Hardlink resident layers during detach ancestor (#10729)

After a detach ancestor operation, we don't want to on-demand download
layers that are already resident. This has shown to impede performance,
sometimes quite a lot (50 seconds:
https://github.com/neondatabase/neon/issues/8828#issuecomment-2643735644)

Fixes #8828.

pageserver/src/tenant/storage_layer/layer.rs

@@ -353,7 +353,6 @@ impl Layer {
     /// while the guard exists.
     ///
     /// Returns None if the layer is currently evicted or becoming evicted.
-    #[cfg(test)]
     pub(crate) async fn keep_resident(&self) -> Option<ResidentLayer> {
         let downloaded = self.0.inner.get().and_then(|rowe| rowe.get())?;
 
@@ -530,7 +529,6 @@ impl ResidentOrWantedEvicted {
     /// This is not used on the read path (anything that calls
     /// [`LayerInner::get_or_maybe_download`]) because it was decided that reads always win
     /// evictions, and part of that winning is using [`ResidentOrWantedEvicted::get_and_upgrade`].
-    #[cfg(test)]
     fn get(&self) -> Option<Arc<DownloadedLayer>> {
         match self {
             ResidentOrWantedEvicted::Resident(strong) => Some(strong.clone()),

pageserver/src/tenant/timeline/detach_ancestor.rs

@@ -6,7 +6,9 @@ use crate::{
     task_mgr::TaskKind,
     tenant::{
         remote_timeline_client::index::GcBlockingReason::DetachAncestor,
-        storage_layer::{AsLayerDesc as _, DeltaLayerWriter, Layer, ResidentLayer},
+        storage_layer::{
+            layer::local_layer_path, AsLayerDesc as _, DeltaLayerWriter, Layer, ResidentLayer,
+        },
         Tenant,
     },
     virtual_file::{MaybeFatalIo, VirtualFile},
@@ -351,18 +353,7 @@ pub(super) async fn prepare(
 
         // FIXME: the fsync should be mandatory, after both rewrites and copies
         if wrote_any {
-            let timeline_dir = VirtualFile::open(
-                &detached
-                    .conf
-                    .timeline_path(&detached.tenant_shard_id, &detached.timeline_id),
-                ctx,
-            )
-            .await
-            .fatal_err("VirtualFile::open for timeline dir fsync");
-            timeline_dir
-                .sync_all()
-                .await
-                .fatal_err("VirtualFile::sync_all timeline dir");
+            fsync_timeline_dir(detached, ctx).await;
         }
     }
 
@@ -376,7 +367,7 @@ pub(super) async fn prepare(
         tasks.spawn(
             async move {
                 let _permit = limiter.acquire().await;
-                let owned = remote_copy(
+                let (owned, did_hardlink) = remote_copy(
                     &adopted,
                     &timeline,
                     timeline.generation,
@@ -384,16 +375,20 @@ pub(super) async fn prepare(
                     &timeline.cancel,
                 )
                 .await?;
-                tracing::info!(layer=%owned, "remote copied");
-                Ok(owned)
+                tracing::info!(layer=%owned, did_hard_link=%did_hardlink, "remote copied");
+                Ok((owned, did_hardlink))
             }
             .in_current_span(),
         );
     }
 
+    let mut should_fsync = false;
     while let Some(res) = tasks.join_next().await {
         match res {
-            Ok(Ok(owned)) => {
+            Ok(Ok((owned, did_hardlink))) => {
+                if did_hardlink {
+                    should_fsync = true;
+                }
                 new_layers.push(owned);
             }
             Ok(Err(failed)) => {
@@ -403,7 +398,10 @@ pub(super) async fn prepare(
         }
     }
 
-    // TODO: fsync directory again if we hardlinked something
+    // fsync directory again if we hardlinked something
+    if should_fsync {
+        fsync_timeline_dir(detached, ctx).await;
+    }
 
     let prepared = PreparedTimelineDetach { layers: new_layers };
 
@@ -629,35 +627,52 @@ async fn copy_lsn_prefix(
     }
 }
 
-/// Creates a new Layer instance for the adopted layer, and ensures it is found from the remote
-/// storage on successful return without the adopted layer being added to `index_part.json`.
+/// Creates a new Layer instance for the adopted layer, and ensures it is found in the remote
+/// storage on successful return. without the adopted layer being added to `index_part.json`.
+/// Returns (Layer, did hardlink)
 async fn remote_copy(
     adopted: &Layer,
     adoptee: &Arc<Timeline>,
     generation: Generation,
     shard_identity: ShardIdentity,
     cancel: &CancellationToken,
-) -> Result<Layer, Error> {
-    // depending if Layer::keep_resident we could hardlink
-
+) -> Result<(Layer, bool), Error> {
     let mut metadata = adopted.metadata();
     debug_assert!(metadata.generation <= generation);
     metadata.generation = generation;
     metadata.shard = shard_identity.shard_index();
 
-    let owned = crate::tenant::storage_layer::Layer::for_evicted(
-        adoptee.conf,
-        adoptee,
-        adopted.layer_desc().layer_name(),
-        metadata,
-    );
+    let conf = adoptee.conf;
+    let file_name = adopted.layer_desc().layer_name();
 
-    adoptee
+    // depending if Layer::keep_resident, do a hardlink
+    let did_hardlink;
+    let owned = if let Some(adopted_resident) = adopted.keep_resident().await {
+        let adopted_path = adopted_resident.local_path();
+        let adoptee_path = local_layer_path(
+            conf,
+            &adoptee.tenant_shard_id,
+            &adoptee.timeline_id,
+            &file_name,
+            &metadata.generation,
+        );
+        std::fs::hard_link(adopted_path, &adoptee_path)
+            .map_err(|e| Error::launder(e.into(), Error::Prepare))?;
+        did_hardlink = true;
+        Layer::for_resident(conf, adoptee, adoptee_path, file_name, metadata).drop_eviction_guard()
+    } else {
+        did_hardlink = false;
+        Layer::for_evicted(conf, adoptee, file_name, metadata)
+    };
+
+    let layer = adoptee
         .remote_client
         .copy_timeline_layer(adopted, &owned, cancel)
         .await
         .map(move |()| owned)
-        .map_err(|e| Error::launder(e, Error::Prepare))
+        .map_err(|e| Error::launder(e, Error::Prepare))?;
+
+    Ok((layer, did_hardlink))
 }
 
 pub(crate) enum DetachingAndReparenting {
@@ -1001,3 +1016,16 @@ fn check_no_archived_children_of_ancestor(
     }
     Ok(())
 }
+
+async fn fsync_timeline_dir(timeline: &Timeline, ctx: &RequestContext) {
+    let path = &timeline
+        .conf
+        .timeline_path(&timeline.tenant_shard_id, &timeline.timeline_id);
+    let timeline_dir = VirtualFile::open(&path, ctx)
+        .await
+        .fatal_err("VirtualFile::open for timeline dir fsync");
+    timeline_dir
+        .sync_all()
+        .await
+        .fatal_err("VirtualFile::sync_all timeline dir");
+}