Merge branch 'yuchen/direct-io-aligned-alloc' into yuchen/direct-io-aligned-alloc-usage-wip

Signed-off-by: Yuchen Liang <yuchen@neon.tech>

libs/pageserver_api/src/config.rs

@@ -104,7 +104,7 @@ pub struct ConfigToml {
     pub image_compression: ImageCompressionAlgorithm,
     pub ephemeral_bytes_per_memory_kb: usize,
     pub l0_flush: Option<crate::models::L0FlushConfig>,
-    pub virtual_file_direct_io: crate::models::virtual_file::DirectIoMode,
+    pub virtual_file_io_mode: Option<crate::models::virtual_file::IoMode>,
     pub io_buffer_alignment: usize,
 }
 
@@ -381,7 +381,7 @@ impl Default for ConfigToml {
             image_compression: (DEFAULT_IMAGE_COMPRESSION),
             ephemeral_bytes_per_memory_kb: (DEFAULT_EPHEMERAL_BYTES_PER_MEMORY_KB),
             l0_flush: None,
-            virtual_file_direct_io: crate::models::virtual_file::DirectIoMode::default(),
+            virtual_file_io_mode: None,
 
             io_buffer_alignment: DEFAULT_IO_BUFFER_ALIGNMENT,
 

libs/pageserver_api/src/models.rs

@@ -972,8 +972,6 @@ pub struct TopTenantShardsResponse {
 }
 
 pub mod virtual_file {
-    use std::path::PathBuf;
-
     #[derive(
         Copy,
         Clone,
@@ -994,50 +992,49 @@ pub mod virtual_file {
     }
 
     /// Direct IO modes for a pageserver.
-    #[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize, serde::Serialize, Default)]
-    #[serde(tag = "mode", rename_all = "kebab-case", deny_unknown_fields)]
-    pub enum DirectIoMode {
-        /// Direct IO disabled (uses usual buffered IO).
-        #[default]
-        Disabled,
-        /// Direct IO disabled (performs checks and perf simulations).
-        Evaluate {
-            /// Alignment check level
-            alignment_check: DirectIoAlignmentCheckLevel,
-            /// Latency padded for performance simulation.
-            latency_padding: DirectIoLatencyPadding,
-        },
-        /// Direct IO enabled.
-        Enabled {
-            /// Actions to perform on alignment error.
-            on_alignment_error: DirectIoOnAlignmentErrorAction,
-        },
+    #[derive(
+        Copy,
+        Clone,
+        PartialEq,
+        Eq,
+        Hash,
+        strum_macros::EnumString,
+        strum_macros::Display,
+        serde_with::DeserializeFromStr,
+        serde_with::SerializeDisplay,
+        Debug,
+    )]
+    #[strum(serialize_all = "kebab-case")]
+    #[repr(u8)]
+    pub enum IoMode {
+        /// Uses buffered IO.
+        Buffered,
+        /// Uses direct IO, error out if the operation fails.
+        #[cfg(target_os = "linux")]
+        Direct,
     }
 
-    #[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize, serde::Serialize, Default)]
-    #[serde(rename_all = "kebab-case")]
-    pub enum DirectIoAlignmentCheckLevel {
-        #[default]
-        Error,
-        Log,
-        None,
+    impl IoMode {
+        pub const fn preferred() -> Self {
+            if cfg!(target_os = "linux") {
+                Self::Direct
+            } else {
+                Self::Buffered
+            }
+        }
     }
 
-    #[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize, serde::Serialize, Default)]
-    #[serde(rename_all = "kebab-case")]
-    pub enum DirectIoOnAlignmentErrorAction {
-        Error,
-        #[default]
-        FallbackToBuffered,
-    }
+    impl TryFrom<u8> for IoMode {
+        type Error = u8;
 
-    #[derive(Debug, PartialEq, Eq, Clone, serde::Deserialize, serde::Serialize, Default)]
-    #[serde(tag = "type", rename_all = "kebab-case")]
-    pub enum DirectIoLatencyPadding {
-        /// Pad virtual file operations with IO to a fake file.
-        FakeFileRW { path: PathBuf },
-        #[default]
-        None,
+        fn try_from(value: u8) -> Result<Self, Self::Error> {
+            Ok(match value {
+                v if v == (IoMode::Buffered as u8) => IoMode::Buffered,
+                #[cfg(target_os = "linux")]
+                v if v == (IoMode::Direct as u8) => IoMode::Direct,
+                x => return Err(x),
+            })
+        }
     }
 }
 

pageserver/benches/bench_ingest.rs

@@ -164,12 +164,10 @@ fn criterion_benchmark(c: &mut Criterion) {
     let conf: &'static PageServerConf = Box::leak(Box::new(
         pageserver::config::PageServerConf::dummy_conf(temp_dir.path().to_path_buf()),
     ));
-    virtual_file::init(
-        16384,
-        virtual_file::io_engine_for_bench(),
-        pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT,
-    );
-    page_cache::init(conf.page_cache_size);
+
+    let align = pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT;
+    virtual_file::init(16384, virtual_file::io_engine_for_bench(), align);
+    page_cache::init(conf.page_cache_size, align);
 
     {
         let mut group = c.benchmark_group("ingest-small-values");

pageserver/client/src/mgmt_api.rs

@@ -550,6 +550,19 @@ impl Client {
             .map_err(Error::ReceiveBody)
     }
 
+    /// Configs io mode at runtime.
+    pub async fn put_io_mode(
+        &self,
+        mode: &pageserver_api::models::virtual_file::IoMode,
+    ) -> Result<()> {
+        let uri = format!("{}/v1/io_mode", self.mgmt_api_endpoint);
+        self.request(Method::PUT, uri, mode)
+            .await?
+            .json()
+            .await
+            .map_err(Error::ReceiveBody)
+    }
+
     pub async fn get_utilization(&self) -> Result<PageserverUtilization> {
         let uri = format!("{}/v1/utilization", self.mgmt_api_endpoint);
         self.get(uri)

pageserver/ctl/src/layer_map_analyzer.rs

@@ -151,13 +151,10 @@ pub(crate) async fn main(cmd: &AnalyzeLayerMapCmd) -> Result<()> {
     let max_holes = cmd.max_holes.unwrap_or(DEFAULT_MAX_HOLES);
     let ctx = RequestContext::new(TaskKind::DebugTool, DownloadBehavior::Error);
 
