From 6146a82771f109c7826bcc9f1ae181e1eb7d5473 Mon Sep 17 00:00:00 2001
From: Christian Schwarz <christian@neon.tech>
Date: Tue, 29 Apr 2025 13:27:39 +0200
Subject: [PATCH] incorporate findings from research

---
 pageserver/src/tenant/ephemeral_file.rs              |  2 +-
 .../src/tenant/remote_timeline_client/download.rs    |  2 +-
 pageserver/src/tenant/storage_layer/delta_layer.rs   |  2 +-
 pageserver/src/tenant/storage_layer/image_layer.rs   |  2 +-
 pageserver/src/virtual_file.rs                       | 10 ++++------
 pageserver/src/virtual_file/io_engine.rs             | 12 +++++++++---
 6 files changed, 17 insertions(+), 13 deletions(-)

diff --git a/pageserver/src/tenant/ephemeral_file.rs b/pageserver/src/tenant/ephemeral_file.rs
index 3bbaeb8946..40d06988ad 100644
--- a/pageserver/src/tenant/ephemeral_file.rs
+++ b/pageserver/src/tenant/ephemeral_file.rs
@@ -88,7 +88,7 @@ impl EphemeralFile {
             gate.enter()?,
         );
 
-        file.fallocate_keep_size(0, 1 * 1024 * 1024 * 1024, ctx)
+        file.fallocate(0, 1 * 1024 * 1024 * 1024, ctx)
             .await
             .unwrap();
 
diff --git a/pageserver/src/tenant/remote_timeline_client/download.rs b/pageserver/src/tenant/remote_timeline_client/download.rs
index 0cee73afba..9fcc059b50 100644
--- a/pageserver/src/tenant/remote_timeline_client/download.rs
+++ b/pageserver/src/tenant/remote_timeline_client/download.rs
@@ -109,7 +109,7 @@ pub async fn download_layer_file<'a>(
 64 * 1024 /* TODO this is the max roundtup size by the buffered writer set_len_then_truncate */
 
             )) {
-                temp_file.fallocate_keep_size(
+                temp_file.fallocate(
                     0,
                     file_size,
                     ctx,
diff --git a/pageserver/src/tenant/storage_layer/delta_layer.rs b/pageserver/src/tenant/storage_layer/delta_layer.rs
index 79bb7b3092..b2e2774446 100644
--- a/pageserver/src/tenant/storage_layer/delta_layer.rs
+++ b/pageserver/src/tenant/storage_layer/delta_layer.rs
@@ -440,7 +440,7 @@ impl DeltaLayerWriterInner {
             gate.enter()?,
         );
 
-        file.fallocate_keep_size(0, 1 * 1024 * 1024 * 1024, ctx)
+        file.fallocate(0, 1 * 1024 * 1024 * 1024, ctx)
             .await
             .unwrap();
 
diff --git a/pageserver/src/tenant/storage_layer/image_layer.rs b/pageserver/src/tenant/storage_layer/image_layer.rs
index 9e8e923414..3456a92cc5 100644
--- a/pageserver/src/tenant/storage_layer/image_layer.rs
+++ b/pageserver/src/tenant/storage_layer/image_layer.rs
@@ -806,7 +806,7 @@ impl ImageLayerWriterInner {
             gate.enter()?,
         );
 
-        file.fallocate_keep_size(0, 1 * 1024 * 1024 * 1024, ctx)
+        file.fallocate(0, 1 * 1024 * 1024 * 1024, ctx)
             .await
             .unwrap();
 
diff --git a/pageserver/src/virtual_file.rs b/pageserver/src/virtual_file.rs
index 7df045310d..e663d13400 100644
--- a/pageserver/src/virtual_file.rs
+++ b/pageserver/src/virtual_file.rs
@@ -161,13 +161,13 @@ impl VirtualFile {
         self.inner.set_len(len, ctx).await
     }
 
-    pub async fn fallocate_keep_size(
+    pub async fn fallocate(
         &self,
         offset: i64,
         size: i64,
         ctx: &RequestContext,
     ) -> Result<(), Error> {
-        self.inner.fallocate_keep_size(offset, size, ctx).await
+        self.inner.fallocate(offset, size, ctx).await
     }
 
     pub async fn metadata(&self) -> Result<Metadata, Error> {
@@ -647,16 +647,14 @@ impl VirtualFileInner {
         })
     }
 
-    pub async fn fallocate_keep_size(
+    pub async fn fallocate(
         &self,
         offset: i64,
         size: i64,
         _ctx: &RequestContext,
     ) -> Result<(), Error> {
         with_file!(self, StorageIoOperation::Fallocate, |file_guard| {
-            let (_file_guard, res) = io_engine::get()
-                .fallocate_keep_size(file_guard, offset, size)
-                .await;
+            let (_file_guard, res) = io_engine::get().fallocate(file_guard, offset, size).await;
             res.maybe_fatal_err("fallocate") // TODO haven't thought about this
         })
     }
diff --git a/pageserver/src/virtual_file/io_engine.rs b/pageserver/src/virtual_file/io_engine.rs
index 7128f4b59a..416240e36c 100644
--- a/pageserver/src/virtual_file/io_engine.rs
+++ b/pageserver/src/virtual_file/io_engine.rs
@@ -232,7 +232,7 @@ impl IoEngine {
         }
     }
 
-    pub(super) async fn fallocate_keep_size(
+    pub(super) async fn fallocate(
         &self,
         file_guard: FileGuard,
         offset: i64,
@@ -250,11 +250,17 @@ impl IoEngine {
                 file_guard.with_std_file(|std_file| {
                     fallocate(
                         std_file.as_raw_fd(),
-                        FallocateFlags::FALLOC_FL_KEEP_SIZE,
+                        // NB: if you ever think of using FALLOC_FL_KEEP_SIZE, keep
+                        // in mind that I have found it to be punting to io_uring worker threads
+                        // on Debian Bookworm Linux 6.1.0-32-amd64 and 6.12.25 mainline.
+                        // => https://gist.github.com/problame/ed876bea40b915ba53267b8265e99352
+                        FallocateFlags::empty(),
                         offset,
                         len,
                     )
-                    .expect("TODO")
+                    .expect("TODO");
+                    std_file.sync_all().unwrap();
+                    ()
                 });
                 (file_guard, Ok(()))
             }