.run=TestZfsCommandHelperProcess", "--", "locale")
+	require.NoError(t, err)
+	assert.Equal(t, "C/C", string(out))
+
+	oldTimeout := commandTimeout
+	commandTimeout = 20 * time.Millisecond
+	t.Cleanup(func() { commandTimeout = oldTimeout })
+	_, err = commandOutput(os.Args[0], "-test.run=TestZfsCommandHelperProcess", "--", "sleep")
+	require.Error(t, err)
+	assert.Contains(t, err.Error(), "timed out")
+}
+
+func TestZfsCommandHelperProcess(t *testing.T) {
+	if os.Getenv("BESZEL_ZFS_COMMAND_HELPER") != "1" {
+		return
+	}
+	mode := ""
+	for i, arg := range os.Args {
+		if arg == "--" && i+1 < len(os.Args) {
+			mode = os.Args[i+1]
+			break
+		}
+	}
+	switch strings.TrimSpace(mode) {
+	case "locale":
+		_, _ = fmt.Printf("%s/%s", os.Getenv("LC_ALL"), os.Getenv("LANG"))
+	case "sleep":
+		time.Sleep(time.Second)
+	}
+	os.Exit(0)
+}

agent/zfs/zfs_unsupported.go

@@ -7,3 +7,7 @@ import "errors"
 func ARCSize() (uint64, error) {
 	return 0, errors.ErrUnsupported
 }
+
+func PoolKernelStats() ([]PoolKernelStat, error) {
+	return nil, errors.ErrUnsupported
+}

agent/zfs_pool.go

@@ -0,0 +1,320 @@
+package agent
+
+import (
+	"log/slog"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/henrygd/beszel/agent/zfs"
+	"github.com/henrygd/beszel/internal/entities/system"
+	zfsentity "github.com/henrygd/beszel/internal/entities/zfs"
+)
+
+// zfsDatasetUsage holds usage values for a ZFS dataset mountpoint.
+type zfsDatasetUsage struct {
+	used  uint64
+	avail uint64
+}
+
+// datasetUsageRefreshInterval controls how often `zfs list` is re-run for the
+// mountpoint usage map. Dataset inventory changes rarely.
+const datasetUsageRefreshInterval = 5 * time.Minute
+
+// poolStatsRefreshInterval controls how often `zpool list` is re-run for pool
+// capacity. Health and I/O are read from procfs on Linux, so the utility only
+// needs to refresh slow-moving space accounting.
+const poolStatsRefreshInterval = time.Minute
+
+type poolKernelSample struct {
+	nread  uint64
+	nwrite uint64
+	at     time.Time
+}
+
+// ZfsManager collects ZFS pool and dataset statistics. Collection functions
+// are fields so unit tests can substitute them (same pattern as
+// diskDiscovery.usageFn). It is safe for concurrent use by a single goroutine
+// only; callers must hold the agent lock like updateDiskUsage does.
+type ZfsManager struct {
+	poolStatsFn    func() ([]zfs.PoolStat, error)       // capacity/health source
+	datasetsFn     func() ([]zfs.Dataset, error)        // dataset inventory source
+	kernelStatsFn  func() ([]zfs.PoolKernelStat, error) // procfs pool state/I/O source
+	poolStatusesFn func() ([]zfs.PoolStatus, error)     // scrub/vdev detail source
+
+	poolData      []zfs.PoolStat // cached pool inventory (TTL below)
+	lastPoolStats time.Time
+	kernelSamples map[string]poolKernelSample
+
+	datasetUsage     map[string]zfsDatasetUsage // mountpoint -> usage
+	lastUsageRefresh time.Time
+
+	// Detail data (pools, vdevs, scrub, datasets) is cached and refreshed on
+	// an interval. Accessed from handler goroutines, so it is mutex-protected.
+	detailMu          sync.Mutex
+	detail            *zfsentity.ZfsData
+	lastDetailRefresh time.Time
+	detailInterval    time.Duration
+}
+
+// newZfsManager creates a ZfsManager wired to the system's ZFS utilities.
