WIP: try to eliminate the raw_vec::finish_grow and bytes::promotable_even-drop

This one doesn't make a big difference.

pageserver/src/tenant/storage_layer.rs

@@ -65,6 +65,7 @@ where
 pub struct ValueReconstructState {
     pub records: Vec<(Lsn, NeonWalRecord)>,
     pub img: Option<(Lsn, Bytes)>,
+    pub(crate) scratch: Vec<u8>,
 }
 
 /// Return value from [`Layer::get_value_reconstruct_data`]

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -778,15 +778,14 @@ impl DeltaLayerInner {
 
         // Ok, 'offsets' now contains the offsets of all the entries we need to read
         let cursor = file.block_cursor();
-        let mut buf = Vec::new();
         for (entry_lsn, pos) in offsets {
             cursor
-                .read_blob_into_buf(pos, &mut buf, ctx)
+                .read_blob_into_buf(pos, &mut reconstruct_state.scratch, ctx)
                 .await
                 .with_context(|| {
                     format!("Failed to read blob from virtual file {}", file.file.path)
                 })?;
-            let val = Value::des(&buf).with_context(|| {
+            let val = Value::des(&reconstruct_state.scratch).with_context(|| {
                 format!(
                     "Failed to deserialize file blob from virtual file {}",
                     file.file.path

pageserver/src/tenant/timeline.rs

@@ -601,6 +601,7 @@ impl Timeline {
         let mut reconstruct_state = ValueReconstructState {
             records: Vec::new(),
             img: None,
+            scratch: Vec::with_capacity(2 * 8192), // for good measure
         };
 
         let timer = crate::metrics::GET_RECONSTRUCT_DATA_TIME.start_timer();