Add comments and make changes suggested by reviewers

refer #1133

pageserver/src/layered_repository.rs

@@ -1612,6 +1612,7 @@ impl LayeredTimeline {
         let global_layer_map = GLOBAL_LAYER_MAP.read().unwrap();
         if let Some(oldest_layer) = global_layer_map.get(&layer_id) {
             let last_lsn = self.get_last_record_lsn();
+            // Count number of layers only if we nned this information: when creation of image layer was not prohibited
             let n_delta_layers = if reconstruct_pages {
                 layers.count_delta_layers(oldest_layer.get_seg_tag(), last_lsn)
             } else {

pageserver/src/layered_repository/inmemory_layer.rs

@@ -28,6 +28,8 @@ use zenith_utils::vec_map::VecMap;
 
 use super::page_versions::PageVersions;
 
+// The garbage collector needs image layers in order to delete files.
+// If this number is too large it can result in too many small files on disk.
 const MAX_DELTA_LAYERS: usize = 10;
 
 pub struct InMemoryLayer {
@@ -626,7 +628,7 @@ impl InMemoryLayer {
                     > inner.page_versions.size() * 2
                 && n_delta_layers < MAX_DELTA_LAYERS)
         {
-            // The segment was dropped. Create just a delta layer containing all the
+            // Create just a delta layer containing all the
             // changes up to and including the drop.
             delta_end_lsn = Some(end_lsn_exclusive);
             image_lsn = None;

pageserver/src/layered_repository/interval_tree.rs

@@ -249,7 +249,7 @@ where
         loop {
             // Return next remaining element from the current point
             if let Some((point_key, elem_iter)) = &mut self.elem_iter {
-                for elem in elem_iter {
+                while let Some(elem) = elem_iter.next_back() {
                     if elem.start_key() == *point_key {
                         return Some(Arc::clone(elem));
                     }

pageserver/src/layered_repository/layer_map.rs

@@ -328,6 +328,8 @@ impl SegEntry {
             .any(|layer| !layer.is_incremental())
     }
 
+    // Count number of delta layers preceeding specified `lsn`.
+    // Perform backward iteration from exclusive upper bound until image layer is reached.
     pub fn count_delta_layers(&self, lsn: Lsn) -> usize {
         let mut count: usize = 0;
         let mut iter = self.historic.iter_older(lsn);