+func newZfsManager() *ZfsManager {
+	return &ZfsManager{
+		poolStatsFn:    zfs.PoolStats,
+		datasetsFn:     zfs.Datasets,
+		kernelStatsFn:  zfs.PoolKernelStats,
+		poolStatusesFn: zfs.PoolStatuses,
+		detailInterval: time.Hour,
+	}
+}
+
+// Update refreshes systemStats.ZfsPools with the latest pool data. I/O
+// throughput and health come from inexpensive kernel kstats on Linux. Pool
+// capacity and dataset usage come from separately cached utility calls. It is
+// a no-op when ZFS is absent.
+func (zm *ZfsManager) Update(systemStats *system.Stats) {
+	pools := zm.poolStats()
+	if len(pools) == 0 {
+		return
+	}
+
+	kernelStats, ioRates := zm.kernelStats()
+
+	if systemStats.ZfsPools == nil {
+		systemStats.ZfsPools = make(map[string]*system.ZfsPool, len(pools))
+	}
+	for i := range pools {
+		pool := &pools[i]
+		// Full precision, matching the dataset values below; the frontend
+		// formats any magnitude.
+		stats := &system.ZfsPool{
+			Total:  float64(pool.Size) / (1024 * 1024 * 1024),
+			Used:   float64(pool.Alloc) / (1024 * 1024 * 1024),
+			Health: pool.Health,
+		}
+		if kernel, exists := kernelStats[pool.Name]; exists && kernel.Health != "" {
+			stats.Health = kernel.Health
+		}
+		if io, exists := ioRates[pool.Name]; exists {
+			stats.ReadBytes = io.NRead
+			stats.WriteBytes = io.NWrite
+		}
+		slog.Debug("ZFS pool sample", "pool", pool.Name, "health", stats.Health, "used_gb", stats.Used, "read_bps", stats.ReadBytes, "write_bps", stats.WriteBytes)
+		systemStats.ZfsPools[pool.Name] = stats
+	}
+
+}
+
+// poolStats returns the cached pool inventory, re-running `zpool list` at most
+// every poolStatsRefreshInterval. On failure the previous inventory is
+// retained and the refresh is retried on the next cadence.
+func (zm *ZfsManager) poolStats() []zfs.PoolStat {
+	if zm.lastPoolStats.IsZero() || time.Since(zm.lastPoolStats) >= poolStatsRefreshInterval {
+		pools, err := zm.poolStatsFn()
+		if err != nil {
+			slog.Debug("ZFS pool stats unavailable", "err", err)
+		} else {
+			zm.poolData = pools
+		}
+		zm.lastPoolStats = time.Now()
+	}
+	return zm.poolData
+}
+
+// kernelStats reads cumulative pool counters and converts them to per-second
+// rates. Counter decreases indicate a pool export/import and reset the
+// baseline instead of producing an underflow spike.
+func (zm *ZfsManager) kernelStats() (map[string]zfs.PoolKernelStat, map[string]zfs.PoolIoStats) {
+	if zm.kernelStatsFn == nil {
+		return nil, nil
+	}
+	stats, err := zm.kernelStatsFn()
+	if err != nil {
+		slog.Debug("ZFS kernel stats unavailable", "err", err)
+		return nil, nil
+	}
+	now := time.Now()
+	byName := make(map[string]zfs.PoolKernelStat, len(stats))
+	rates := make(map[string]zfs.PoolIoStats, len(stats))
+	nextSamples := make(map[string]poolKernelSample, len(stats))
+	for _, stat := range stats {
+		byName[stat.Name] = stat
+		if previous, ok := zm.kernelSamples[stat.Name]; ok && now.After(previous.at) &&
+			stat.NRead >= previous.nread && stat.NWrite >= previous.nwrite {
+			seconds := now.Sub(previous.at).Seconds()
+			rates[stat.Name] = zfs.PoolIoStats{
+				NRead:  uint64(float64(stat.NRead-previous.nread) / seconds),
+				NWrite: uint64(float64(stat.NWrite-previous.nwrite) / seconds),
+			}
+		}
+		nextSamples[stat.Name] = poolKernelSample{nread: stat.NRead, nwrite: stat.NWrite, at: now}
+	}
+	zm.kernelSamples = nextSamples
+	return byName, rates
+}
+
+// refreshDatasetUsage re-runs `zfs list` when the refresh window has elapsed
+// and rebuilds the mountpoint-keyed usage map.
