storage: set default value separation minimum size to 256 bytes

Lower the default minimum size that will trigger separation of value from 1 KiB
to 256 bytes. Benchmarking kv0, kv50 and kv95 workloads showed consistent net
benefit with values sized around ~256 bytes.

Epic: none
Release note: none

Author: jbowens

pkg/storage/pebble.go

@@ -430,7 +430,7 @@ var (
 		"storage.value_separation.minimum_size",
 		"the minimum size of a value that will be separated into a blob file",
 		int64(metamorphic.ConstantWithTestRange("storage.value_separation.minimum_size",
-			1<<10 /* 1 KiB (default) */, 25 /* 25 bytes (minimum) */, 1<<20 /* 1 MiB (maximum) */)),
+			256 /* 256 bytes (default) */, 25 /* 25 bytes (minimum) */, 1<<20 /* 1 MiB (maximum) */)),
 		settings.IntWithMinimum(1),
 	)
 	valueSeparationMaxReferenceDepth = settings.RegisterIntSetting(