pageserver: respect no_sync in VirtualFile (#9772)

## Problem

`no_sync` initially just skipped syncfs on startup (#9677). I'm also
interested in flaky tests that time out during pageserver shutdown while
flushing l0s, so to eliminate disk throughput as a source of issues
there,

## Summary of changes

- Drive-by change for test timeouts: add a couple more ::info logs
during pageserver startup so it's obvious which part got stuck.
- Add a SyncMode enum to configure VirtualFile and respect it in
sync_all and sync_data functions
- During pageserver startup, set SyncMode according to `no_sync`

pageserver/benches/bench_ingest.rs

@@ -167,6 +167,7 @@ fn criterion_benchmark(c: &mut Criterion) {
         16384,
         virtual_file::io_engine_for_bench(),
         conf.virtual_file_io_mode,
+        virtual_file::SyncMode::Sync,
     );
     page_cache::init(conf.page_cache_size);
 

pageserver/ctl/src/layer_map_analyzer.rs

@@ -138,6 +138,7 @@ pub(crate) async fn main(cmd: &AnalyzeLayerMapCmd) -> Result<()> {
         10,
         virtual_file::api::IoEngineKind::StdFs,
         IoMode::preferred(),
+        virtual_file::SyncMode::Sync,
     );
     pageserver::page_cache::init(100);
 

pageserver/ctl/src/layers.rs

@@ -51,6 +51,7 @@ async fn read_delta_file(path: impl AsRef<Path>, ctx: &RequestContext) -> Result
         10,
         virtual_file::api::IoEngineKind::StdFs,
         IoMode::preferred(),
+        virtual_file::SyncMode::Sync,
     );
     page_cache::init(100);
     let path = Utf8Path::from_path(path.as_ref()).expect("non-Unicode path");
@@ -65,6 +66,7 @@ async fn read_image_file(path: impl AsRef<Path>, ctx: &RequestContext) -> Result
         10,
         virtual_file::api::IoEngineKind::StdFs,
         IoMode::preferred(),
+        virtual_file::SyncMode::Sync,
     );
     page_cache::init(100);
     let path = Utf8Path::from_path(path.as_ref()).expect("non-Unicode path");
@@ -171,6 +173,7 @@ pub(crate) async fn main(cmd: &LayerCmd) -> Result<()> {
                 10,
                 virtual_file::api::IoEngineKind::StdFs,
                 IoMode::preferred(),
+                virtual_file::SyncMode::Sync,
             );
             pageserver::page_cache::init(100);
 

pageserver/ctl/src/main.rs

@@ -209,6 +209,7 @@ async fn print_layerfile(path: &Utf8Path) -> anyhow::Result<()> {
         10,
         virtual_file::api::IoEngineKind::StdFs,
         IoMode::preferred(),
+        virtual_file::SyncMode::Sync,
     );
     page_cache::init(100);
     let ctx = RequestContext::new(TaskKind::DebugTool, DownloadBehavior::Error);

pageserver/src/bin/pageserver.rs

@@ -171,11 +171,18 @@ fn main() -> anyhow::Result<()> {
     let scenario = failpoint_support::init();
 
     // Basic initialization of things that don't change after startup
+    tracing::info!("Initializing virtual_file...");
     virtual_file::init(
         conf.max_file_descriptors,
         conf.virtual_file_io_engine,
         conf.virtual_file_io_mode,
+        if conf.no_sync {
+            virtual_file::SyncMode::UnsafeNoSync
+        } else {
+            virtual_file::SyncMode::Sync
+        },
     );
+    tracing::info!("Initializing page_cache...");
     page_cache::init(conf.page_cache_size);
 
     start_pageserver(launch_ts, conf).context("Failed to start pageserver")?;

pageserver/src/virtual_file.rs

@@ -175,10 +175,16 @@ impl VirtualFile {
     }
 
     pub async fn sync_all(&self) -> Result<(), Error> {
+        if SYNC_MODE.load(std::sync::atomic::Ordering::Relaxed) == SyncMode::UnsafeNoSync as u8 {
+            return Ok(());
+        }
         self.inner.sync_all().await
     }
 
     pub async fn sync_data(&self) -> Result<(), Error> {
+        if SYNC_MODE.load(std::sync::atomic::Ordering::Relaxed) == SyncMode::UnsafeNoSync as u8 {
+            return Ok(());
+        }
         self.inner.sync_data().await
     }
 
@@ -233,6 +239,27 @@ impl VirtualFile {
     }
 }
 
+/// Indicates whether to enable fsync, fdatasync, or O_SYNC/O_DSYNC when writing
+/// files. Switching this off is unsafe and only used for testing on machines
+/// with slow drives.
+#[repr(u8)]
+pub enum SyncMode {
+    Sync,
+    UnsafeNoSync,
+}
+
+impl TryFrom<u8> for SyncMode {
+    type Error = u8;
+
+    fn try_from(value: u8) -> Result<Self, Self::Error> {
+        Ok(match value {
+            v if v == (SyncMode::Sync as u8) => SyncMode::Sync,
+            v if v == (SyncMode::UnsafeNoSync as u8) => SyncMode::UnsafeNoSync,
+            x => return Err(x),
+        })
+    }
+}
+
 ///
 /// A virtual file descriptor. You can use this just like std::fs::File, but internally
 /// the underlying file is closed if the system is low on file descriptors,
@@ -1332,12 +1359,13 @@ impl OpenFiles {
 /// server startup.
 ///
 #[cfg(not(test))]
-pub fn init(num_slots: usize, engine: IoEngineKind, mode: IoMode) {
+pub fn init(num_slots: usize, engine: IoEngineKind, mode: IoMode, sync_mode: SyncMode) {
     if OPEN_FILES.set(OpenFiles::new(num_slots)).is_err() {
         panic!("virtual_file::init called twice");
     }
     set_io_mode(mode);
     io_engine::init(engine);
+    SYNC_MODE.store(sync_mode as u8, std::sync::atomic::Ordering::Relaxed);
     crate::metrics::virtual_file_descriptor_cache::SIZE_MAX.set(num_slots as u64);
 }
 
@@ -1379,6 +1407,9 @@ pub(crate) fn set_io_mode(mode: IoMode) {
 pub(crate) fn get_io_mode() -> IoMode {
     IoMode::try_from(IO_MODE.load(Ordering::Relaxed)).unwrap()
 }
+
+static SYNC_MODE: AtomicU8 = AtomicU8::new(SyncMode::Sync as u8);
+
 #[cfg(test)]
 mod tests {
     use crate::context::DownloadBehavior;