vfs: add --vfs-fast-fingerprint for less accurate but faster fingerprints

vfs/help.go

@@ -184,6 +184,38 @@ FAT/exFAT do not. Rclone will perform very badly if the cache
 directory is on a filesystem which doesn't support sparse files and it
 will log an ERROR message if one is detected.
 
+#### Fingerprinting
+
+Various parts of the VFS use fingerprinting to see if a local file
+copy has changed relative to a remote file. Fingerprints are made
+from:
+
+- size
+- modification time
+- hash
+
+where available on an object.
+
+On some backends some of these attributes are slow to read (they take
+an extra API call per object, or extra work per object).
+
+For example !hash! is slow with the !local! and !sftp! backends as
+they have to read the entire file and hash it, and !modtime! is slow
+with the !s3!, !swift!, !ftp! and !qinqstor! backends because they
+need to do an extra API call to fetch it.
+
+If you use the !--vfs-fast-fingerprint! flag then rclone will not
+include the slow operations in the fingerprint. This makes the
+fingerprinting less accurate but much faster and will improve the
+opening time of cached files.
+
+If you are running a vfs cache over !local!, !s3! or !swift! backends
+then using this flag is recommended.
+
+Note that if you change the value of this flag, the fingerprints of
+the files in the cache may be invalidated and the files will need to
+be downloaded again.
+
 ### VFS Chunked Reading
 
 When rclone reads files from a remote it reads them in chunks. This

vfs/vfscache/item.go

@@ -784,7 +784,7 @@ func (item *Item) _checkObject(o fs.Object) error {
 			// OK
 		}
 	} else {
-		remoteFingerprint := fs.Fingerprint(context.TODO(), o, false)
+		remoteFingerprint := fs.Fingerprint(context.TODO(), o, item.c.opt.FastFingerprint)
 		fs.Debugf(item.name, "vfs cache: checking remote fingerprint %q against cached fingerprint %q", remoteFingerprint, item.info.Fingerprint)
 		if item.info.Fingerprint != "" {
 			// remote object && local object
@@ -1159,7 +1159,7 @@ func (item *Item) _updateFingerprint() {
 		return
 	}
 	oldFingerprint := item.info.Fingerprint
-	item.info.Fingerprint = fs.Fingerprint(context.TODO(), item.o, false)
+	item.info.Fingerprint = fs.Fingerprint(context.TODO(), item.o, item.c.opt.FastFingerprint)
 	if oldFingerprint != item.info.Fingerprint {
 		fs.Debugf(item.o, "vfs cache: fingerprint now %q", item.info.Fingerprint)
 	}

vfs/vfscommon/options.go

@@ -33,6 +33,7 @@ type Options struct {
 	WriteBack         time.Duration // time to wait before writing back dirty files
 	ReadAhead         fs.SizeSuffix // bytes to read ahead in cache mode "full"
 	UsedIsSize        bool          // if true, use the `rclone size` algorithm for Used size
+	FastFingerprint   bool          // if set use fast fingerprints
 }
 
 // DefaultOpt is the default values uses for Opt

vfs/vfsflags/vfsflags.go

@@ -38,5 +38,6 @@ func AddFlags(flagSet *pflag.FlagSet) {
 	flags.DurationVarP(flagSet, &Opt.WriteBack, "vfs-write-back", "", Opt.WriteBack, "Time to writeback files after last use when using cache")
 	flags.FVarP(flagSet, &Opt.ReadAhead, "vfs-read-ahead", "", "Extra read ahead over --buffer-size when using cache-mode full")
 	flags.BoolVarP(flagSet, &Opt.UsedIsSize, "vfs-used-is-size", "", Opt.UsedIsSize, "Use the `rclone size` algorithm for Used size")
+	flags.BoolVarP(flagSet, &Opt.FastFingerprint, "vfs-fast-fingerprint", "", Opt.FastFingerprint, "Use fast (less accurate) fingerprints for change detection")
 	platformFlags(flagSet)
 }