+    let align = pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT;
     // Initialize virtual_file (file desriptor cache) and page cache which are needed to access layer persistent B-Tree.
-    pageserver::virtual_file::init(
-        10,
-        virtual_file::api::IoEngineKind::StdFs,
-        pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT,
-    );
-    pageserver::page_cache::init(100);
+    pageserver::virtual_file::init(10, virtual_file::api::IoEngineKind::StdFs, align);
+    pageserver::page_cache::init(100, align);
 
     let mut total_delta_layers = 0usize;
     let mut total_image_layers = 0usize;

pageserver/ctl/src/layers.rs

@@ -59,8 +59,9 @@ pub(crate) enum LayerCmd {
 
 async fn read_delta_file(path: impl AsRef<Path>, ctx: &RequestContext) -> Result<()> {
     let path = Utf8Path::from_path(path.as_ref()).expect("non-Unicode path");
-    virtual_file::init(10, virtual_file::api::IoEngineKind::StdFs, 1);
-    page_cache::init(100);
+    let align = pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT;
+    virtual_file::init(10, virtual_file::api::IoEngineKind::StdFs, align);
+    page_cache::init(100, align);
     let file = VirtualFile::open(path, ctx).await?;
     let file_id = page_cache::next_file_id();
     let block_reader = FileBlockReader::new(&file, file_id);
@@ -190,12 +191,10 @@ pub(crate) async fn main(cmd: &LayerCmd) -> Result<()> {
             new_tenant_id,
             new_timeline_id,
         } => {
-            pageserver::virtual_file::init(
-                10,
-                virtual_file::api::IoEngineKind::StdFs,
-                pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT,
-            );
-            pageserver::page_cache::init(100);
+            let align = pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT;
+
+            pageserver::virtual_file::init(10, virtual_file::api::IoEngineKind::StdFs, align);
+            pageserver::page_cache::init(100, align);
 
             let ctx = RequestContext::new(TaskKind::DebugTool, DownloadBehavior::Error);
 

pageserver/ctl/src/main.rs

@@ -205,12 +205,9 @@ fn read_pg_control_file(control_file_path: &Utf8Path) -> anyhow::Result<()> {
 
 async fn print_layerfile(path: &Utf8Path) -> anyhow::Result<()> {
     // Basic initialization of things that don't change after startup
-    virtual_file::init(
-        10,
-        virtual_file::api::IoEngineKind::StdFs,
-        DEFAULT_IO_BUFFER_ALIGNMENT,
-    );
-    page_cache::init(100);
+    let align = DEFAULT_IO_BUFFER_ALIGNMENT;
+    virtual_file::init(10, virtual_file::api::IoEngineKind::StdFs, align);
+    page_cache::init(100, align);
     let ctx = RequestContext::new(TaskKind::DebugTool, DownloadBehavior::Error);
     dump_layerfile_from_path(path, true, &ctx).await
 }

pageserver/pagebench/src/cmd/getpage_latest_lsn.rs

@@ -63,6 +63,10 @@ pub(crate) struct Args {
     #[clap(long)]
     set_io_alignment: Option<usize>,
 
+    /// Before starting the benchmark, live-reconfigure the pageserver to use specified io mode (buffered vs. direct).
+    #[clap(long)]
+    set_io_mode: Option<pageserver_api::models::virtual_file::IoMode>,
+
     targets: Option<Vec<TenantTimelineId>>,
 }
 
@@ -133,6 +137,10 @@ async fn main_impl(
         mgmt_api_client.put_io_alignment(align).await?;
     }
 
+    if let Some(mode) = &args.set_io_mode {
+        mgmt_api_client.put_io_mode(mode).await?;
+    }
+
     // discover targets
     let timelines: Vec<TenantTimelineId> = crate::util::cli::targets::discover(
         &mgmt_api_client,

pageserver/src/bin/pageserver.rs

@@ -125,7 +125,7 @@ fn main() -> anyhow::Result<()> {
 
     // after setting up logging, log the effective IO engine choice and read path implementations
     info!(?conf.virtual_file_io_engine, "starting with virtual_file IO engine");
-    info!(?conf.virtual_file_direct_io, "starting with virtual_file Direct IO settings");
+    info!(?conf.virtual_file_io_mode, "starting with virtual_file Direct IO settings");
     info!(?conf.io_buffer_alignment, "starting with setting for IO buffer alignment");
 
     // The tenants directory contains all the pageserver local disk state.
@@ -173,7 +173,7 @@ fn main() -> anyhow::Result<()> {
         conf.virtual_file_io_engine,
         conf.io_buffer_alignment,
     );
-    page_cache::init(conf.page_cache_size);
+    page_cache::init(conf.page_cache_size, conf.io_buffer_alignment);
 
     start_pageserver(launch_ts, conf).context("Failed to start pageserver")?;
 

pageserver/src/config.rs

@@ -174,7 +174,7 @@ pub struct PageServerConf {
     pub l0_flush: crate::l0_flush::L0FlushConfig,
 
     /// Direct IO settings
-    pub virtual_file_direct_io: virtual_file::DirectIoMode,
+    pub virtual_file_io_mode: virtual_file::IoMode,
 
     pub io_buffer_alignment: usize,
 }
@@ -325,7 +325,7 @@ impl PageServerConf {
             image_compression,
             ephemeral_bytes_per_memory_kb,
             l0_flush,
-            virtual_file_direct_io,
+            virtual_file_io_mode,
             concurrent_tenant_warmup,
             concurrent_tenant_size_logical_size_queries,
             virtual_file_io_engine,
@@ -368,7 +368,6 @@ impl PageServerConf {
             max_vectored_read_bytes,
             image_compression,
             ephemeral_bytes_per_memory_kb,
-            virtual_file_direct_io,
             io_buffer_alignment,
 
