add compatibility logic during convert table opts pb to table opts.

src/analytic_engine/src/memtable/mod.rs

@@ -103,9 +103,24 @@ impl Default for LayeredMemtableOptions {
 
 impl From<manifest::LayeredMemtableOptions> for LayeredMemtableOptions {
     fn from(value: manifest::LayeredMemtableOptions) -> Self {
+        let mutable_segment_switch_threshold = ReadableSize(value.mutable_segment_switch_threshold);
+
         let enable = match value.enable {
             Some(Enable::EnableOpt(enable)) => enable,
-            None => false,
+            None => {
+                // For compatibility, mutable_segment_switch_threshold is used to decision
+                // whether use the layered memtable in former.
+                // So this situation is possible to exist:
+                //
+                //  mutable_segment_switch_threshold > 0 and enable none
+                //
+                // And in this situation, layered memtable should be used.
+                if mutable_segment_switch_threshold.0 > 0 {
+                    true
+                } else {
+                    false
+                }
+            }
         };
 
         Self {