+func (zm *ZfsManager) refreshDatasetUsage() {
+	if !zm.lastUsageRefresh.IsZero() && time.Since(zm.lastUsageRefresh) < datasetUsageRefreshInterval {
+		return
+	}
+	datasets, err := zm.datasetsFn()
+	if err != nil {
+		slog.Debug("ZFS dataset usage unavailable", "err", err)
+	} else {
+		usage := make(map[string]zfsDatasetUsage, len(datasets))
+		for _, ds := range datasets {
+			if ds.Mountpoint != "" && ds.Mountpoint != "-" {
+				usage[ds.Mountpoint] = zfsDatasetUsage{used: ds.Used, avail: ds.Avail}
+			}
+		}
+		zm.datasetUsage = usage
+	}
+	zm.lastUsageRefresh = time.Now()
+}
+
+// DatasetUsage returns ZFS dataset usage keyed by mountpoint, refreshed at
+// most every datasetUsageRefreshInterval. On failure the previous map is
+// retained and a debug log is emitted.
+func (zm *ZfsManager) DatasetUsage() map[string]zfsDatasetUsage {
+	zm.refreshDatasetUsage()
+	return zm.datasetUsage
+}
+
+// GetDetail returns ZFS detail data (pool health, scrub, vdevs, datasets).
+// Scheduled requests use the cached snapshot until stale; manual requests can
+// force collection. On failure the previous snapshot is retained.
+func (zm *ZfsManager) GetDetail(force bool) *zfsentity.ZfsData {
+	zm.detailMu.Lock()
+	defer zm.detailMu.Unlock()
+
+	if force || zm.detail == nil || time.Since(zm.lastDetailRefresh) >= zm.detailInterval {
+		if data, err := zm.collectDetail(zm.detail); err != nil {
+			slog.Debug("ZFS detail collection failed", "err", err)
+			if zm.detail == nil {
+				return &zfsentity.ZfsData{}
+			}
+			return &zfsentity.ZfsData{Pools: zm.detail.Pools}
+		} else {
+			zm.detail = data
+			zm.lastDetailRefresh = time.Now()
+		}
+	}
+	if zm.detail == nil {
+		return &zfsentity.ZfsData{}
+	}
+	return zm.detail
+}
+
+// collectDetail builds a ZfsData payload from the current system state.
+func (zm *ZfsManager) collectDetail(previous *zfsentity.ZfsData) (*zfsentity.ZfsData, error) {
+	pools, err := zm.poolStatsFn()
+	if err != nil {
+		return nil, err
+	}
+	if len(pools) == 0 {
+		return &zfsentity.ZfsData{Pools: []*zfsentity.PoolDetail{}, Complete: true}, nil
+	}
+
+	statuses, statusErr := zm.poolStatusesFn()
+	if statusErr != nil {
+		slog.Debug("ZFS pool status unavailable", "err", statusErr)
+	}
+	datasets, datasetsErr := zm.datasetsFn()
+	if datasetsErr != nil {
+		slog.Debug("ZFS datasets unavailable", "err", datasetsErr)
+	}
+
+	statusByPool := make(map[string]zfs.PoolStatus, len(statuses))
+	for _, st := range statuses {
+		statusByPool[st.Name] = st
+	}
+
+	previousByPool := make(map[string]*zfsentity.PoolDetail)
+	if previous != nil {
+		for _, pool := range previous.Pools {
+			if pool != nil {
+				previousByPool[pool.Name] = pool
+			}
+		}
+	}
+
+	data := &zfsentity.ZfsData{Pools: make([]*zfsentity.PoolDetail, 0, len(pools)), Complete: true}
+	for i := range pools {
+		p := &pools[i]
+		detail := &zfsentity.PoolDetail{
+			Name:   p.Name,
+			Health: p.Health,
+			Size:   p.Size,
+			Alloc:  p.Alloc,
+			Free:   p.Free,
+		}
+		if st, ok := statusByPool[p.Name]; statusErr == nil && ok {
+			if st.Scrub.State != "" && st.Scrub.State != "NONE" {
+				detail.Scrub = &zfsentity.Scrub{
+					State:    st.Scrub.State,
+					Progress: st.Scrub.Progress,
+					Errors:   st.Scrub.Errors,
+				}
+			}
+			for _, v := range st.Vdevs {
+				detail.Vdevs = append(detail.Vdevs, &zfsentity.Vdev{
+					Name:         v.Name,
+					State:        v.State,
+					ReadErrs:     v.ReadErrs,
