From b9c08d3814685bc8c4a322756e042b9852e321b4 Mon Sep 17 00:00:00 2001
From: Jakob Borg <jakob@kastelo.net>
Date: Fri, 4 Aug 2023 19:57:30 +0200
Subject: [PATCH] all: Add Prometheus-style metrics to expose some internal
 performance counters (fixes #5175) (#9003)

---
 lib/api/api.go                       |  25 ++
 lib/events/events.go                 |   3 +
 lib/events/metrics.go                |  25 ++
 lib/fs/filesystem.go                 |   3 +
 lib/fs/filesystem_copy_range_test.go |  10 +-
 lib/fs/metrics.go                    | 339 +++++++++++++++++++++++++++
 lib/fs/mtimefs_test.go               |   6 +-
 lib/model/folder.go                  |   8 +
 lib/model/folder_sendrecv.go         |  36 +--
 lib/model/folder_sendrecv_test.go    |   2 +-
 lib/model/folder_summary.go          |  18 ++
 lib/model/folderstate.go             |   8 +
 lib/model/metrics.go                 |  93 ++++++++
 lib/model/progressemitter_test.go    |   2 +-
 lib/model/sharedpullerstate.go       |  24 +-
 lib/model/util.go                    |  30 ++-
 lib/protocol/counting.go             |  12 +-
 lib/protocol/metrics.go              |  62 +++++
 lib/protocol/protocol.go             |  23 +-
 lib/scanner/blockqueue.go            |  34 +--
 lib/scanner/metrics.go               |  35 +++
 lib/scanner/virtualfs_test.go        |   2 +-
 lib/scanner/walk.go                  |   9 +-
 lib/scanner/walk_test.go             |   2 +-
 script/find-metrics.go               | 187 +++++++++++++++
 25 files changed, 945 insertions(+), 53 deletions(-)
 create mode 100644 lib/events/metrics.go
 create mode 100644 lib/fs/metrics.go
 create mode 100644 lib/model/metrics.go
 create mode 100644 lib/protocol/metrics.go
 create mode 100644 lib/scanner/metrics.go
 create mode 100644 script/find-metrics.go

diff --git a/lib/api/api.go b/lib/api/api.go
index 1d4116ef7..363c6a162 100644
--- a/lib/api/api.go
+++ b/lib/api/api.go
@@ -32,6 +32,7 @@ import (
 
 	"github.com/calmh/incontainer"
 	"github.com/julienschmidt/httprouter"
+	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/rcrowley/go-metrics"
 	"github.com/thejerf/suture/v4"
 	"github.com/vitrun/qart/qr"
@@ -351,6 +352,15 @@ func (s *service) Serve(ctx context.Context) error {
 	// Handle the special meta.js path
 	mux.HandleFunc("/meta.js", s.getJSMetadata)
 
+	// Handle Prometheus metrics
+	promHttpHandler := promhttp.Handler()
+	mux.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
+		// fetching metrics counts as an event, for the purpose of whether
+		// we should prepare folder summaries etc.
+		s.fss.OnEventRequest()
+		promHttpHandler.ServeHTTP(w, req)
+	})
+
 	guiCfg := s.cfg.GUI()
 
 	// Wrap everything in CSRF protection. The /rest prefix should be
@@ -1214,6 +1224,12 @@ func (s *service) getSupportBundle(w http.ResponseWriter, r *http.Request) {
 		}
 	}
 
+	// Metrics data as text
+	buf := bytes.NewBuffer(nil)
+	wr := bufferedResponseWriter{Writer: buf}
+	promhttp.Handler().ServeHTTP(wr, &http.Request{Method: http.MethodGet})
+	files = append(files, fileEntry{name: "metrics.txt", data: buf.Bytes()})
+
 	// Heap and CPU Proofs as a pprof extension
 	var heapBuffer, cpuBuffer bytes.Buffer
 	filename := fmt.Sprintf("syncthing-heap-%s-%s-%s-%s.pprof", runtime.GOOS, runtime.GOARCH, build.Version, time.Now().Format("150405")) // hhmmss
@@ -2043,3 +2059,12 @@ func httpError(w http.ResponseWriter, err error) {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 	}
 }
+
+type bufferedResponseWriter struct {
+	io.Writer
+}
+
+func (w bufferedResponseWriter) WriteHeader(int) {}
+func (w bufferedResponseWriter) Header() http.Header {
+	return http.Header{}
+}
diff --git a/lib/events/events.go b/lib/events/events.go
index d10c397f4..a3c233761 100644
--- a/lib/events/events.go
+++ b/lib/events/events.go
@@ -297,6 +297,7 @@ loop:
 		case e := <-l.events:
 			// Incoming events get sent
 			l.sendEvent(e)
+			metricEvents.WithLabelValues(e.Type.String(), metricEventStateCreated).Inc()
 
 		case fn := <-l.funcs:
 			// Subscriptions are handled here.
@@ -345,9 +346,11 @@ func (l *logger) sendEvent(e Event) {
 
 			select {
 			case s.events <- e:
+				metricEvents.WithLabelValues(e.Type.String(), metricEventStateDelivered).Inc()
 			case <-l.timeout.C:
 				// if s.events is not ready, drop the event
 				timedOut = true
+				metricEvents.WithLabelValues(e.Type.String(), metricEventStateDropped).Inc()
 			}
 
 			// If stop returns false it already sent something to the
diff --git a/lib/events/metrics.go b/lib/events/metrics.go
new file mode 100644
index 000000000..dd583362a
--- /dev/null
+++ b/lib/events/metrics.go
@@ -0,0 +1,25 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package events
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var metricEvents = promauto.NewCounterVec(prometheus.CounterOpts{
+	Namespace: "syncthing",
+	Subsystem: "events",
+	Name:      "total",
+	Help:      "Total number of created/forwarded/dropped events",
+}, []string{"event", "state"})
+
+const (
+	metricEventStateCreated   = "created"
+	metricEventStateDelivered = "delivered"
+	metricEventStateDropped   = "dropped"
+)
diff --git a/lib/fs/filesystem.go b/lib/fs/filesystem.go
index 970195a93..959407739 100644
--- a/lib/fs/filesystem.go
+++ b/lib/fs/filesystem.go
@@ -28,6 +28,7 @@ const (
 	filesystemWrapperTypeError
 	filesystemWrapperTypeWalk
 	filesystemWrapperTypeLog
+	filesystemWrapperTypeMetrics
 )
 
 type XattrFilter interface {
@@ -275,6 +276,8 @@ func NewFilesystem(fsType FilesystemType, uri string, opts ...Option) Filesystem
 		fs = mtimeOpt.apply(fs)
 	}
 
+	fs = &metricsFS{next: fs}
+
 	if l.ShouldDebug("walkfs") {
 		return NewWalkFilesystem(&logFilesystem{fs})
 	}
diff --git a/lib/fs/filesystem_copy_range_test.go b/lib/fs/filesystem_copy_range_test.go
index 5831753df..04328be0f 100644
--- a/lib/fs/filesystem_copy_range_test.go
+++ b/lib/fs/filesystem_copy_range_test.go
@@ -320,7 +320,15 @@ func TestCopyRange(tttt *testing.T) {
 								t.Fatal(err)
 							}
 
