Add low-level WAL scan and compaction metrics

CHANGELOG.md

@@ -1,3 +1,7 @@
+## 8.18.6 (unreleased)
+### Implementation changes and bug fixes
+- [PR #1656](https://github.com/rqlite/rqlite/pull/1656): Add low-level WAL compaction timings and metrics.
+
 ## 8.18.5 (January 30th 2024)
 ### Implementation changes and bug fixes
 - [PR #1646](https://github.com/rqlite/rqlite/pull/1646): Expose BUSY TIMEOUT on /status.

db/wal/compacting_scanner.go

@@ -3,9 +3,11 @@ package wal
 import (
 	"encoding/binary"
 	"errors"
+	"expvar"
 	"fmt"
 	"io"
 	"sort"
+	"time"
 )
 
 var (
@@ -77,9 +79,11 @@ func NewCompactingScanner(r io.ReadSeeker, fullScan bool) (*CompactingScanner, e
 		header:     hdr,
 		fullScan:   fullScan,
 	}
+	startT := time.Now()
 	if err := s.scan(); err != nil {
 		return nil, err
 	}
+	stats.Get(compactScanDuration).(*expvar.Int).Set(time.Since(startT).Milliseconds())
 
 	return s, nil
 }
@@ -115,6 +119,7 @@ func (c *CompactingScanner) Next() (*Frame, error) {
 // Bytes returns a byte slice containing the entire contents of the compacted WAL file.
 // The byte slice is suitable for writing to a new WAL file.
 func (c *CompactingScanner) Bytes() ([]byte, error) {
+	startT := time.Now()
 	pageSz := int(c.header.PageSize)
 	buf := make([]byte, WALHeaderSize+(len(c.frames)*WALFrameHeaderSize)+len(c.frames)*pageSz)
 	c.header.Copy(buf)
@@ -157,6 +162,8 @@ func (c *CompactingScanner) Bytes() ([]byte, error) {
 
 		frmHdr += WALFrameHeaderSize + pageSz
 	}
+	stats.Get(compactLoadDuration).(*expvar.Int).Set(time.Since(startT).Milliseconds())
+	stats.Get(compactLoadPageCount).(*expvar.Int).Set(int64(len(c.frames)))
 	return buf, nil
 }
 

db/wal/reader.go

@@ -2,6 +2,7 @@ package wal
 
 import (
 	"encoding/binary"
+	"expvar"
 	"fmt"
 	"io"
 )
@@ -14,6 +15,28 @@ const (
 	WALSupportedVersion = 3007000
 )
 
+const (
+	compactScanDuration  = "compact_index_duration_ms"
+	compactLoadDuration  = "compact_load_duration_ms"
+	compactLoadPageCount = "compact_load_page_count"
+)
+
+// stats captures stats for the DB layer.
+var stats *expvar.Map
+
+func init() {
+	stats = expvar.NewMap("wal")
+	ResetStats()
+}
+
+// ResetStats resets the expvar stats for this module. Mostly for test purposes.
+func ResetStats() {
+	stats.Init()
+	stats.Add(compactScanDuration, 0)
+	stats.Add(compactLoadDuration, 0)
+	stats.Add(compactLoadPageCount, 0)
+}
+
 // Reader wraps an io.Reader and parses SQLite WAL frames.
 //
 // This reader verifies the salt & checksum integrity while it reads. It does