             // ------------------------------------------------------------
@@ -408,6 +407,7 @@ impl PageServerConf {
             l0_flush: l0_flush
                 .map(crate::l0_flush::L0FlushConfig::from)
                 .unwrap_or_default(),
+            virtual_file_io_mode: virtual_file_io_mode.unwrap_or(virtual_file::IoMode::preferred()),
         };
 
         // ------------------------------------------------------------

pageserver/src/http/routes.rs

@@ -17,6 +17,7 @@ use hyper::header;
 use hyper::StatusCode;
 use hyper::{Body, Request, Response, Uri};
 use metrics::launch_timestamp::LaunchTimestamp;
+use pageserver_api::models::virtual_file::IoMode;
 use pageserver_api::models::AuxFilePolicy;
 use pageserver_api::models::DownloadRemoteLayersTaskSpawnRequest;
 use pageserver_api::models::IngestAuxFilesRequest;
@@ -2381,6 +2382,16 @@ async fn put_io_alignment_handler(
     json_response(StatusCode::OK, ())
 }
 
+async fn put_io_mode_handler(
+    mut r: Request<Body>,
+    _cancel: CancellationToken,
+) -> Result<Response<Body>, ApiError> {
+    check_permission(&r, None)?;
+    let mode: IoMode = json_request(&mut r).await?;
+    crate::virtual_file::set_io_mode(mode);
+    json_response(StatusCode::OK, ())
+}
+
 /// Polled by control plane.
 ///
 /// See [`crate::utilization`].
@@ -3071,6 +3082,7 @@ pub fn make_router(
         .put("/v1/io_alignment", |r| {
             api_handler(r, put_io_alignment_handler)
         })
+        .put("/v1/io_mode", |r| api_handler(r, put_io_mode_handler))
         .put(
             "/v1/tenant/:tenant_shard_id/timeline/:timeline_id/force_aux_policy_switch",
             |r| api_handler(r, force_aux_policy_switch_handler),

pageserver/src/page_cache.rs

@@ -82,6 +82,7 @@ use once_cell::sync::OnceCell;
 use crate::{
     context::RequestContext,
     metrics::{page_cache_eviction_metrics, PageCacheSizeMetrics},
+    virtual_file::{self, dio::IoBufferMut},
 };
 
 static PAGE_CACHE: OnceCell<PageCache> = OnceCell::new();
@@ -90,8 +91,8 @@ const TEST_PAGE_CACHE_SIZE: usize = 50;
 ///
 /// Initialize the page cache. This must be called once at page server startup.
 ///
-pub fn init(size: usize) {
-    if PAGE_CACHE.set(PageCache::new(size)).is_err() {
+pub fn init(size: usize, align: usize) {
+    if PAGE_CACHE.set(PageCache::new(size, align)).is_err() {
         panic!("page cache already initialized");
     }
 }
@@ -106,7 +107,12 @@ pub fn get() -> &'static PageCache {
     // page cache is usable in unit tests.
     //
     if cfg!(test) {
-        PAGE_CACHE.get_or_init(|| PageCache::new(TEST_PAGE_CACHE_SIZE))
+        PAGE_CACHE.get_or_init(|| {
+            PageCache::new(
+                TEST_PAGE_CACHE_SIZE,
+                virtual_file::get_io_buffer_alignment(),
+            )
+        })
     } else {
         PAGE_CACHE.get().expect("page cache not initialized")
     }
@@ -637,13 +643,11 @@ impl PageCache {
     /// Initialize a new page cache
     ///
     /// This should be called only once at page server startup.
-    fn new(num_pages: usize) -> Self {
+    fn new(num_pages: usize, align: usize) -> Self {
         assert!(num_pages > 0, "page cache size must be > 0");
 
-        // We could use Vec::leak here, but that potentially also leaks
-        // uninitialized reserved capacity. With into_boxed_slice and Box::leak
-        // this is avoided.
-        let page_buffer = Box::leak(vec![0u8; num_pages * PAGE_SZ].into_boxed_slice());
+        let page_buffer =
+            IoBufferMut::with_capacity_aligned_zeroed(num_pages * PAGE_SZ, align).leak();
 
         let size_metrics = &crate::metrics::PAGE_CACHE_SIZE;
         size_metrics.max_bytes.set_page_sz(num_pages);

pageserver/src/tenant/ephemeral_file.rs

@@ -84,7 +84,7 @@ impl Drop for EphemeralFile {
     fn drop(&mut self) {
         // unlink the file
         // we are clear to do this, because we have entered a gate
-        let path = &self.buffered_writer.as_inner().as_inner().path;
+        let path = self.buffered_writer.as_inner().as_inner().path();
         let res = std::fs::remove_file(path);
         if let Err(e) = res {
             if e.kind() != std::io::ErrorKind::NotFound {
@@ -356,7 +356,7 @@ mod tests {
         }
 
         let file_contents =
-            std::fs::read(&file.buffered_writer.as_inner().as_inner().path).unwrap();
+            std::fs::read(file.buffered_writer.as_inner().as_inner().path()).unwrap();
         assert_eq!(file_contents, &content[0..cap]);
 
         let buffer_contents = file.buffered_writer.inspect_buffer();
@@ -392,7 +392,7 @@ mod tests {
             .buffered_writer
             .as_inner()
             .as_inner()
-            .path
+            .path()
             .metadata()
             .unwrap();
         assert_eq!(