-							if err := impl(src.(basicFile), dst.(basicFile), testCase.srcOffset, testCase.dstOffset, testCase.copySize); err != nil {
+							srcBasic, ok := unwrap(src).(basicFile)
+							if !ok {
+								t.Fatal("src file is not a basic file")
+							}
+							dstBasic, ok := unwrap(dst).(basicFile)
+							if !ok {
+								t.Fatal("dst file is not a basic file")
+							}
+							if err := impl(srcBasic, dstBasic, testCase.srcOffset, testCase.dstOffset, testCase.copySize); err != nil {
 								if err == syscall.ENOTSUP {
 									// Test runner can adjust directory in which to run the tests, that allow broader tests.
 									t.Skip("Not supported on the current filesystem, set STFSTESTPATH env var.")
diff --git a/lib/fs/metrics.go b/lib/fs/metrics.go
new file mode 100644
index 000000000..ffde77721
--- /dev/null
+++ b/lib/fs/metrics.go
@@ -0,0 +1,339 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package fs
+
+import (
+	"context"
+	"time"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+	"github.com/syncthing/syncthing/lib/protocol"
+)
+
+var (
+	metricTotalOperationSeconds = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "fs",
+		Name:      "operation_seconds_total",
+		Help:      "Total time spent in filesystem operations, per filesystem root and operation",
+	}, []string{"root", "operation"})
+	metricTotalOperationsCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "fs",
+		Name:      "operations_total",
+		Help:      "Total number of filesystem operations, per filesystem root and operation",
+	}, []string{"root", "operation"})
+	metricTotalBytesCount = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "fs",
+		Name:      "operation_bytes_total",
+		Help:      "Total number of filesystem bytes transferred, per filesystem root and operation",
+	}, []string{"root", "operation"})
+)
+
+const (
+	// fs operations
+	metricOpChmod             = "chmod"
+	metricOpLchmod            = "lchmod"
+	metricOpChtimes           = "chtimes"
+	metricOpCreate            = "create"
+	metricOpCreateSymlink     = "createsymlink"
+	metricOpDirNames          = "dirnames"
+	metricOpLstat             = "lstat"
+	metricOpMkdir             = "mdkir"
+	metricOpMkdirAll          = "mkdirall"
+	metricOpOpen              = "open"
+	metricOpOpenFile          = "openfile"
+	metricOpReadSymlink       = "readsymlink"
+	metricOpRemove            = "remove"
+	metricOpRemoveAll         = "removeall"
+	metricOpRename            = "rename"
+	metricOpStat              = "stat"
+	metricOpSymlinksSupported = "symlinkssupported"
+	metricOpWalk              = "walk"
+	metricOpWatch             = "watch"
+	metricOpHide              = "hide"
+	metricOpUnhide            = "unhide"
+	metricOpGlob              = "glob"
+	metricOpRoots             = "roots"
+	metricOpUsage             = "usage"
+	metricOpType              = "type"
+	metricOpURI               = "uri"
+	metricOpOptions           = "options"
+	metricOpSameFile          = "samefile"
+	metricOpPlatformData      = "platformdata"
+	metricOpGetXattr          = "getxattr"
+	metricOpSetXattr          = "setxattr"
+
+	// file operations
+	metricOpRead     = "read"
+	metricOpReadAt   = "readat"
+	metricOpWrite    = "write"
+	metricOpWriteAt  = "writeat"
+	metricOpTruncate = "truncate"
+	metricOpSeek     = "seek"
+	metricOpSync     = "sync"
+	metricOpClose    = "close"
+	metricOpName     = "name"
+)
+
+type metricsFS struct {
+	next Filesystem
+}
+
+var _ Filesystem = (*metricsFS)(nil)
+
+func (m *metricsFS) account(op string) func(bytes int) {
+	t0 := time.Now()
+	root := m.next.URI()
+	return func(bytes int) {
+		metricTotalOperationSeconds.WithLabelValues(root, op).Add(time.Since(t0).Seconds())
+		metricTotalOperationsCount.WithLabelValues(root, op).Inc()
+		if bytes >= 0 {
+			metricTotalBytesCount.WithLabelValues(root, op).Add(float64(bytes))
+		}
+	}
+}
+
+func (m *metricsFS) Chmod(name string, mode FileMode) error {
+	defer m.account(metricOpChmod)(-1)
+	return m.next.Chmod(name, mode)
+}
+
+func (m *metricsFS) Lchown(name string, uid, gid string) error {
+	defer m.account(metricOpLchmod)(-1)
+	return m.next.Lchown(name, uid, gid)
+}
+
+func (m *metricsFS) Chtimes(name string, atime time.Time, mtime time.Time) error {
+	defer m.account(metricOpChtimes)(-1)
+	return m.next.Chtimes(name, atime, mtime)
+}
+
+func (m *metricsFS) Create(name string) (File, error) {
+	defer m.account(metricOpCreate)(-1)
+	f, err := m.next.Create(name)
+	if err != nil {
+		return nil, err
+	}
+	return &metricsFile{next: f, fs: m}, nil
+}
+
+func (m *metricsFS) CreateSymlink(target, name string) error {
+	defer m.account(metricOpCreateSymlink)(-1)
+	return m.next.CreateSymlink(target, name)
+}
+
+func (m *metricsFS) DirNames(name string) ([]string, error) {
+	defer m.account(metricOpDirNames)(-1)
+	return m.next.DirNames(name)
+}
+
+func (m *metricsFS) Lstat(name string) (FileInfo, error) {
+	defer m.account(metricOpLstat)(-1)
+	return m.next.Lstat(name)
+}
+
+func (m *metricsFS) Mkdir(name string, perm FileMode) error {
+	defer m.account(metricOpMkdir)(-1)
+	return m.next.Mkdir(name, perm)
+}
+
+func (m *metricsFS) MkdirAll(name string, perm FileMode) error {
+	defer m.account(metricOpMkdirAll)(-1)
+	return m.next.MkdirAll(name, perm)
+}
+
+func (m *metricsFS) Open(name string) (File, error) {
+	defer m.account(metricOpOpen)(-1)
+	f, err := m.next.Open(name)
+	if err != nil {
+		return nil, err
+	}
+	return &metricsFile{next: f, fs: m}, nil
+}
+
+func (m *metricsFS) OpenFile(name string, flags int, mode FileMode) (File, error) {
+	defer m.account(metricOpOpenFile)(-1)
+	f, err := m.next.OpenFile(name, flags, mode)
+	if err != nil {
+		return nil, err
+	}
+	return &metricsFile{next: f, fs: m}, nil
+}
+
+func (m *metricsFS) ReadSymlink(name string) (string, error) {
+	defer m.account(metricOpReadSymlink)(-1)
+	return m.next.ReadSymlink(name)
+}
+
+func (m *metricsFS) Remove(name string) error {
+	defer m.account(metricOpRemove)(-1)
+	return m.next.Remove(name)
+}
+
+func (m *metricsFS) RemoveAll(name string) error {
+	defer m.account(metricOpRemoveAll)(-1)
+	return m.next.RemoveAll(name)
+}
+
+func (m *metricsFS) Rename(oldname, newname string) error {
+	defer m.account(metricOpRename)(-1)
+	return m.next.Rename(oldname, newname)
+}
+
+func (m *metricsFS) Stat(name string) (FileInfo, error) {
+	defer m.account(metricOpStat)(-1)
+	return m.next.Stat(name)
+}
+
+func (m *metricsFS) SymlinksSupported() bool {
+	defer m.account(metricOpSymlinksSupported)(-1)
+	return m.next.SymlinksSupported()
+}
+
+func (m *metricsFS) Walk(name string, walkFn WalkFunc) error {
+	defer m.account(metricOpWalk)(-1)
+	return m.next.Walk(name, walkFn)
+}
+
+func (m *metricsFS) Watch(path string, ignore Matcher, ctx context.Context, ignorePerms bool) (<-chan Event, <-chan error, error) {
+	defer m.account(metricOpWatch)(-1)
+	return m.next.Watch(path, ignore, ctx, ignorePerms)
+}
+
+func (m *metricsFS) Hide(name string) error {
+	defer m.account(metricOpHide)(-1)
+	return m.next.Hide(name)
+}
+
+func (m *metricsFS) Unhide(name string) error {
+	defer m.account(metricOpUnhide)(-1)
+	return m.next.Unhide(name)
+}
+
+func (m *metricsFS) Glob(pattern string) ([]string, error) {
+	defer m.account(metricOpGlob)(-1)
+	return m.next.Glob(pattern)
+}
+
+func (m *metricsFS) Roots() ([]string, error) {
+	defer m.account(metricOpRoots)(-1)
+	return m.next.Roots()
+}
+
+func (m *metricsFS) Usage(name string) (Usage, error) {
+	defer m.account(metricOpUsage)(-1)
+	return m.next.Usage(name)
+}
+
+func (m *metricsFS) Type() FilesystemType {
+	defer m.account(metricOpType)(-1)
+	return m.next.Type()
+}
+
+func (m *metricsFS) URI() string {
+	defer m.account(metricOpURI)(-1)
+	return m.next.URI()
+}
+
+func (m *metricsFS) Options() []Option {
+	defer m.account(metricOpOptions)(-1)
+	return m.next.Options()
+}
+
+func (m *metricsFS) SameFile(fi1, fi2 FileInfo) bool {
+	defer m.account(metricOpSameFile)(-1)
+	return m.next.SameFile(fi1, fi2)
+}
+
+func (m *metricsFS) PlatformData(name string, withOwnership, withXattrs bool, xattrFilter XattrFilter) (protocol.PlatformData, error) {
+	defer m.account(metricOpPlatformData)(-1)
+	return m.next.PlatformData(name, withOwnership, withXattrs, xattrFilter)
+}
+
+func (m *metricsFS) GetXattr(name string, xattrFilter XattrFilter) ([]protocol.Xattr, error) {
+	defer m.account(metricOpGetXattr)(-1)
+	return m.next.GetXattr(name, xattrFilter)
+}
+
+func (m *metricsFS) SetXattr(path string, xattrs []protocol.Xattr, xattrFilter XattrFilter) error {
+	defer m.account(metricOpSetXattr)(-1)
+	return m.next.SetXattr(path, xattrs, xattrFilter)
+}
+
+func (m *metricsFS) underlying() (Filesystem, bool) {
+	return m.next, true
+}
+
+func (m *metricsFS) wrapperType() filesystemWrapperType {
+	return filesystemWrapperTypeMetrics
+}
+
+type metricsFile struct {
+	fs   *metricsFS
+	next File
+}
+
+func (m *metricsFile) Read(p []byte) (n int, err error) {
+	acc := m.fs.account(metricOpRead)
+	defer func() { acc(n) }()
+	return m.next.Read(p)
+}
+
+func (m *metricsFile) ReadAt(p []byte, off int64) (n int, err error) {
+	acc := m.fs.account(metricOpReadAt)
+	defer func() { acc(n) }()
+	return m.next.ReadAt(p, off)
+}
+
+func (m *metricsFile) Seek(offset int64, whence int) (int64, error) {
+	defer m.fs.account(metricOpSeek)(-1)
+	return m.next.Seek(offset, whence)
+}
+
+func (m *metricsFile) Stat() (FileInfo, error) {
+	defer m.fs.account(metricOpStat)(-1)
+	return m.next.Stat()
+}
+
+func (m *metricsFile) Sync() error {
+	defer m.fs.account(metricOpSync)(-1)
+	return m.next.Sync()
+}
+
+func (m *metricsFile) Truncate(size int64) error {
+	defer m.fs.account(metricOpTruncate)(-1)
+	return m.next.Truncate(size)
+}
+
+func (m *metricsFile) Write(p []byte) (n int, err error) {
+	acc := m.fs.account(metricOpWrite)
+	defer func() { acc(n) }()
+	return m.next.Write(p)
+}
+
+func (m *metricsFile) WriteAt(p []byte, off int64) (n int, err error) {
+	acc := m.fs.account(metricOpWriteAt)
+	defer func() { acc(n) }()
+	return m.next.WriteAt(p, off)
+}
+
+func (m *metricsFile) Close() error {
+	defer m.fs.account(metricOpClose)(-1)
+	return m.next.Close()
+}
+
+func (m *metricsFile) Name() string {
+	defer m.fs.account(metricOpName)(-1)
+	return m.next.Name()
+}
+
+func (m *metricsFile) unwrap() File {
+	return m.next
+}
diff --git a/lib/fs/mtimefs_test.go b/lib/fs/mtimefs_test.go
index 1ce5407ec..22f02b383 100644
--- a/lib/fs/mtimefs_test.go
+++ b/lib/fs/mtimefs_test.go
@@ -260,6 +260,8 @@ func newMtimeFS(path string, db database, options ...MtimeFSOption) *mtimeFS {
 }
 
