compute read amp for tiered compaction

Signed-off-by: Alex Chi Z <iskyzh@gmail.com>

mini-lsm/src/bin/compaction_simulator.rs

@@ -138,7 +138,7 @@ fn main() {
             level0_file_num_compaction_trigger,
             max_levels,
         } => {
-            let mut controller =
+            let controller =
                 SimpleLeveledCompactionController::new(SimpleLeveledCompactionOptions {
                     size_ratio_percent,
                     level0_file_num_compaction_trigger,
@@ -294,6 +294,16 @@ fn main() {
                     storage.total_flushes,
                     max_space as f64 / storage.total_flushes as f64
                 );
+                println!(
+                    "Read Amplification: {}x",
+                    storage.snapshot.l0_sstables.len()
+                        + storage
+                            .snapshot
+                            .levels
+                            .iter()
+                            .filter(|(_, f)| !f.is_empty())
+                            .count()
+                );
                 println!();
             }
         }

mini-lsm/src/compact/simple_leveled.rs

@@ -26,7 +26,7 @@ impl SimpleLeveledCompactionController {
     }
 
     pub fn generate_compaction_task(
-        &mut self,
+        &self,
         snapshot: &LsmStorageInner,
     ) -> Option<SimpleLeveledCompactionTask> {
         let mut level_sizes = Vec::new();

mini-lsm/src/compact/tiered.rs

@@ -119,6 +119,9 @@ impl TieredCompactionController {
                 levels.push((output[0], output.to_vec()));
             }
         }
+        if !tier_to_remove.is_empty() {
+            unreachable!("some tiers not found??");
+        }
         snapshot.levels = levels;
         (snapshot, files_to_remove)
     }