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -43,12 +43,12 @@ use crate::tenant::vectored_blob_io::{
     VectoredReadPlanner,
 };
 use crate::tenant::PageReconstructError;
+use crate::virtual_file::dio::IoBufferMut;
 use crate::virtual_file::owned_buffers_io::io_buf_ext::{FullSlice, IoBufExt};
 use crate::virtual_file::{self, MaybeFatalIo, VirtualFile};
 use crate::{walrecord, TEMP_FILE_SUFFIX};
 use crate::{DELTA_FILE_MAGIC, STORAGE_FORMAT_VERSION};
 use anyhow::{anyhow, bail, ensure, Context, Result};
-use bytes::BytesMut;
 use camino::{Utf8Path, Utf8PathBuf};
 use futures::StreamExt;
 use itertools::Itertools;
@@ -572,7 +572,7 @@ impl DeltaLayerWriterInner {
         ensure!(
             metadata.len() <= S3_UPLOAD_LIMIT,
             "Created delta layer file at {} of size {} above limit {S3_UPLOAD_LIMIT}!",
-            file.path,
+            file.path(),
             metadata.len()
         );
 
@@ -790,7 +790,7 @@ impl DeltaLayerInner {
         max_vectored_read_bytes: Option<MaxVectoredReadBytes>,
         ctx: &RequestContext,
     ) -> anyhow::Result<Self> {
-        let file = VirtualFile::open(path, ctx)
+        let file = VirtualFile::open_v2(path, ctx)
             .await
             .context("open layer file")?;
 
@@ -991,7 +991,8 @@ impl DeltaLayerInner {
             .0
             .into();
         let buf_size = Self::get_min_read_buffer_size(&reads, max_vectored_read_bytes);
-        let mut buf = Some(BytesMut::with_capacity(buf_size));
+        let align = virtual_file::get_io_buffer_alignment();
+        let mut buf = Some(IoBufferMut::with_capacity_aligned(buf_size, align));
 
         // Note that reads are processed in reverse order (from highest key+lsn).
         // This is the order that `ReconstructState` requires such that it can
@@ -1010,7 +1011,7 @@ impl DeltaLayerInner {
                             blob_meta.key,
                             PageReconstructError::Other(anyhow!(
                                 "Failed to read blobs from virtual file {}: {}",
-                                self.file.path,
+                                self.file.path(),
                                 kind
                             )),
                         );
@@ -1018,7 +1019,7 @@ impl DeltaLayerInner {
 
                     // We have "lost" the buffer since the lower level IO api
                     // doesn't return the buffer on error. Allocate a new one.
-                    buf = Some(BytesMut::with_capacity(buf_size));
+                    buf = Some(IoBufferMut::with_capacity_aligned(buf_size, align));
 
                     continue;
                 }
@@ -1036,7 +1037,7 @@ impl DeltaLayerInner {
                             meta.meta.key,
                             PageReconstructError::Other(anyhow!(e).context(format!(
                                 "Failed to decompress blob from virtual file {}",
-                                self.file.path,
+                                self.file.path(),
                             ))),
                         );
 
@@ -1054,7 +1055,7 @@ impl DeltaLayerInner {
                             meta.meta.key,
                             PageReconstructError::Other(anyhow!(e).context(format!(
                                 "Failed to deserialize blob from virtual file {}",
-                                self.file.path,
+                                self.file.path(),
                             ))),
                         );
 
@@ -1186,14 +1187,14 @@ impl DeltaLayerInner {
         let mut prev: Option<(Key, Lsn, BlobRef)> = None;
 
         let mut read_builder: Option<ChunkedVectoredReadBuilder> = None;
-        let align = virtual_file::get_io_buffer_alignment();
 
         let max_read_size = self
             .max_vectored_read_bytes
             .map(|x| x.0.get())
             .unwrap_or(8192);
 
-        let mut buffer = Some(BytesMut::with_capacity(max_read_size));
+        let align = virtual_file::get_io_buffer_alignment();
+        let mut buffer = Some(IoBufferMut::with_capacity_aligned(max_read_size, align));
 
         // FIXME: buffering of DeltaLayerWriter
         let mut per_blob_copy = Vec::new();
@@ -1552,12 +1553,12 @@ impl<'a> DeltaLayerIterator<'a> {
         let vectored_blob_reader = VectoredBlobReader::new(&self.delta_layer.file);
         let mut next_batch = std::collections::VecDeque::new();
         let buf_size = plan.size();
-        let buf = BytesMut::with_capacity(buf_size);
+        let align = virtual_file::get_io_buffer_alignment();
+        let buf = IoBufferMut::with_capacity_aligned(buf_size, align);
         let blobs_buf = vectored_blob_reader
             .read_blobs(&plan, buf, self.ctx)
             .await?;
-        let frozen_buf = blobs_buf.buf.freeze();
-        let view = BufView::new_bytes(frozen_buf);
+        let view = BufView::new_slice(&blobs_buf.buf);
         for meta in blobs_buf.blobs.iter() {
             let blob_read = meta.read(&view).await?;
             let value = Value::des(&blob_read)?;
@@ -1932,7 +1933,9 @@ pub(crate) mod test {
                 &vectored_reads,
                 constants::MAX_VECTORED_READ_BYTES,
             );
-            let mut buf = Some(BytesMut::with_capacity(buf_size));
+
+            let align = virtual_file::get_io_buffer_alignment();
+            let mut buf = Some(IoBufferMut::with_capacity_aligned(buf_size, align));
 
             for read in vectored_reads {
                 let blobs_buf = vectored_blob_reader

pageserver/src/tenant/storage_layer/image_layer.rs

@@ -40,11 +40,12 @@ use crate::tenant::vectored_blob_io::{
     VectoredReadPlanner,
 };
 use crate::tenant::PageReconstructError;
+use crate::virtual_file::dio::IoBufferMut;
 use crate::virtual_file::owned_buffers_io::io_buf_ext::IoBufExt;
 use crate::virtual_file::{self, MaybeFatalIo, VirtualFile};
 use crate::{IMAGE_FILE_MAGIC, STORAGE_FORMAT_VERSION, TEMP_FILE_SUFFIX};
 use anyhow::{anyhow, bail, ensure, Context, Result};
-use bytes::{Bytes, BytesMut};
+use bytes::Bytes;
 use camino::{Utf8Path, Utf8PathBuf};
 use hex;
 use itertools::Itertools;
@@ -388,7 +389,7 @@ impl ImageLayerInner {
         max_vectored_read_bytes: Option<MaxVectoredReadBytes>,
         ctx: &RequestContext,
     ) -> anyhow::Result<Self> {
-        let file = VirtualFile::open(path, ctx)
+        let file = VirtualFile::open_v2(path, ctx)
             .await
             .context("open layer file")?;
         let file_id = page_cache::next_file_id();
@@ -542,14 +543,15 @@ impl ImageLayerInner {
             .await?;
 
         let vectored_blob_reader = VectoredBlobReader::new(&self.file);
+        let align = virtual_file::get_io_buffer_alignment();
         let mut key_count = 0;
         for read in plan.into_iter() {
             let buf_size = read.size();
 