 func newMtimeFSWithWalk(path string, db database, options ...MtimeFSOption) (*mtimeFS, *walkFilesystem) {
-	wfs := NewFilesystem(FilesystemTypeBasic, path, NewMtimeOption(db, options...)).(*walkFilesystem)
-	return wfs.Filesystem.(*mtimeFS), wfs
+	fs := NewFilesystem(FilesystemTypeBasic, path, NewMtimeOption(db, options...))
+	wfs, _ := unwrapFilesystem(fs, filesystemWrapperTypeWalk)
+	mfs, _ := unwrapFilesystem(fs, filesystemWrapperTypeMtime)
+	return mfs.(*mtimeFS), wfs.(*walkFilesystem)
 }
diff --git a/lib/model/folder.go b/lib/model/folder.go
index d71fbd508..b62281562 100644
--- a/lib/model/folder.go
+++ b/lib/model/folder.go
@@ -137,6 +137,9 @@ func newFolder(model *model, fset *db.FileSet, ignores *ignore.Matcher, cfg conf
 	f.pullPause = f.pullBasePause()
 	f.pullFailTimer = time.NewTimer(0)
 	<-f.pullFailTimer.C
+
+	registerFolderMetrics(f.ID)
+
 	return f
 }
 
@@ -459,6 +462,11 @@ func (f *folder) scanSubdirs(subDirs []string) error {
 	}
 	defer f.ioLimiter.Give(1)
 
+	metricFolderScans.WithLabelValues(f.ID).Inc()
+	ctx, cancel := context.WithCancel(f.ctx)
+	defer cancel()
+	go addTimeUntilCancelled(ctx, metricFolderScanSeconds.WithLabelValues(f.ID))
+
 	for i := range subDirs {
 		sub := osutil.NativeFilename(subDirs[i])
 
diff --git a/lib/model/folder_sendrecv.go b/lib/model/folder_sendrecv.go
index 218124c6a..4050f06ff 100644
--- a/lib/model/folder_sendrecv.go
+++ b/lib/model/folder_sendrecv.go
@@ -8,6 +8,7 @@ package model
 
 import (
 	"bytes"
+	"context"
 	"errors"
 	"fmt"
 	"io"
@@ -162,11 +163,12 @@ func (f *sendReceiveFolder) pull() (bool, error) {
 
 	scanChan := make(chan string)
 	go f.pullScannerRoutine(scanChan)
+	defer close(scanChan)
 
-	defer func() {
-		close(scanChan)
-		f.setState(FolderIdle)
-	}()
+	metricFolderPulls.WithLabelValues(f.ID).Inc()
+	ctx, cancel := context.WithCancel(f.ctx)
+	defer cancel()
+	go addTimeUntilCancelled(ctx, metricFolderPullSeconds.WithLabelValues(f.ID))
 
 	changed := 0
 
@@ -573,9 +575,9 @@ func (f *sendReceiveFolder) handleDir(file protocol.FileInfo, snap *db.Snapshot,
 		})
 	}()
 