+					WriteErrs:    v.WriteErrs,
+					ChecksumErrs: v.ChecksumErrs,
+				})
+			}
+		} else {
+			if cached := previousByPool[p.Name]; cached != nil {
+				detail.Scrub = cached.Scrub
+				detail.Vdevs = cached.Vdevs
+			}
+		}
+		if datasetsErr == nil {
+			foundDataset := false
+			for _, ds := range datasets {
+				if poolOfDataset(ds.Name) == p.Name {
+					foundDataset = true
+					detail.Datasets = append(detail.Datasets, &zfsentity.Dataset{
+						Name:       ds.Name,
+						Used:       ds.Used,
+						Avail:      ds.Avail,
+						Mountpoint: ds.Mountpoint,
+					})
+				}
+			}
+			if !foundDataset {
+				if cached := previousByPool[p.Name]; cached != nil {
+					detail.Datasets = cached.Datasets
+				}
+			}
+		} else if cached := previousByPool[p.Name]; cached != nil {
+			detail.Datasets = cached.Datasets
+		}
+		data.Pools = append(data.Pools, detail)
+	}
+	return data, nil
+}
+
+// poolOfDataset returns the pool name for a dataset name (everything before
+// the first '/'). Datasets without a separator belong to a pool of the same
+// name.
+func poolOfDataset(name string) string {
+	if idx := strings.IndexByte(name, '/'); idx >= 0 {
+		return name[:idx]
+	}
+	return name
+}
+
+// ZfsMountpoints returns the set of mountpoints backed by ZFS datasets.
+func (zm *ZfsManager) ZfsMountpoints() map[string]bool {
+	usage := zm.DatasetUsage()
+	mountpoints := make(map[string]bool, len(usage))
+	for mountpoint := range usage {
+		mountpoints[mountpoint] = true
+	}
+	return mountpoints
+}

agent/zfs_pool_test.go

@@ -0,0 +1,245 @@
+//go:build testing
+
+package agent
+
+import (
+	"testing"
+	"time"
+
+	"github.com/henrygd/beszel/agent/zfs"
+	"github.com/henrygd/beszel/internal/entities/system"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestUpdatePopulatesZfsPools(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		return []zfs.PoolStat{{Name: "tank", Size: 23999000000000, Alloc: 12000000000000, Free: 11999000000000, Health: "DEGRADED"}}, nil
+	}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{
+			{Name: "tank/apps", Used: 5000000000000, Avail: 11999000000000, Mountpoint: "/tank/apps"},
+			{Name: "tank/backup", Used: 6000000000000, Avail: 11999000000000, Mountpoint: "/tank/backup"},
+			// Small zvol (Proxmox VM EFI disk): must not round to zero.
+			{Name: "rpool/vm-100-disk-2", Used: 4194304, Avail: 0, Mountpoint: "-"},
+		}, nil
+	}
+	var kernelCalls int
+	zm.kernelStatsFn = func() ([]zfs.PoolKernelStat, error) {
+		kernelCalls++
+		return []zfs.PoolKernelStat{{
+			Name: "tank", Health: "ONLINE",
+			NRead: uint64(kernelCalls-1) * 1250, NWrite: uint64(kernelCalls-1) * 5120,
+		}}, nil
+	}
+
+	var stats system.Stats
+	// The first kernel sample establishes the cumulative-counter baseline.
+	zm.Update(&stats)
+	zm.kernelSamples["tank"] = poolKernelSample{at: time.Now().Add(-time.Second)}
+	zm.Update(&stats)
+	require.NotNil(t, stats.ZfsPools)
+	require.Contains(t, stats.ZfsPools, "tank")
+	assert.InDelta(t, 22350.8105, stats.ZfsPools["tank"].Total, 0.0001) // Size in GiB
+	assert.InDelta(t, 11175.8709, stats.ZfsPools["tank"].Used, 0.0001)  // Alloc in GiB
+	assert.Equal(t, "ONLINE", stats.ZfsPools["tank"].Health)
+	assert.InDelta(t, 1250, stats.ZfsPools["tank"].ReadBytes, 5)
+	assert.InDelta(t, 5120, stats.ZfsPools["tank"].WriteBytes, 5)
+
+}
+
+// TestUpdateKernelStatsMissing verifies pools without a kernel sample report zero
+// I/O instead of erroring.