-            let buf = BytesMut::with_capacity(buf_size);
+            let buf = IoBufferMut::with_capacity_aligned(buf_size, align);
             let blobs_buf = vectored_blob_reader.read_blobs(&read, buf, ctx).await?;
-            let frozen_buf = blobs_buf.buf.freeze();
-            let view = BufView::new_bytes(frozen_buf);
+
+            let view = BufView::new_slice(&blobs_buf.buf);
 
             for meta in blobs_buf.blobs.iter() {
                 let img_buf = meta.read(&view).await?;
@@ -597,13 +599,13 @@ impl ImageLayerInner {
                 );
             }
 
-            let buf = BytesMut::with_capacity(buf_size);
+            let align = virtual_file::get_io_buffer_alignment();
+            let buf = IoBufferMut::with_capacity_aligned(buf_size, align);
             let res = vectored_blob_reader.read_blobs(&read, buf, ctx).await;
 
             match res {
                 Ok(blobs_buf) => {
-                    let frozen_buf = blobs_buf.buf.freeze();
-                    let view = BufView::new_bytes(frozen_buf);
+                    let view = BufView::new_slice(&blobs_buf.buf);
                     for meta in blobs_buf.blobs.iter() {
                         let img_buf = meta.read(&view).await;
 
@@ -614,7 +616,7 @@ impl ImageLayerInner {
                                     meta.meta.key,
                                     PageReconstructError::Other(anyhow!(e).context(format!(
                                         "Failed to decompress blob from virtual file {}",
-                                        self.file.path,
+                                        self.file.path(),
                                     ))),
                                 );
 
@@ -635,7 +637,7 @@ impl ImageLayerInner {
                             blob_meta.key,
                             PageReconstructError::from(anyhow!(
                                 "Failed to read blobs from virtual file {}: {}",
-                                self.file.path,
+                                self.file.path(),
                                 kind
                             )),
                         );
@@ -1039,12 +1041,12 @@ impl<'a> ImageLayerIterator<'a> {
         let vectored_blob_reader = VectoredBlobReader::new(&self.image_layer.file);
         let mut next_batch = std::collections::VecDeque::new();
         let buf_size = plan.size();
-        let buf = BytesMut::with_capacity(buf_size);
+        let align = virtual_file::get_io_buffer_alignment();
+        let buf = IoBufferMut::with_capacity_aligned(buf_size, align);
         let blobs_buf = vectored_blob_reader
             .read_blobs(&plan, buf, self.ctx)
             .await?;
-        let frozen_buf = blobs_buf.buf.freeze();
-        let view = BufView::new_bytes(frozen_buf);
+        let view = BufView::new_slice(&blobs_buf.buf);
         for meta in blobs_buf.blobs.iter() {
             let img_buf = meta.read(&view).await?;
             next_batch.push_back((

pageserver/src/tenant/vectored_blob_io.rs

@@ -18,7 +18,7 @@
 use std::collections::BTreeMap;
 use std::ops::Deref;
 
-use bytes::{Bytes, BytesMut};
+use bytes::Bytes;
 use pageserver_api::key::Key;
 use tokio::io::AsyncWriteExt;
 use tokio_epoll_uring::BoundedBuf;
@@ -27,6 +27,7 @@ use utils::vec_map::VecMap;
 
 use crate::context::RequestContext;
 use crate::tenant::blob_io::{BYTE_UNCOMPRESSED, BYTE_ZSTD, LEN_COMPRESSION_BIT_MASK};
+use crate::virtual_file::dio::IoBufferMut;
 use crate::virtual_file::{self, VirtualFile};
 
 /// Metadata bundled with the start and end offset of a blob.
@@ -158,7 +159,7 @@ impl std::fmt::Display for VectoredBlob {
 /// Return type of [`VectoredBlobReader::read_blobs`]
 pub struct VectoredBlobsBuf {
     /// Buffer for all blobs in this read
-    pub buf: BytesMut,
+    pub buf: IoBufferMut,
     /// Offsets into the buffer and metadata for all blobs in this read
     pub blobs: Vec<VectoredBlob>,
 }
@@ -460,7 +461,7 @@ impl<'a> VectoredBlobReader<'a> {
     pub async fn read_blobs(
         &self,
         read: &VectoredRead,
-        buf: BytesMut,
+        buf: IoBufferMut,
         ctx: &RequestContext,
     ) -> Result<VectoredBlobsBuf, std::io::Error> {
         assert!(read.size() > 0);
@@ -945,7 +946,8 @@ mod tests {
 
