pageserver: don't evict metadata layers

pageserver/src/tenant/storage_layer/layer_desc.rs

@@ -5,7 +5,7 @@ use utils::{
     lsn::Lsn,
 };
 
-use crate::repository::Key;
+use crate::{pgdatadir_mapping::METADATA_CUT, repository::Key};
 
 use super::{DeltaFileName, ImageFileName, LayerFileName};
 
@@ -49,6 +49,20 @@ impl PersistentLayerDesc {
         }
     }
 
+    /// Does this layer consist exclusively of metadata
+    /// content such as dbdir & relation sizes?  This is a
+    /// hint that the layer is likely to be small and should
+    /// not be a candidate for eviction under normal circumstances.
+    pub fn is_metadata_pages(&self) -> bool {
+        self.key_range.start >= METADATA_CUT
+    }
+
+    /// Does this layer consist exclusively of content
+    /// required to serve a basebackup request?
+    pub fn is_basebackup_pages(&self) -> bool {
+        self.key_range.start >= METADATA_CUT
+    }
+
     pub fn short_id(&self) -> impl Display {
         self.filename()
     }

pageserver/src/tenant/timeline.rs

@@ -4274,6 +4274,11 @@ impl Timeline {
             let file_size = l.file_size();
             max_layer_size = max_layer_size.map_or(Some(file_size), |m| Some(m.max(file_size)));
 
+            // Don't evict small layers required to serve a basebackup
+            if l.is_basebackup_pages() {
+                continue;
+            }
+
             let l = guard.get_from_desc(&l);
 
             let l = match l.keep_resident().await {

pageserver/src/tenant/timeline/eviction_task.rs

@@ -197,6 +197,11 @@ impl Timeline {
             let layers = guard.layer_map();
             let mut candidates = Vec::new();
             for hist_layer in layers.iter_historic_layers() {
+                // Don't evict the small layers needed to serve a basebackup request.
+                if hist_layer.is_basebackup_pages() {
+                    continue;
+                }
+
                 let hist_layer = guard.get_from_desc(&hist_layer);
 
                 // guard against eviction while we inspect it; it might be that eviction_task and