+func TestUpdateKernelStatsMissing(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		return []zfs.PoolStat{{Name: "tank", Size: 1, Alloc: 1, Health: "ONLINE"}}, nil
+	}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, nil }
+	zm.kernelStatsFn = func() ([]zfs.PoolKernelStat, error) {
+		return nil, zfs.ErrNoZfs
+	}
+
+	var stats system.Stats
+	zm.Update(&stats)
+	require.NotNil(t, stats.ZfsPools)
+	assert.Equal(t, uint64(0), stats.ZfsPools["tank"].ReadBytes)
+	assert.Equal(t, uint64(0), stats.ZfsPools["tank"].WriteBytes)
+}
+
+func TestUpdateKernelCounterReset(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		return []zfs.PoolStat{{Name: "tank", Health: "ONLINE"}}, nil
+	}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, nil }
+	zm.kernelSamples = map[string]poolKernelSample{
+		"tank": {nread: 100, nwrite: 200, at: time.Now().Add(-time.Second)},
+	}
+	zm.kernelStatsFn = func() ([]zfs.PoolKernelStat, error) {
+		return []zfs.PoolKernelStat{{Name: "tank", Health: "ONLINE", NRead: 10, NWrite: 20}}, nil
+	}
+
+	var stats system.Stats
+	zm.Update(&stats)
+	assert.Equal(t, uint64(0), stats.ZfsPools["tank"].ReadBytes)
+	assert.Equal(t, uint64(0), stats.ZfsPools["tank"].WriteBytes)
+}
+
+func TestUpdateNoZfs(t *testing.T) {
+	zm := &ZfsManager{}
+	calls := 0
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		calls++
+		return nil, zfs.ErrNoZfs
+	}
+
+	var stats system.Stats
+	zm.Update(&stats)
+	zm.Update(&stats)
+	assert.Nil(t, stats.ZfsPools)
+	assert.Equal(t, 1, calls, "failed pool discovery should be cached until the next refresh interval")
+}
+
+func TestUpdateEmptyPools(t *testing.T) {
+	zm := &ZfsManager{}
+	calls := 0
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		calls++
+		return nil, nil
+	}
+
+	var stats system.Stats
+	zm.Update(&stats)
+	zm.Update(&stats)
+	assert.Nil(t, stats.ZfsPools)
+	assert.Equal(t, 1, calls, "an empty pool inventory should be cached until the next refresh interval")
+}
+
+func TestDatasetUsage(t *testing.T) {
+	zm := &ZfsManager{}
+	calls := 0
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		calls++
+		return []zfs.Dataset{
+			{Name: "tank", Used: 12000000000000, Avail: 11999000000000, Mountpoint: "/tank"},
+			{Name: "tank/apps", Used: 1000000000000, Avail: 11999000000000, Mountpoint: "/tank/apps"},
+			{Name: "rpool", Used: 900000000000, Avail: 300000000000, Mountpoint: "-"}, // zvol/unmounted: excluded
+		}, nil
+	}
+
+	usage := zm.DatasetUsage()
+	require.Len(t, usage, 2)
+	assert.Equal(t, zfsDatasetUsage{used: 12000000000000, avail: 11999000000000}, usage["/tank"])
+	assert.Equal(t, zfsDatasetUsage{used: 1000000000000, avail: 11999000000000}, usage["/tank/apps"])
+	assert.Equal(t, 1, calls)
+
+	// Second call within the refresh window must not re-run the collector.
+	zm.DatasetUsage()
+	assert.Equal(t, 1, calls)
+}
+
+func TestDatasetUsageRefreshOnErrorKeepsPrevious(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{{Name: "tank", Used: 1, Avail: 1, Mountpoint: "/tank"}}, nil
+	}
+	assert.Len(t, zm.DatasetUsage(), 1)
+
+	// Force refresh window expiry, then a failing collector.