         // Multiply by two (compressed data might need more space), and add a few bytes for the header
         let reserved_bytes = blobs.iter().map(|bl| bl.len()).max().unwrap() * 2 + 16;
-        let mut buf = BytesMut::with_capacity(reserved_bytes);
+        let align = virtual_file::get_io_buffer_alignment();
+        let mut buf = IoBufferMut::with_capacity_aligned(reserved_bytes, align);
 
         let align = virtual_file::get_io_buffer_alignment();
         let vectored_blob_reader = VectoredBlobReader::new(&file);

pageserver/src/virtual_file.rs

@@ -23,10 +23,12 @@ use pageserver_api::config::defaults::DEFAULT_IO_BUFFER_ALIGNMENT;
 use pageserver_api::shard::TenantShardId;
 use std::fs::File;
 use std::io::{Error, ErrorKind, Seek, SeekFrom};
+#[cfg(target_os = "linux")]
+use std::os::unix::fs::OpenOptionsExt;
 use tokio_epoll_uring::{BoundedBuf, IoBuf, IoBufMut, Slice};
 
 use std::os::fd::{AsRawFd, FromRawFd, IntoRawFd, OwnedFd, RawFd};
-use std::sync::atomic::{AtomicBool, AtomicUsize, Ordering};
+use std::sync::atomic::{AtomicBool, AtomicU8, AtomicUsize, Ordering};
 use tokio::sync::{RwLock, RwLockReadGuard, RwLockWriteGuard};
 use tokio::time::Instant;
 
@@ -38,10 +40,11 @@ pub use io_engine::FeatureTestResult as IoEngineFeatureTestResult;
 mod metadata;
 mod open_options;
 use self::owned_buffers_io::write::OwnedAsyncWriter;
-pub(crate) use api::DirectIoMode;
+pub(crate) use api::IoMode;
 pub(crate) use io_engine::IoEngineKind;
 pub(crate) use metadata::Metadata;
 pub(crate) use open_options::*;
+pub(crate) mod dio;
 
 pub(crate) mod owned_buffers_io {
     //! Abstractions for IO with owned buffers.
@@ -53,6 +56,7 @@ pub(crate) mod owned_buffers_io {
     //! but for the time being we're proving out the primitives in the neon.git repo
     //! for faster iteration.
 
+    pub(crate) mod io_buf_aligned;
     pub(crate) mod io_buf_ext;
     pub(crate) mod slice;
     pub(crate) mod write;
@@ -61,6 +65,176 @@ pub(crate) mod owned_buffers_io {
     }
 }
 
+#[derive(Debug)]
+pub enum VirtualFile {
+    Buffered(VirtualFileInner),
+    Direct(VirtualFileInner),
+}
+
+impl VirtualFile {
+    fn inner(&self) -> &VirtualFileInner {
+        match self {
+            Self::Buffered(file) => file,
+            Self::Direct(file) => file,
+        }
+    }
+
+    fn inner_mut(&mut self) -> &mut VirtualFileInner {
+        match self {
+            Self::Buffered(file) => file,
+            Self::Direct(file) => file,
+        }
+    }
+
+    fn into_inner(self) -> VirtualFileInner {
+        match self {
+            Self::Buffered(file) => file,
+            Self::Direct(file) => file,
+        }
+    }
+    /// Open a file in read-only mode. Like File::open.
+    pub async fn open<P: AsRef<Utf8Path>>(
+        path: P,
+        ctx: &RequestContext,
+    ) -> Result<Self, std::io::Error> {
+        let file = VirtualFileInner::open(path, ctx).await?;
+        Ok(Self::Buffered(file))
+    }
+
+    /// Open a file in read-only mode. Like File::open.
+    ///
+    /// `O_DIRECT` will be enabled base on `virtual_file_io_mode`.
+    pub async fn open_v2<P: AsRef<Utf8Path>>(
+        path: P,
+        ctx: &RequestContext,
+    ) -> Result<Self, std::io::Error> {
+        Self::open_with_options_v2(path.as_ref(), OpenOptions::new().read(true), ctx).await
+    }
+
+    pub async fn create<P: AsRef<Utf8Path>>(
+        path: P,
+        ctx: &RequestContext,
+    ) -> Result<Self, std::io::Error> {
+        let file = VirtualFileInner::create(path, ctx).await?;
+        Ok(Self::Buffered(file))
+    }
+
+    pub async fn create_v2<P: AsRef<Utf8Path>>(
+        path: P,
+        ctx: &RequestContext,
+    ) -> Result<Self, std::io::Error> {
+        VirtualFile::open_with_options_v2(
+            path.as_ref(),
+            OpenOptions::new().write(true).create(true).truncate(true),
+            ctx,
+        )
+        .await
+    }
+
+    pub async fn open_with_options<P: AsRef<Utf8Path>>(
+        path: P,
+        open_options: &OpenOptions,
+        ctx: &RequestContext, /* TODO: carry a pointer to the metrics in the RequestContext instead of the parsing https://github.com/neondatabase/neon/issues/6107 */
+    ) -> Result<Self, std::io::Error> {
+        let file = VirtualFileInner::open_with_options(path, open_options, ctx).await?;
+        Ok(Self::Buffered(file))
+    }
+
+    pub async fn open_with_options_v2<P: AsRef<Utf8Path>>(
+        path: P,
+        open_options: &mut OpenOptions, // Uses `&mut` here to add `O_DIRECT`.
+        ctx: &RequestContext, /* TODO: carry a pointer to the metrics in the RequestContext instead of the parsing https://github.com/neondatabase/neon/issues/6107 */
+    ) -> Result<Self, std::io::Error> {
+        let file = match get_io_mode() {
+            IoMode::Buffered => {
+                let file = VirtualFileInner::open_with_options(path, open_options, ctx).await?;
+                Self::Buffered(file)
+            }
+            #[cfg(target_os = "linux")]
+            IoMode::Direct => {
+                let file = VirtualFileInner::open_with_options(
+                    path,
+                    open_options.custom_flags(nix::libc::O_DIRECT),
+                    ctx,
+                )
+                .await?;
+                Self::Direct(file)
+            }
+        };
+        Ok(file)
+    }
+
+    pub fn path(&self) -> &Utf8Path {
+        self.inner().path.as_path()
+    }
+
+    pub async fn crashsafe_overwrite<B: BoundedBuf<Buf = Buf> + Send, Buf: IoBuf + Send>(
+        final_path: Utf8PathBuf,
+        tmp_path: Utf8PathBuf,
+        content: B,
+    ) -> std::io::Result<()> {
+        VirtualFileInner::crashsafe_overwrite(final_path, tmp_path, content).await
+    }
+
+    pub async fn sync_all(&self) -> Result<(), Error> {
+        self.inner().sync_all().await
+    }
+
+    pub async fn sync_data(&self) -> Result<(), Error> {
+        self.inner().sync_data().await
+    }
+
+    pub async fn metadata(&self) -> Result<Metadata, Error> {
+        self.inner().metadata().await
+    }
+
+    pub fn remove(self) {
+        self.into_inner().remove();
+    }
+
+    pub async fn seek(&mut self, pos: SeekFrom) -> Result<u64, Error> {
+        self.inner_mut().seek(pos).await
+    }
+
+    pub async fn read_exact_at<Buf>(
+        &self,
+        slice: Slice<Buf>,
+        offset: u64,
+        ctx: &RequestContext,
+    ) -> Result<Slice<Buf>, Error>
+    where
+        Buf: IoBufMut + Send,
+    {
+        self.inner().read_exact_at(slice, offset, ctx).await
+    }
+
+    pub async fn read_exact_at_page(
+        &self,
+        page: PageWriteGuard<'static>,
+        offset: u64,
+        ctx: &RequestContext,
+    ) -> Result<PageWriteGuard<'static>, Error> {
+        self.inner().read_exact_at_page(page, offset, ctx).await
+    }
+
+    pub async fn write_all_at<Buf: IoBuf + Send>(
+        &self,
+        buf: FullSlice<Buf>,
+        offset: u64,
+        ctx: &RequestContext,
+    ) -> (FullSlice<Buf>, Result<(), Error>) {
+        self.inner().write_all_at(buf, offset, ctx).await
+    }
+
+    pub async fn write_all<Buf: IoBuf + Send>(
+        &mut self,
+        buf: FullSlice<Buf>,
+        ctx: &RequestContext,
+    ) -> (FullSlice<Buf>, Result<usize, Error>) {
+        self.inner_mut().write_all(buf, ctx).await
+    }
+}
+
 ///
 /// 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,
@@ -77,7 +251,7 @@ pub(crate) mod owned_buffers_io {
 /// 'tag' field is used to detect whether the handle still is valid or not.
 ///
 #[derive(Debug)]
-pub struct VirtualFile {
+pub struct VirtualFileInner {
     /// Lazy handle to the global file descriptor cache. The slot that this points to
     /// might contain our File, or it may be empty, or it may contain a File that
     /// belongs to a different VirtualFile.
@@ -350,12 +524,12 @@ macro_rules! with_file {
     }};
 }
 