-	mode := fs.FileMode(file.Permissions & 0777)
+	mode := fs.FileMode(file.Permissions & 0o777)
 	if f.IgnorePerms || file.NoPermissions {
-		mode = 0777
+		mode = 0o777
 	}
 
 	if shouldDebug() {
@@ -705,7 +707,7 @@ func (f *sendReceiveFolder) checkParent(file string, scanChan chan<- string) boo
 		return true
 	}
 	l.Debugf("%v creating parent directory of %v", f, file)
-	if err := f.mtimefs.MkdirAll(parent, 0755); err != nil {
+	if err := f.mtimefs.MkdirAll(parent, 0o755); err != nil {
 		f.newPullError(file, fmt.Errorf("creating parent dir: %w", err))
 		return false
 	}
@@ -1136,12 +1138,12 @@ func (f *sendReceiveFolder) handleFile(file protocol.FileInfo, snap *db.Snapshot
 func (f *sendReceiveFolder) reuseBlocks(blocks []protocol.BlockInfo, reused []int, file protocol.FileInfo, tempName string) ([]protocol.BlockInfo, []int) {
 	// Check for an old temporary file which might have some blocks we could
 	// reuse.
-	tempBlocks, err := scanner.HashFile(f.ctx, f.mtimefs, tempName, file.BlockSize(), nil, false)
+	tempBlocks, err := scanner.HashFile(f.ctx, f.ID, f.mtimefs, tempName, file.BlockSize(), nil, false)
 	if err != nil {
 		var caseErr *fs.ErrCaseConflict
 		if errors.As(err, &caseErr) {
 			if rerr := f.mtimefs.Rename(caseErr.Real, tempName); rerr == nil {
-				tempBlocks, err = scanner.HashFile(f.ctx, f.mtimefs, tempName, file.BlockSize(), nil, false)
+				tempBlocks, err = scanner.HashFile(f.ctx, f.ID, f.mtimefs, tempName, file.BlockSize(), nil, false)
 			}
 		}
 	}
@@ -1235,7 +1237,7 @@ func (f *sendReceiveFolder) shortcutFile(file protocol.FileInfo, dbUpdateChan ch
 	f.queue.Done(file.Name)
 
 	if !f.IgnorePerms && !file.NoPermissions {
-		if err = f.mtimefs.Chmod(file.Name, fs.FileMode(file.Permissions&0777)); err != nil {
+		if err = f.mtimefs.Chmod(file.Name, fs.FileMode(file.Permissions&0o777)); err != nil {
 			f.newPullError(file.Name, fmt.Errorf("shortcut file (setting permissions): %w", err))
 			return
 		}
@@ -1249,7 +1251,7 @@ func (f *sendReceiveFolder) shortcutFile(file protocol.FileInfo, dbUpdateChan ch
 	// Still need to re-write the trailer with the new encrypted fileinfo.
 	if f.Type == config.FolderTypeReceiveEncrypted {
 		err = inWritableDir(func(path string) error {
-			fd, err := f.mtimefs.OpenFile(path, fs.OptReadWrite, 0666)
+			fd, err := f.mtimefs.OpenFile(path, fs.OptReadWrite, 0o666)
 			if err != nil {
 				return err
 			}
@@ -1329,7 +1331,7 @@ func (f *sendReceiveFolder) copierRoutine(in <-chan copyBlocksState, pullChan ch
 				// block of all zeroes, so then we should not skip it.
 
 				// Pretend we copied it.
-				state.copiedFromOrigin()
+				state.skippedSparseBlock(block.Size)
 				state.copyDone(block)
 				continue
 			}
@@ -1348,9 +1350,9 @@ func (f *sendReceiveFolder) copierRoutine(in <-chan copyBlocksState, pullChan ch
 						state.fail(fmt.Errorf("dst write: %w", err))
 					}
 					if offset == block.Offset {
-						state.copiedFromOrigin()
+						state.copiedFromOrigin(block.Size)
 					} else {
-						state.copiedFromOriginShifted()
+						state.copiedFromOriginShifted(block.Size)
 					}
 
 					return false
@@ -1398,7 +1400,9 @@ func (f *sendReceiveFolder) copierRoutine(in <-chan copyBlocksState, pullChan ch
 						state.fail(fmt.Errorf("dst write: %w", err))
 					}
 					if path == state.file.Name {
-						state.copiedFromOrigin()
+						state.copiedFromOrigin(block.Size)
+					} else {
+						state.copiedFromElsewhere(block.Size)
 					}
 					return true
 				})
@@ -1608,7 +1612,7 @@ loop:
 func (f *sendReceiveFolder) performFinish(file, curFile protocol.FileInfo, hasCurFile bool, tempName string, snap *db.Snapshot, dbUpdateChan chan<- dbUpdateJob, scanChan chan<- string) error {
 	// Set the correct permission bits on the new file
 	if !f.IgnorePerms && !file.NoPermissions {
-		if err := f.mtimefs.Chmod(tempName, fs.FileMode(file.Permissions&0777)); err != nil {
+		if err := f.mtimefs.Chmod(tempName, fs.FileMode(file.Permissions&0o777)); err != nil {
 			return fmt.Errorf("setting permissions: %w", err)
 		}
 	}
diff --git a/lib/model/folder_sendrecv_test.go b/lib/model/folder_sendrecv_test.go
index 02a9e6b0b..9b3edffaa 100644
--- a/lib/model/folder_sendrecv_test.go
+++ b/lib/model/folder_sendrecv_test.go
@@ -299,7 +299,7 @@ func TestCopierFinder(t *testing.T) {
 	}
 
 	// Verify that the fetched blocks have actually been written to the temp file
-	blks, err := scanner.HashFile(context.TODO(), f.Filesystem(nil), tempFile, protocol.MinBlockSize, nil, false)
+	blks, err := scanner.HashFile(context.TODO(), f.ID, f.Filesystem(nil), tempFile, protocol.MinBlockSize, nil, false)
 	if err != nil {
 		t.Log(err)
 	}
diff --git a/lib/model/folder_summary.go b/lib/model/folder_summary.go
index 3fed5c55c..708c96ae3 100644
--- a/lib/model/folder_summary.go
+++ b/lib/model/folder_summary.go
@@ -396,6 +396,24 @@ func (c *folderSummaryService) sendSummary(ctx context.Context, folder string) {
 		Summary: data,
 	})
 
+	metricFolderSummary.WithLabelValues(folder, metricScopeGlobal, metricTypeFiles).Set(float64(data.GlobalFiles))
+	metricFolderSummary.WithLabelValues(folder, metricScopeGlobal, metricTypeDirectories).Set(float64(data.GlobalDirectories))
+	metricFolderSummary.WithLabelValues(folder, metricScopeGlobal, metricTypeSymlinks).Set(float64(data.GlobalSymlinks))
+	metricFolderSummary.WithLabelValues(folder, metricScopeGlobal, metricTypeDeleted).Set(float64(data.GlobalDeleted))
+	metricFolderSummary.WithLabelValues(folder, metricScopeGlobal, metricTypeBytes).Set(float64(data.GlobalBytes))
+
+	metricFolderSummary.WithLabelValues(folder, metricScopeLocal, metricTypeFiles).Set(float64(data.LocalFiles))
+	metricFolderSummary.WithLabelValues(folder, metricScopeLocal, metricTypeDirectories).Set(float64(data.LocalDirectories))
+	metricFolderSummary.WithLabelValues(folder, metricScopeLocal, metricTypeSymlinks).Set(float64(data.LocalSymlinks))
+	metricFolderSummary.WithLabelValues(folder, metricScopeLocal, metricTypeDeleted).Set(float64(data.LocalDeleted))
+	metricFolderSummary.WithLabelValues(folder, metricScopeLocal, metricTypeBytes).Set(float64(data.LocalBytes))
+
+	metricFolderSummary.WithLabelValues(folder, metricScopeNeed, metricTypeFiles).Set(float64(data.NeedFiles))
+	metricFolderSummary.WithLabelValues(folder, metricScopeNeed, metricTypeDirectories).Set(float64(data.NeedDirectories))
+	metricFolderSummary.WithLabelValues(folder, metricScopeNeed, metricTypeSymlinks).Set(float64(data.NeedSymlinks))
+	metricFolderSummary.WithLabelValues(folder, metricScopeNeed, metricTypeDeleted).Set(float64(data.NeedDeletes))
+	metricFolderSummary.WithLabelValues(folder, metricScopeNeed, metricTypeBytes).Set(float64(data.NeedBytes))
+
 	for _, devCfg := range c.cfg.Folders()[folder].Devices {
 		select {
 		case <-ctx.Done():
diff --git a/lib/model/folderstate.go b/lib/model/folderstate.go
index 0437e259a..5082a1c51 100644
--- a/lib/model/folderstate.go
+++ b/lib/model/folderstate.go
@@ -111,6 +111,10 @@ func (s *stateTracker) setState(newState folderState) {
 		return
 	}
 
