convert metacache struct to tuple as its transient (minio#13609)

Additionally re-arrange structs for less alignment size

Author: harshavardhana

cmd/data-scanner.go

@@ -22,6 +22,7 @@ import (
 	"context"
 	"encoding/binary"
 	"errors"
+	"io/fs"
 	"math"
 	"math/rand"
 	"net/http"
@@ -816,14 +817,13 @@ func (f *folderScanner) scanFolder(ctx context.Context, folder cachedFolder, int
 
 // scannerItem represents each file while walking.
 type scannerItem struct {
-	Path string
-	Typ  os.FileMode
-
+	Path        string
 	bucket      string // Bucket.
 	prefix      string // Only the prefix if any, does not have final object name.
 	objectName  string // Only the object name without prefixes.
-	lifeCycle   *lifecycle.Lifecycle
 	replication replicationConfig
+	lifeCycle   *lifecycle.Lifecycle
+	Typ         fs.FileMode
 	heal        bool // Has the object been selected for heal check?
 	debug       bool
 }

cmd/metacache-stream.go

@@ -56,14 +56,13 @@ const metacacheStreamVersion = 2
 
 // metacacheWriter provides a serializer of metacache objects.
 type metacacheWriter struct {
+	streamErr   error
 	mw          *msgp.Writer
 	creator     func() error
 	closer      func() error
 	blockSize   int
+	streamWg    sync.WaitGroup
 	reuseBlocks bool
-
-	streamErr error
-	streamWg  sync.WaitGroup
 }
 
 // newMetacacheWriter will create a serializer that will write objects in given order to the output.

cmd/metacache.go

@@ -56,18 +56,21 @@ const (
 
 // metacache contains a tracked cache entry.
 type metacache struct {
-	id           string     `msg:"id"`
+	// do not re-arrange the struct this struct has been ordered to use less
+	// space - if you do so please run https://github.com/orijtech/structslop
+	// and verify if your changes are optimal.
+	ended        time.Time  `msg:"end"`
+	started      time.Time  `msg:"st"`
+	lastHandout  time.Time  `msg:"lh"`
+	lastUpdate   time.Time  `msg:"u"`
 	bucket       string     `msg:"b"`
-	root         string     `msg:"root"`
-	recursive    bool       `msg:"rec"`
 	filter       string     `msg:"flt"`
-	status       scanStatus `msg:"stat"`
-	fileNotFound bool       `msg:"fnf"`
+	id           string     `msg:"id"`
 	error        string     `msg:"err"`
-	started      time.Time  `msg:"st"`
-	ended        time.Time  `msg:"end"`
-	lastUpdate   time.Time  `msg:"u"`
-	lastHandout  time.Time  `msg:"lh"`
+	root         string     `msg:"root"`
+	fileNotFound bool       `msg:"fnf"`
+	status       scanStatus `msg:"stat"`
+	recursive    bool       `msg:"rec"`
 	dataVersion  uint8      `msg:"v"`
 }