-impl VirtualFile {
+impl VirtualFileInner {
     /// Open a file in read-only mode. Like File::open.
     pub async fn open<P: AsRef<Utf8Path>>(
         path: P,
         ctx: &RequestContext,
-    ) -> Result<VirtualFile, std::io::Error> {
+    ) -> Result<VirtualFileInner, std::io::Error> {
         Self::open_with_options(path.as_ref(), OpenOptions::new().read(true), ctx).await
     }
 
@@ -364,7 +538,7 @@ impl VirtualFile {
     pub async fn create<P: AsRef<Utf8Path>>(
         path: P,
         ctx: &RequestContext,
-    ) -> Result<VirtualFile, std::io::Error> {
+    ) -> Result<VirtualFileInner, std::io::Error> {
         Self::open_with_options(
             path.as_ref(),
             OpenOptions::new().write(true).create(true).truncate(true),
@@ -382,7 +556,7 @@ impl VirtualFile {
         path: P,
         open_options: &OpenOptions,
         _ctx: &RequestContext, /* TODO: carry a pointer to the metrics in the RequestContext instead of the parsing https://github.com/neondatabase/neon/issues/6107 */
-    ) -> Result<VirtualFile, std::io::Error> {
+    ) -> Result<VirtualFileInner, std::io::Error> {
         let path_ref = path.as_ref();
         let path_str = path_ref.to_string();
         let parts = path_str.split('/').collect::<Vec<&str>>();
@@ -413,7 +587,7 @@ impl VirtualFile {
             open_options.open(path_ref.as_std_path()).await?
         });
 
-        // Strip all options other than read and write.
+        // Strip all options other than read and write (O_DIRECT).
         //
         // It would perhaps be nicer to check just for the read and write flags
         // explicitly, but OpenOptions doesn't contain any functions to read flags,
@@ -423,7 +597,7 @@ impl VirtualFile {
         reopen_options.create_new(false);
         reopen_options.truncate(false);
 
-        let vfile = VirtualFile {
+        let vfile = VirtualFileInner {
             handle: RwLock::new(handle),
             pos: 0,
             path: path_ref.to_path_buf(),
@@ -1034,6 +1208,21 @@ impl tokio_epoll_uring::IoFd for FileGuard {
 
 #[cfg(test)]
 impl VirtualFile {
+    pub(crate) async fn read_blk(
+        &self,
+        blknum: u32,
+        ctx: &RequestContext,
+    ) -> Result<crate::tenant::block_io::BlockLease<'_>, std::io::Error> {
+        self.inner().read_blk(blknum, ctx).await
+    }
+
+    async fn read_to_end(&mut self, buf: &mut Vec<u8>, ctx: &RequestContext) -> Result<(), Error> {
+        self.inner_mut().read_to_end(buf, ctx).await
+    }
+}
+
+#[cfg(test)]
+impl VirtualFileInner {
     pub(crate) async fn read_blk(
         &self,
         blknum: u32,
@@ -1067,7 +1256,7 @@ impl VirtualFile {
     }
 }
 
-impl Drop for VirtualFile {
+impl Drop for VirtualFileInner {
     /// If a VirtualFile is dropped, close the underlying file if it was open.
     fn drop(&mut self) {
         let handle = self.handle.get_mut();
@@ -1216,6 +1405,15 @@ pub(crate) fn get_io_buffer_alignment() -> usize {
     }
 }
 