+	defer func() {
+		metricFolderState.WithLabelValues(s.folderID).Set(float64(s.current))
+	}()
+
 	/* This should hold later...
 	if s.current != FolderIdle && (newState == FolderScanning || newState == FolderSyncing) {
 		panic("illegal state transition " + s.current.String() + " -> " + newState.String())
@@ -148,6 +152,10 @@ func (s *stateTracker) setError(err error) {
 	s.mut.Lock()
 	defer s.mut.Unlock()
 
+	defer func() {
+		metricFolderState.WithLabelValues(s.folderID).Set(float64(s.current))
+	}()
+
 	eventData := map[string]interface{}{
 		"folder": s.folderID,
 		"from":   s.current.String(),
diff --git a/lib/model/metrics.go b/lib/model/metrics.go
new file mode 100644
index 000000000..ad211aecb
--- /dev/null
+++ b/lib/model/metrics.go
@@ -0,0 +1,93 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package model
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricFolderState = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_state",
+		Help:      "Current folder state",
+	}, []string{"folder"})
+	metricFolderSummary = promauto.NewGaugeVec(prometheus.GaugeOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_summary",
+		Help:      "Current folder summary data (counts for global/local/need files/directories/symlinks/deleted/bytes)",
+	}, []string{"folder", "scope", "type"})
+
+	metricFolderPulls = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_pulls_total",
+		Help:      "Total number of folder pull iterations, per folder ID",
+	}, []string{"folder"})
+	metricFolderPullSeconds = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_pull_seconds_total",
+		Help:      "Total time spent in folder pull iterations, per folder ID",
+	}, []string{"folder"})
+
+	metricFolderScans = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_scans_total",
+		Help:      "Total number of folder scan iterations, per folder ID",
+	}, []string{"folder"})
+	metricFolderScanSeconds = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_scan_seconds_total",
+		Help:      "Total time spent in folder scan iterations, per folder ID",
+	}, []string{"folder"})
+
+	metricFolderProcessedBytesTotal = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "model",
+		Name:      "folder_processed_bytes_total",
+		Help:      "Total amount of data processed during folder syncing, per folder ID and data source (network/local_origin/local_other/local_shifted/skipped)",
+	}, []string{"folder", "source"})
+)
+
+const (
+	metricSourceNetwork      = "network"       // from the network
+	metricSourceLocalOrigin  = "local_origin"  // from the existing version of the local file
+	metricSourceLocalOther   = "local_other"   // from a different local file
+	metricSourceLocalShifted = "local_shifted" // from the existing version of the local file, rolling hash shifted
+	metricSourceSkipped      = "skipped"       // block of all zeroes, invented out of thin air
+
+	metricScopeGlobal = "global"
+	metricScopeLocal  = "local"
+	metricScopeNeed   = "need"
+
+	metricTypeFiles       = "files"
+	metricTypeDirectories = "directories"
+	metricTypeSymlinks    = "symlinks"
+	metricTypeDeleted     = "deleted"
+	metricTypeBytes       = "bytes"
+)
+
+func registerFolderMetrics(folderID string) {
+	// Register metrics for this folder, so that counters are present even
+	// when zero.
+	metricFolderState.WithLabelValues(folderID)
+	metricFolderPulls.WithLabelValues(folderID)
+	metricFolderPullSeconds.WithLabelValues(folderID)
+	metricFolderScans.WithLabelValues(folderID)
+	metricFolderScanSeconds.WithLabelValues(folderID)
+	metricFolderProcessedBytesTotal.WithLabelValues(folderID, metricSourceNetwork)
+	metricFolderProcessedBytesTotal.WithLabelValues(folderID, metricSourceLocalOrigin)
+	metricFolderProcessedBytesTotal.WithLabelValues(folderID, metricSourceLocalOther)
+	metricFolderProcessedBytesTotal.WithLabelValues(folderID, metricSourceLocalShifted)
+	metricFolderProcessedBytesTotal.WithLabelValues(folderID, metricSourceSkipped)
+}
diff --git a/lib/model/progressemitter_test.go b/lib/model/progressemitter_test.go
index e103a69ae..62d980f8c 100644
--- a/lib/model/progressemitter_test.go
+++ b/lib/model/progressemitter_test.go
@@ -91,7 +91,7 @@ func TestProgressEmitter(t *testing.T) {
 	expectEvent(w, t, 1)
 	expectTimeout(w, t)
 
-	s.copiedFromOrigin()
+	s.copiedFromOrigin(1)
 
 	expectEvent(w, t, 1)
 	expectTimeout(w, t)
diff --git a/lib/model/sharedpullerstate.go b/lib/model/sharedpullerstate.go
index 92598e562..6679909ba 100644
--- a/lib/model/sharedpullerstate.go
+++ b/lib/model/sharedpullerstate.go
@@ -152,11 +152,11 @@ func (s *sharedPullerState) tempFileInWritableDir(_ string) error {
 	// permissions will be set to the final value later, but in the meantime
 	// we don't want to have a temporary file with looser permissions than
 	// the final outcome.
-	mode := fs.FileMode(s.file.Permissions) | 0600
+	mode := fs.FileMode(s.file.Permissions) | 0o600
 	if s.ignorePerms {
 		// When ignorePerms is set we use a very permissive mode and let the
 		// system umask filter it.
-		mode = 0666
+		mode = 0o666
 	}
 
 	// Attempt to create the temp file
@@ -261,19 +261,34 @@ func (s *sharedPullerState) copyDone(block protocol.BlockInfo) {
 	s.mut.Unlock()
 }
 
-func (s *sharedPullerState) copiedFromOrigin() {
+func (s *sharedPullerState) copiedFromOrigin(bytes int) {
 	s.mut.Lock()
 	s.copyOrigin++
 	s.updated = time.Now()
 	s.mut.Unlock()
+	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceLocalOrigin).Add(float64(bytes))
 }
 
-func (s *sharedPullerState) copiedFromOriginShifted() {
+func (s *sharedPullerState) copiedFromElsewhere(bytes int) {
+	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceLocalOther).Add(float64(bytes))
+}
+
+func (s *sharedPullerState) skippedSparseBlock(bytes int) {
+	// pretend we copied it, historical
+	s.mut.Lock()
+	s.copyOrigin++
+	s.updated = time.Now()
+	s.mut.Unlock()
+	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceSkipped).Add(float64(bytes))
+}
+
+func (s *sharedPullerState) copiedFromOriginShifted(bytes int) {
 	s.mut.Lock()
 	s.copyOrigin++
 	s.copyOriginShifted++
 	s.updated = time.Now()
 	s.mut.Unlock()
+	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceLocalShifted).Add(float64(bytes))
 }
 
 func (s *sharedPullerState) pullStarted() {
@@ -295,6 +310,7 @@ func (s *sharedPullerState) pullDone(block protocol.BlockInfo) {
 	s.availableUpdated = time.Now()
 	l.Debugln("sharedPullerState", s.folder, s.file.Name, "pullNeeded done ->", s.pullNeeded)
 	s.mut.Unlock()
+	metricFolderProcessedBytesTotal.WithLabelValues(s.folder, metricSourceNetwork).Add(float64(block.Size))
 }
 
