From 12545b4b6dd17b81d5069b7bb669d7568b047fef Mon Sep 17 00:00:00 2001
From: henrygd <hank@henrygd.me>
Date: Tue, 24 Feb 2026 14:36:53 -0500
Subject: [PATCH] fix: dedupe root-mirrored extra filesystems during disk
 discovery (#1428)

---
 agent/disk.go      | 51 ++++++++++++++++++++++++++++++++++++++++++++++
 agent/disk_test.go | 34 +++++++++++++++++++++++++++++++
 2 files changed, 85 insertions(+)

diff --git a/agent/disk.go b/agent/disk.go
index 5a30bb6b..6df5d73f 100644
--- a/agent/disk.go
+++ b/agent/disk.go
@@ -197,9 +197,60 @@ func (a *Agent) initializeDiskInfo() {
 		a.fsStats[rootKey] = &system.FsStats{Root: true, Mountpoint: rootMountPoint}
 	}
 
+	a.pruneDuplicateRootExtraFilesystems()
 	a.initializeDiskIoStats(diskIoCounters)
 }
 
+// Removes extra filesystems that mirror root usage (https://github.com/henrygd/beszel/issues/1428).
+func (a *Agent) pruneDuplicateRootExtraFilesystems() {
+	var rootMountpoint string
+	for _, stats := range a.fsStats {
+		if stats != nil && stats.Root {
+			rootMountpoint = stats.Mountpoint
+			break
+		}
+	}
+	if rootMountpoint == "" {
+		return
+	}
+	rootUsage, err := disk.Usage(rootMountpoint)
+	if err != nil {
+		return
+	}
+	for name, stats := range a.fsStats {
+		if stats == nil || stats.Root {
+			continue
+		}
+		extraUsage, err := disk.Usage(stats.Mountpoint)
+		if err != nil {
+			continue
+		}
+		if hasSameDiskUsage(rootUsage, extraUsage) {
+			slog.Info("Ignoring duplicate FS", "name", name, "mount", stats.Mountpoint)
+			delete(a.fsStats, name)
+		}
+	}
+}
+
+// hasSameDiskUsage compares root/extra usage with a small byte tolerance.
+func hasSameDiskUsage(a, b *disk.UsageStat) bool {
+	if a == nil || b == nil || a.Total == 0 || b.Total == 0 {
+		return false
+	}
+	// Allow minor drift between sequential disk usage calls.
+	const toleranceBytes uint64 = 16 * 1024 * 1024
+	return withinUsageTolerance(a.Total, b.Total, toleranceBytes) &&
+		withinUsageTolerance(a.Used, b.Used, toleranceBytes)
+}
+
+// withinUsageTolerance reports whether two byte values differ by at most tolerance.
+func withinUsageTolerance(a, b, tolerance uint64) bool {
+	if a >= b {
+		return a-b <= tolerance
+	}
+	return b-a <= tolerance
+}
+
 // Returns matching device from /proc/diskstats.
 // bool is true if a match was found.
 func findIoDevice(filesystem string, diskIoCounters map[string]disk.IOCountersStat) (string, bool) {
diff --git a/agent/disk_test.go b/agent/disk_test.go
index 7ca58cae..b4a58624 100644
--- a/agent/disk_test.go
+++ b/agent/disk_test.go
@@ -372,3 +372,37 @@ func TestDiskUsageCaching(t *testing.T) {
 			"lastDiskUsageUpdate should be refreshed when cache expires")
 	})
 }
+
+func TestHasSameDiskUsage(t *testing.T) {
+	const toleranceBytes uint64 = 16 * 1024 * 1024
+
+	t.Run("returns true when totals and usage are equal", func(t *testing.T) {
+		a := &disk.UsageStat{Total: 100 * 1024 * 1024 * 1024, Used: 42 * 1024 * 1024 * 1024}
+		b := &disk.UsageStat{Total: 100 * 1024 * 1024 * 1024, Used: 42 * 1024 * 1024 * 1024}
+		assert.True(t, hasSameDiskUsage(a, b))
+	})
+
+	t.Run("returns true within tolerance", func(t *testing.T) {
+		a := &disk.UsageStat{Total: 100 * 1024 * 1024 * 1024, Used: 42 * 1024 * 1024 * 1024}
+		b := &disk.UsageStat{
+			Total: a.Total + toleranceBytes - 1,
+			Used:  a.Used - toleranceBytes + 1,
+		}
+		assert.True(t, hasSameDiskUsage(a, b))
+	})
+
+	t.Run("returns false when total exceeds tolerance", func(t *testing.T) {
+		a := &disk.UsageStat{Total: 100 * 1024 * 1024 * 1024, Used: 42 * 1024 * 1024 * 1024}
+		b := &disk.UsageStat{
+			Total: a.Total + toleranceBytes + 1,
+			Used:  a.Used,
+		}
+		assert.False(t, hasSameDiskUsage(a, b))
+	})
+
+	t.Run("returns false for nil or zero total", func(t *testing.T) {
+		assert.False(t, hasSameDiskUsage(nil, &disk.UsageStat{Total: 1, Used: 1}))
+		assert.False(t, hasSameDiskUsage(&disk.UsageStat{Total: 1, Used: 1}, nil))
+		assert.False(t, hasSameDiskUsage(&disk.UsageStat{Total: 0, Used: 0}, &disk.UsageStat{Total: 1, Used: 1}))
+	})
+}