+	zm.lastUsageRefresh = time.Now().Add(-10 * time.Minute)
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return nil, zfs.ErrNoZfs
+	}
+	usage := zm.DatasetUsage()
+	assert.Len(t, usage, 1, "previous usage should be retained on error")
+}
+
+func TestGetDetailForceRefresh(t *testing.T) {
+	zm := &ZfsManager{detailInterval: time.Hour}
+	poolCalls := 0
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		poolCalls++
+		return []zfs.PoolStat{{Name: "tank", Alloc: uint64(poolCalls)}}, nil
+	}
+	zm.poolStatusesFn = func() ([]zfs.PoolStatus, error) { return nil, nil }
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, nil }
+
+	first := zm.GetDetail(false)
+	assert.True(t, first.Complete)
+	require.Len(t, first.Pools, 1)
+	assert.Equal(t, uint64(1), first.Pools[0].Alloc)
+
+	cached := zm.GetDetail(false)
+	require.Len(t, cached.Pools, 1)
+	assert.Equal(t, uint64(1), cached.Pools[0].Alloc)
+	assert.Equal(t, 1, poolCalls)
+
+	refreshed := zm.GetDetail(true)
+	assert.True(t, refreshed.Complete)
+	require.Len(t, refreshed.Pools, 1)
+	assert.Equal(t, uint64(2), refreshed.Pools[0].Alloc)
+	assert.Equal(t, 2, poolCalls)
+}
+
+func TestGetDetailSuccessfulEmptyInventoryClearsCache(t *testing.T) {
+	zm := &ZfsManager{detailInterval: time.Hour}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		return []zfs.PoolStat{{Name: "tank"}}, nil
+	}
+	zm.poolStatusesFn = func() ([]zfs.PoolStatus, error) { return nil, nil }
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, nil }
+
+	require.Len(t, zm.GetDetail(false).Pools, 1)
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) { return nil, nil }
+	empty := zm.GetDetail(true)
+	assert.True(t, empty.Complete)
+	assert.Empty(t, empty.Pools)
+}
+
+func TestGetDetailFailureReturnsIncompleteCachedInventory(t *testing.T) {
+	zm := &ZfsManager{detailInterval: time.Hour}
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) {
+		return []zfs.PoolStat{{Name: "tank"}}, nil
+	}
+	zm.poolStatusesFn = func() ([]zfs.PoolStatus, error) {
+		return []zfs.PoolStatus{{Name: "tank", Vdevs: []zfs.VdevStatus{{Name: "mirror-0"}}}}, nil
+	}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{{Name: "tank/data"}}, nil
+	}
+	first := zm.GetDetail(false)
+	require.True(t, first.Complete)
+	require.Len(t, first.Pools[0].Vdevs, 1)
+	require.Len(t, first.Pools[0].Datasets, 1)
+
+	zm.poolStatusesFn = func() ([]zfs.PoolStatus, error) { return nil, zfs.ErrNoZfs }
+	zm.datasetsFn = func() ([]zfs.Dataset, error) { return nil, zfs.ErrNoZfs }
+	partial := zm.GetDetail(true)
+	require.True(t, partial.Complete)
+	require.Len(t, partial.Pools[0].Vdevs, 1)
+	require.Len(t, partial.Pools[0].Datasets, 1)
+
+	zm.poolStatsFn = func() ([]zfs.PoolStat, error) { return nil, zfs.ErrNoZfs }
+	lastSuccessfulRefresh := zm.lastDetailRefresh
+	failed := zm.GetDetail(true)
+	assert.False(t, failed.Complete)
+	require.Len(t, failed.Pools, 1)
+	assert.Equal(t, "tank", failed.Pools[0].Name)
+	assert.Equal(t, lastSuccessfulRefresh, zm.lastDetailRefresh)
+}
+
+func TestZfsMountpoints(t *testing.T) {
+	zm := &ZfsManager{}
+	zm.datasetsFn = func() ([]zfs.Dataset, error) {
+		return []zfs.Dataset{
+			{Name: "tank", Mountpoint: "/tank"},
+			{Name: "rpool/ROOT/pve-1", Mountpoint: "/"},
+		}, nil
+	}
+	mountpoints := zm.ZfsMountpoints()
+	assert.Len(t, mountpoints, 2)
+	assert.True(t, mountpoints["/tank"])
+	assert.True(t, mountpoints["/"])
+}