 // finalClose atomically closes and returns closed status of a file. A true
diff --git a/lib/model/util.go b/lib/model/util.go
index 77b542226..2364c5e80 100644
--- a/lib/model/util.go
+++ b/lib/model/util.go
@@ -7,6 +7,7 @@
 package model
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"path/filepath"
@@ -14,6 +15,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/prometheus/client_golang/prometheus"
 	"github.com/syncthing/syncthing/lib/events"
 	"github.com/syncthing/syncthing/lib/fs"
 	"github.com/syncthing/syncthing/lib/ur"
@@ -117,11 +119,11 @@ func inWritableDir(fn func(string) error, targetFs fs.Filesystem, path string, i
 
 	const permBits = fs.ModePerm | fs.ModeSetuid | fs.ModeSetgid | fs.ModeSticky
 	var parentErr error
-	if mode := info.Mode() & permBits; mode&0200 == 0 {
+	if mode := info.Mode() & permBits; mode&0o200 == 0 {
 		// A non-writeable directory (for this user; we assume that's the
 		// relevant part). Temporarily change the mode so we can delete the
 		// file or directory inside it.
-		parentErr = targetFs.Chmod(dir, mode|0700)
+		parentErr = targetFs.Chmod(dir, mode|0o700)
 		if parentErr != nil {
 			l.Debugf("Failed to make parent directory writable: %v", parentErr)
 		} else {
@@ -148,3 +150,27 @@ func inWritableDir(fn func(string) error, targetFs fs.Filesystem, path string, i
 	}
 	return err
 }
+
+// addTimeUntilCancelled adds time to the counter for the duration of the
+// Context. We do this piecemeal so that polling the counter during a long
+// operation shows a relevant value, instead of the counter just increasing
+// by a large amount at the end of the operation.
+func addTimeUntilCancelled(ctx context.Context, counter prometheus.Counter) {
+	t0 := time.Now()
+	defer func() {
+		counter.Add(time.Since(t0).Seconds())
+	}()
+
+	ticker := time.NewTicker(time.Second)
+	defer ticker.Stop()
+
+	for {
+		select {
+		case t := <-ticker.C:
+			counter.Add(t.Sub(t0).Seconds())
+			t0 = t
+		case <-ctx.Done():
+			return
+		}
+	}
+}
diff --git a/lib/protocol/counting.go b/lib/protocol/counting.go
index e54c593ad..d7cba25da 100644
--- a/lib/protocol/counting.go
+++ b/lib/protocol/counting.go
@@ -10,8 +10,9 @@ import (
 
 type countingReader struct {
 	io.Reader
-	tot  atomic.Int64 // bytes
-	last atomic.Int64 // unix nanos
+	idString string
+	tot      atomic.Int64 // bytes
+	last     atomic.Int64 // unix nanos
 }
 
 var (
@@ -24,6 +25,7 @@ func (c *countingReader) Read(bs []byte) (int, error) {
 	c.tot.Add(int64(n))
 	totalIncoming.Add(int64(n))
 	c.last.Store(time.Now().UnixNano())
+	metricDeviceRecvBytes.WithLabelValues(c.idString).Add(float64(n))
 	return n, err
 }
 
@@ -35,8 +37,9 @@ func (c *countingReader) Last() time.Time {
 
 type countingWriter struct {
 	io.Writer
-	tot  atomic.Int64 // bytes
-	last atomic.Int64 // unix nanos
+	idString string
+	tot      atomic.Int64 // bytes
+	last     atomic.Int64 // unix nanos
 }
 
 func (c *countingWriter) Write(bs []byte) (int, error) {
@@ -44,6 +47,7 @@ func (c *countingWriter) Write(bs []byte) (int, error) {
 	c.tot.Add(int64(n))
 	totalOutgoing.Add(int64(n))
 	c.last.Store(time.Now().UnixNano())
+	metricDeviceSentBytes.WithLabelValues(c.idString).Add(float64(n))
 	return n, err
 }
 
diff --git a/lib/protocol/metrics.go b/lib/protocol/metrics.go
new file mode 100644
index 000000000..1f57e51fd
--- /dev/null
+++ b/lib/protocol/metrics.go
@@ -0,0 +1,62 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package protocol
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricDeviceSentBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "sent_bytes_total",
+		Help:      "Total amount of data sent, per device",
+	}, []string{"device"})
+	metricDeviceSentUncompressedBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "sent_uncompressed_bytes_total",
+		Help:      "Total amount of data sent, before compression, per device",
+	}, []string{"device"})
+	metricDeviceSentMessages = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "sent_messages_total",
+		Help:      "Total number of messages sent, per device",
+	}, []string{"device"})
+
+	metricDeviceRecvBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "recv_bytes_total",
+		Help:      "Total amount of data received, per device",
+	}, []string{"device"})
+	metricDeviceRecvDecompressedBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "recv_decompressed_bytes_total",
+		Help:      "Total amount of data received, after decompression, per device",
+	}, []string{"device"})
+	metricDeviceRecvMessages = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "protocol",
+		Name:      "recv_messages_total",
+		Help:      "Total number of messages received, per device",
+	}, []string{"device"})
+)
+
+func registerDeviceMetrics(deviceID string) {
+	// Register metrics for this device, so that counters are present even
+	// when zero.
+	metricDeviceSentBytes.WithLabelValues(deviceID)
+	metricDeviceSentUncompressedBytes.WithLabelValues(deviceID)
+	metricDeviceSentMessages.WithLabelValues(deviceID)
+	metricDeviceRecvBytes.WithLabelValues(deviceID)
+	metricDeviceRecvMessages.WithLabelValues(deviceID)
+}
diff --git a/lib/protocol/protocol.go b/lib/protocol/protocol.go
index 6c6581c1a..9189151c2 100644
--- a/lib/protocol/protocol.go
+++ b/lib/protocol/protocol.go
@@ -183,6 +183,7 @@ type rawConnection struct {
 	ConnectionInfo
 
 	deviceID  DeviceID
+	idString  string
 	model     contextLessModel
 	startTime time.Time
 
@@ -263,12 +264,15 @@ func NewConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, closer
 }
 
 func newRawConnection(deviceID DeviceID, reader io.Reader, writer io.Writer, closer io.Closer, receiver contextLessModel, connInfo ConnectionInfo, compress Compression) *rawConnection {
-	cr := &countingReader{Reader: reader}
-	cw := &countingWriter{Writer: writer}
+	idString := deviceID.String()
+	cr := &countingReader{Reader: reader, idString: idString}
+	cw := &countingWriter{Writer: writer, idString: idString}
+	registerDeviceMetrics(idString)
 
 	return &rawConnection{
 		ConnectionInfo:        connInfo,
 		deviceID:              deviceID,
+		idString:              deviceID.String(),
 		model:                 receiver,
 		cr:                    cr,
 		cw:                    cw,
@@ -445,6 +449,8 @@ func (c *rawConnection) dispatcherLoop() (err error) {
 			return ErrClosed
 		}
 
+		metricDeviceRecvMessages.WithLabelValues(c.idString).Inc()
+
 		msgContext, err := messageContext(msg)
 		if err != nil {
 			return fmt.Errorf("protocol error: %w", err)
@@ -553,6 +559,8 @@ func (c *rawConnection) readMessageAfterHeader(hdr Header, fourByteBuf []byte) (
 
