Tweak the database block cache size, and add config for it

cmd/syncthing/main.go

@@ -715,22 +715,21 @@ func syncthingMain() {
 }
 
 func dbOpts() *opt.Options {
-	// Calculate a sutiable database block cache capacity. We start at the
-	// default of 8 MiB and use larger values for machines with more memory.
+	// Calculate a sutiable database block cache capacity. Default is 8 MiB.
 	// In reality, the database will use twice the amount we calculate here,
 	// as it also has two write buffers each sized at half the block cache.
-
 	blockCacheCapacity := 8 << 20
-	if bytes, err := memorySize(); err == nil {
-		if bytes > 74<<30 {
-			// At 74 GiB of RAM, we hit a 256 MiB block cache (per the
-			// calculations below). There's probably no point in growing the
-			// cache beyond this point.
-			blockCacheCapacity = 256 << 20
-		} else if bytes > 8<<30 {
-			// Slowly grow from 128 MiB at 8 GiB of RAM up to 256 MiB for a
-			// ~74 GiB RAM machine
-			blockCacheCapacity = int(bytes/512) + 128 - 16
+
+	if v := cfg.Options().DatabaseBlockCacheMiB; v != 0 {
+		// Use the value from the config, if it's set.
+		blockCacheCapacity = v << 20
+	} else if bytes, err := memorySize(); err == nil {
+		// We start at the default of 8 MiB and use larger values for machines
+		// with more memory.
+
+		if bytes > 8<<30 {
+			// Cap the cache at 128 MB when we reach 8 GiB of RAM
+			blockCacheCapacity = 128 << 20
 		} else if bytes > 512<<20 {
 			// Grow from 8 MiB at start to 128 MiB of cache at 8 GiB of RAM.
 			blockCacheCapacity = int(bytes / 64)

internal/config/config.go

@@ -240,6 +240,7 @@ type OptionsConfiguration struct {
 	ProgressUpdateIntervalS int      `xml:"progressUpdateIntervalS" json:"progressUpdateIntervalS" default:"5"`
 	SymlinksEnabled         bool     `xml:"symlinksEnabled" json:"symlinksEnabled" default:"true"`
 	LimitBandwidthInLan     bool     `xml:"limitBandwidthInLan" json:"limitBandwidthInLan" default:"false"`
+	DatabaseBlockCacheMiB   int      `xml:"databaseBlockCacheMiB" json:"databaseBlockCacheMiB" default:"0"`
 }
 
 func (orig OptionsConfiguration) Copy() OptionsConfiguration {

internal/config/config_test.go

@@ -52,6 +52,7 @@ func TestDefaultValues(t *testing.T) {
 		ProgressUpdateIntervalS: 5,
 		SymlinksEnabled:         true,
 		LimitBandwidthInLan:     false,
+		DatabaseBlockCacheMiB:   0,
 	}
 
 	cfg := New(device1)
@@ -158,6 +159,7 @@ func TestOverriddenValues(t *testing.T) {
 		ProgressUpdateIntervalS: 10,
 		SymlinksEnabled:         false,
 		LimitBandwidthInLan:     true,
+		DatabaseBlockCacheMiB:   42,
 	}
 
 	cfg, err := Load("testdata/overridenvalues.xml", device1)

internal/config/testdata/overridenvalues.xml

@@ -23,5 +23,6 @@
         <progressUpdateIntervalS>10</progressUpdateIntervalS>
         <symlinksEnabled>false</symlinksEnabled>
         <limitBandwidthInLan>true</limitBandwidthInLan>
+        <databaseBlockCacheMiB>42</databaseBlockCacheMiB>
     </options>
 </configuration>