 	// ... and is then unmarshalled
 
+	metricDeviceRecvDecompressedBytes.WithLabelValues(c.idString).Add(float64(4 + len(buf)))
+
 	msg, err := newMessage(hdr.Type)
 	if err != nil {
 		BufferPool.Put(buf)
@@ -593,6 +601,8 @@ func (c *rawConnection) readHeader(fourByteBuf []byte) (Header, error) {
 		return Header{}, fmt.Errorf("unmarshalling header: %w", err)
 	}
 
+	metricDeviceRecvDecompressedBytes.WithLabelValues(c.idString).Add(float64(2 + len(buf)))
+
 	return hdr, nil
 }
 
@@ -758,6 +768,10 @@ func (c *rawConnection) writeMessage(msg message) error {
 	msgContext, _ := messageContext(msg)
 	l.Debugf("Writing %v", msgContext)
 
+	defer func() {
+		metricDeviceSentMessages.WithLabelValues(c.idString).Inc()
+	}()
+
 	size := msg.ProtoSize()
 	hdr := Header{
 		Type: typeOf(msg),
@@ -784,6 +798,8 @@ func (c *rawConnection) writeMessage(msg message) error {
 		}
 	}
 
+	metricDeviceSentUncompressedBytes.WithLabelValues(c.idString).Add(float64(totSize))
+
 	// Header length
 	binary.BigEndian.PutUint16(buf, uint16(hdrSize))
 	// Header
@@ -817,6 +833,9 @@ func (c *rawConnection) writeCompressedMessage(msg message, marshaled []byte) (o
 	}
 
 	cOverhead := 2 + hdrSize + 4
+
+	metricDeviceSentUncompressedBytes.WithLabelValues(c.idString).Add(float64(cOverhead + len(marshaled)))
+
 	// The compressed size may be at most n-n/32 = .96875*n bytes,
 	// I.e., if we can't save at least 3.125% bandwidth, we forgo compression.
 	// This number is arbitrary but cheap to compute.
diff --git a/lib/scanner/blockqueue.go b/lib/scanner/blockqueue.go
index 4e4517628..911b482f0 100644
--- a/lib/scanner/blockqueue.go
+++ b/lib/scanner/blockqueue.go
@@ -16,7 +16,7 @@ import (
 )
 
 // HashFile hashes the files and returns a list of blocks representing the file.
-func HashFile(ctx context.Context, fs fs.Filesystem, path string, blockSize int, counter Counter, useWeakHashes bool) ([]protocol.BlockInfo, error) {
+func HashFile(ctx context.Context, folderID string, fs fs.Filesystem, path string, blockSize int, counter Counter, useWeakHashes bool) ([]protocol.BlockInfo, error) {
 	fd, err := fs.Open(path)
 	if err != nil {
 		l.Debugln("open:", err)
@@ -42,6 +42,8 @@ func HashFile(ctx context.Context, fs fs.Filesystem, path string, blockSize int,
 		return nil, err
 	}
 
+	metricHashedBytes.WithLabelValues(folderID).Add(float64(size))
+
 	// Recheck the size and modtime again. If they differ, the file changed
 	// while we were reading it and our hash results are invalid.
 
@@ -62,22 +64,24 @@ func HashFile(ctx context.Context, fs fs.Filesystem, path string, blockSize int,
 // workers are used in parallel. The outbox will become closed when the inbox
 // is closed and all items handled.
 type parallelHasher struct {
-	fs      fs.Filesystem
-	outbox  chan<- ScanResult
-	inbox   <-chan protocol.FileInfo
-	counter Counter
-	done    chan<- struct{}
-	wg      sync.WaitGroup
+	folderID string
+	fs       fs.Filesystem
+	outbox   chan<- ScanResult
+	inbox    <-chan protocol.FileInfo
+	counter  Counter
+	done     chan<- struct{}
+	wg       sync.WaitGroup
 }
 
-func newParallelHasher(ctx context.Context, fs fs.Filesystem, workers int, outbox chan<- ScanResult, inbox <-chan protocol.FileInfo, counter Counter, done chan<- struct{}) {
+func newParallelHasher(ctx context.Context, folderID string, fs fs.Filesystem, workers int, outbox chan<- ScanResult, inbox <-chan protocol.FileInfo, counter Counter, done chan<- struct{}) {
 	ph := &parallelHasher{
-		fs:      fs,
-		outbox:  outbox,
-		inbox:   inbox,
-		counter: counter,
-		done:    done,
-		wg:      sync.NewWaitGroup(),
+		folderID: folderID,
+		fs:       fs,
+		outbox:   outbox,
+		inbox:    inbox,
+		counter:  counter,
+		done:     done,
+		wg:       sync.NewWaitGroup(),
 	}
 
 	ph.wg.Add(workers)
@@ -104,7 +108,7 @@ func (ph *parallelHasher) hashFiles(ctx context.Context) {
 				panic("Bug. Asked to hash a directory or a deleted file.")
 			}
 
-			blocks, err := HashFile(ctx, ph.fs, f.Name, f.BlockSize(), ph.counter, true)
+			blocks, err := HashFile(ctx, ph.folderID, ph.fs, f.Name, f.BlockSize(), ph.counter, true)
 			if err != nil {
 				handleError(ctx, "hashing", f.Name, err, ph.outbox)
 				continue
diff --git a/lib/scanner/metrics.go b/lib/scanner/metrics.go
new file mode 100644
index 000000000..06c555a78
--- /dev/null
+++ b/lib/scanner/metrics.go
@@ -0,0 +1,35 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+package scanner
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+var (
+	metricHashedBytes = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "scanner",
+		Name:      "hashed_bytes_total",
+		Help:      "Total amount of data hashed, per folder",
+	}, []string{"folder"})
+
+	metricScannedItems = promauto.NewCounterVec(prometheus.CounterOpts{
+		Namespace: "syncthing",
+		Subsystem: "scanner",
+		Name:      "scanned_items_total",
+		Help:      "Total number of items (files/directories) inspected, per folder",
+	}, []string{"folder"})
+)
+
+func registerFolderMetrics(folderID string) {
+	// Register metrics for this folder, so that counters are present even
+	// when zero.
+	metricHashedBytes.WithLabelValues(folderID)
+	metricScannedItems.WithLabelValues(folderID)
+}
diff --git a/lib/scanner/virtualfs_test.go b/lib/scanner/virtualfs_test.go
index 4b97e2e52..d1c5515b5 100644
--- a/lib/scanner/virtualfs_test.go
+++ b/lib/scanner/virtualfs_test.go
@@ -115,7 +115,7 @@ type fakeInfo struct {
 }
 
 func (f fakeInfo) Name() string     { return f.name }
-func (fakeInfo) Mode() fs.FileMode  { return 0755 }
+func (fakeInfo) Mode() fs.FileMode  { return 0o755 }
 func (f fakeInfo) Size() int64      { return f.size }
 func (fakeInfo) ModTime() time.Time { return time.Unix(1234567890, 0) }
 func (f fakeInfo) IsDir() bool {
diff --git a/lib/scanner/walk.go b/lib/scanner/walk.go
index 2d8dde469..c92427a1f 100644
--- a/lib/scanner/walk.go
+++ b/lib/scanner/walk.go
@@ -104,6 +104,7 @@ func newWalker(cfg Config) *walker {
 		w.Matcher = ignore.New(w.Filesystem)
 	}
 
+	registerFolderMetrics(w.Folder)
 	return w
 }
 
@@ -132,7 +133,7 @@ func (w *walker) walk(ctx context.Context) chan ScanResult {
 	// We're not required to emit scan progress events, just kick off hashers,
 	// and feed inputs directly from the walker.
 	if w.ProgressTickIntervalS < 0 {
-		newParallelHasher(ctx, w.Filesystem, w.Hashers, finishedChan, toHashChan, nil, nil)
+		newParallelHasher(ctx, w.Folder, w.Filesystem, w.Hashers, finishedChan, toHashChan, nil, nil)
 		return finishedChan
 	}
 
@@ -163,7 +164,7 @@ func (w *walker) walk(ctx context.Context) chan ScanResult {
 		done := make(chan struct{})
 		progress := newByteCounter()
 
-		newParallelHasher(ctx, w.Filesystem, w.Hashers, finishedChan, realToHashChan, progress, done)
+		newParallelHasher(ctx, w.Folder, w.Filesystem, w.Hashers, finishedChan, realToHashChan, progress, done)
 
 		// A routine which actually emits the FolderScanProgress events
 		// every w.ProgressTicker ticks, until the hasher routines terminate.
@@ -255,6 +256,8 @@ func (w *walker) walkAndHashFiles(ctx context.Context, toHashChan chan<- protoco
 		default:
 		}
 
+		metricScannedItems.WithLabelValues(w.Folder).Inc()
+
 		// Return value used when we are returning early and don't want to
 		// process the item. For directories, this means do-not-descend.
 		var skip error // nil
@@ -599,7 +602,7 @@ func (w *walker) updateFileInfo(dst, src protocol.FileInfo) protocol.FileInfo {
 	if dst.Type == protocol.FileInfoTypeFile && build.IsWindows {
 		// If we have an existing index entry, copy the executable bits
 		// from there.
-		dst.Permissions |= (src.Permissions & 0111)
+		dst.Permissions |= (src.Permissions & 0o111)
 	}
 	dst.Version = src.Version.Update(w.ShortID)
 	dst.ModifiedBy = w.ShortID
diff --git a/lib/scanner/walk_test.go b/lib/scanner/walk_test.go
index 47d5d3f4d..bde35897b 100644
--- a/lib/scanner/walk_test.go
+++ b/lib/scanner/walk_test.go
@@ -635,7 +635,7 @@ func BenchmarkHashFile(b *testing.B) {
 	b.ResetTimer()
 
 	for i := 0; i < b.N; i++ {
-		if _, err := HashFile(context.TODO(), testFs, testdataName, protocol.MinBlockSize, nil, true); err != nil {
+		if _, err := HashFile(context.TODO(), "", testFs, testdataName, protocol.MinBlockSize, nil, true); err != nil {
 			b.Fatal(err)
 		}
 	}
diff --git a/script/find-metrics.go b/script/find-metrics.go
new file mode 100644
index 000000000..dbb6549b0
--- /dev/null
+++ b/script/find-metrics.go
@@ -0,0 +1,187 @@
+// Copyright (C) 2023 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// Usage: go run script/find-metrics.go > metrics.md
+//
+// This script finds all of the metrics in the Syncthing codebase and prints
+// them in Markdown format. It's used to generate the metrics documentation
+// for the Syncthing docs.
+package main
+
+import (
+	"fmt"
+	"go/ast"
+	"go/token"
+	"log"
+	"strconv"
+	"strings"
+
+	"golang.org/x/exp/slices"
+	"golang.org/x/tools/go/packages"
+)
+
+type metric struct {
+	subsystem string
+	name      string
+	help      string
+	kind      string
+}
+
+func main() {
+	opts := &packages.Config{
+		Mode: packages.NeedSyntax | packages.NeedName | packages.NeedTypes | packages.NeedTypesInfo | packages.NeedImports | packages.NeedDeps,
+	}
+
+	pkgs, err := packages.Load(opts, "github.com/syncthing/syncthing/...")
+	if err != nil {
+		log.Fatalln(err)
+	}
+
+	var coll metricCollector
+	for _, pkg := range pkgs {
+		for _, file := range pkg.Syntax {
+			ast.Inspect(file, coll.Visit)
+		}
+	}
+	coll.print()
+}
+
+type metricCollector struct {
+	metrics []metric
+}
+
+func (c *metricCollector) Visit(n ast.Node) bool {
+	if gen, ok := n.(*ast.GenDecl); ok {
+		// We're only interested in var declarations (var metricWhatever =
+		// promauto.NewCounter(...) etc).
+		if gen.Tok != token.VAR {
+			return false
+		}
+
+		for _, spec := range gen.Specs {
+			// We want to look at the value given to a var (the NewCounter()
+			// etc call).
+			if vsp, ok := spec.(*ast.ValueSpec); ok {
+				// There should be only one value.
+				if len(vsp.Values) != 1 {
+					continue
+				}
+
+				// The value should be a function call.
+				call, ok := vsp.Values[0].(*ast.CallExpr)
+				if !ok {
+					continue
+				}
+
+				// The call should be a selector expression
+				// (package.Identifer).
+				sel, ok := call.Fun.(*ast.SelectorExpr)
+				if !ok {
+					continue
+				}
+
+				// The package selector should be `promauto`.
+				selID, ok := sel.X.(*ast.Ident)
+				if !ok || selID.Name != "promauto" {
+					continue
+				}
+
+				// The function should be one of the New* functions.
+				var kind string
+				switch sel.Sel.Name {
+				case "NewCounter":
+					kind = "counter"
+				case "NewGauge":
+					kind = "gauge"
+				case "NewCounterVec":
+					kind = "counter vector"
+				case "NewGaugeVec":
+					kind = "gauge vector"
+				default:
+					continue
+				}
+
+				// The arguments to the function should be a single
+				// composite (struct literal). Grab all of the fields in the
+				// declaration into a map so we can easily access them.
+				args := make(map[string]string)
+				for _, el := range call.Args[0].(*ast.CompositeLit).Elts {
+					kv := el.(*ast.KeyValueExpr)
+					key := kv.Key.(*ast.Ident).Name       // e.g., "Name"
+					val := kv.Value.(*ast.BasicLit).Value // e.g., `"foo"`
+					args[key], _ = strconv.Unquote(val)
+				}
+
+				// Build the full name of the metric from the namespace +
+				// subsystem + name, like Prometheus does.
+				var parts []string
+				if v := args["Namespace"]; v != "" {
+					parts = append(parts, v)
+				}
+				if v := args["Subsystem"]; v != "" {
+					parts = append(parts, v)
+				}
+				if v := args["Name"]; v != "" {
+					parts = append(parts, v)
+				}
+				fullName := strings.Join(parts, "_")
+
+				// Add the metric to the list.
+				c.metrics = append(c.metrics, metric{
+					subsystem: args["Subsystem"],
+					name:      fullName,
+					help:      args["Help"],
+					kind:      kind,
+				})
+			}
+		}
+	}
+	return true
+}
+
+func (c *metricCollector) print() {
+	slices.SortFunc(c.metrics, func(a, b metric) bool {
+		if a.subsystem != b.subsystem {
+			return a.subsystem < b.subsystem
+		}
+		return a.name < b.name
+	})
+
+	var prevSubsystem string
+	for _, m := range c.metrics {
+		if m.subsystem != prevSubsystem {
+			fmt.Printf("## Package `%s`\n\n", m.subsystem)
+			prevSubsystem = m.subsystem
+		}
+		fmt.Printf("### `%v` (%s)\n\n%s\n\n", m.name, m.kind, wordwrap(sentenceize(m.help), 72))
+	}
+}
+
+func sentenceize(s string) string {
+	if s == "" {
+		return ""
+	}
+	if !strings.HasSuffix(s, ".") {
+		return s + "."
+	}
+	return s
+}
+
+func wordwrap(s string, width int) string {
+	var lines []string
+	for _, line := range strings.Split(s, "\n") {
+		for len(line) > width {
+			i := strings.LastIndex(line[:width], " ")
+			if i == -1 {
+				i = width
+			}
+			lines = append(lines, line[:i])
+			line = line[i+1:]
+		}
+		lines = append(lines, line)
+	}
+	return strings.Join(lines, "\n")
+}