+static IO_MODE: AtomicU8 = AtomicU8::new(IoMode::preferred() as u8);
+
+pub(crate) fn set_io_mode(mode: IoMode) {
+    IO_MODE.store(mode as u8, std::sync::atomic::Ordering::Relaxed);
+}
+
+pub(crate) fn get_io_mode() -> IoMode {
+    IoMode::try_from(IO_MODE.load(Ordering::Relaxed)).unwrap()
+}
 #[cfg(test)]
 mod tests {
     use crate::context::DownloadBehavior;
@@ -1524,7 +1722,7 @@ mod tests {
         // Open the file many times.
         let mut files = Vec::new();
         for _ in 0..VIRTUAL_FILES {
-            let f = VirtualFile::open_with_options(
+            let f = VirtualFileInner::open_with_options(
                 &test_file_path,
                 OpenOptions::new().read(true),
                 &ctx,
@@ -1576,7 +1774,7 @@ mod tests {
         let path = testdir.join("myfile");
         let tmp_path = testdir.join("myfile.tmp");
 
-        VirtualFile::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"foo".to_vec())
+        VirtualFileInner::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"foo".to_vec())
             .await
             .unwrap();
         let mut file = MaybeVirtualFile::from(VirtualFile::open(&path, &ctx).await.unwrap());
@@ -1585,7 +1783,7 @@ mod tests {
         assert!(!tmp_path.exists());
         drop(file);
 
-        VirtualFile::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"bar".to_vec())
+        VirtualFileInner::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"bar".to_vec())
             .await
             .unwrap();
         let mut file = MaybeVirtualFile::from(VirtualFile::open(&path, &ctx).await.unwrap());
@@ -1608,7 +1806,7 @@ mod tests {
         std::fs::write(&tmp_path, "some preexisting junk that should be removed").unwrap();
         assert!(tmp_path.exists());
 
-        VirtualFile::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"foo".to_vec())
+        VirtualFileInner::crashsafe_overwrite(path.clone(), tmp_path.clone(), b"foo".to_vec())
             .await
             .unwrap();
 

pageserver/src/virtual_file/dio.rs

@@ -0,0 +1,410 @@
+#![allow(unused)]
+
+use core::slice;
+use std::{
+    alloc::{self, Layout},
+    cmp,
+    mem::{ManuallyDrop, MaybeUninit},
+    ops::{Deref, DerefMut},
+    ptr::{addr_of_mut, NonNull},
+};
+
+use bytes::buf::UninitSlice;
+
+struct IoBufferPtr(*mut u8);
+
+// SAFETY: We gurantees no one besides `IoBufferPtr` itself has the raw pointer.
+unsafe impl Send for IoBufferPtr {}
+
+/// An aligned buffer type used for I/O.
+pub struct IoBufferMut {
+    ptr: IoBufferPtr,
+    capacity: usize,
+    len: usize,
+    align: usize,
+}
+
+impl IoBufferMut {
+    /// Constructs a new, empty `IoBufferMut` with at least the specified capacity and alignment.
+    ///
+    /// The buffer will be able to hold at most `capacity` elements and will never resize.
+    ///
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` _bytes_, or if the following alignment requirement is not met:
+    /// * `align` must not be zero,
+    ///
+    /// * `align` must be a power of two,
+    ///
+    /// * `capacity`, when rounded up to the nearest multiple of `align`,
+    ///    must not overflow isize (i.e., the rounded value must be
+    ///    less than or equal to `isize::MAX`).
+    pub fn with_capacity_aligned(capacity: usize, align: usize) -> Self {
+        let layout = Layout::from_size_align(capacity, align).expect("Invalid layout");
+
+        // SAFETY:  Making an allocation with a sized and aligned layout. The memory is manually freed with the same layout.
+        let ptr = unsafe {
+            let ptr = alloc::alloc(layout);
+            if ptr.is_null() {
+                alloc::handle_alloc_error(layout);
+            }
+            IoBufferPtr(ptr)
+        };
+
+        IoBufferMut {
+            ptr,
+            capacity,
+            len: 0,
+            align,
+        }
+    }
+
+
+    /// Constructs a new `IoBufferMut` with at least the specified capacity and alignment, filled with zeros.
+    pub fn with_capacity_aligned_zeroed(capacity: usize, align: usize) -> Self {
+        use bytes::BufMut;
+        let mut buf = Self::with_capacity_aligned(capacity, align);
+        buf.put_bytes(0, capacity);
+        buf.len = capacity;
+        buf
+    }
+
+    /// Returns the total number of bytes the buffer can hold.
+    #[inline]
+    pub fn capacity(&self) -> usize {
+        self.capacity
+    }
+
+    /// Returns the alignment of the buffer.
+    #[inline]
+    pub fn align(&self) -> usize {
+        self.align
+    }
+
+    /// Returns the number of bytes in the buffer, also referred to as its 'length'.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.len
+    }
+
+    /// Force the length of the buffer to `new_len`.
+    #[inline]
+    unsafe fn set_len(&mut self, new_len: usize) {
+        debug_assert!(new_len <= self.capacity());
+        self.len = new_len;
+    }
+
+    #[inline]
+    fn as_ptr(&self) -> *const u8 {
+        self.ptr.0
+    }
+
+    #[inline]
+    fn as_mut_ptr(&mut self) -> *mut u8 {
+        self.ptr.0
+    }
+
+    /// Extracts a slice containing the entire buffer.
+    ///
+    /// Equivalent to `&s[..]`.
+    #[inline]
+    fn as_slice(&self) -> &[u8] {
+        // SAFETY: The pointer is valid and `len` bytes are initialized.
+        unsafe { slice::from_raw_parts(self.as_ptr(), self.len) }
+    }
+
+    /// Extracts a mutable slice of the entire buffer.
+    ///
+    /// Equivalent to `&mut s[..]`.
+    fn as_mut_slice(&mut self) -> &mut [u8] {
+        // SAFETY: The pointer is valid and `len` bytes are initialized.
+        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr(), self.len) }
+    }
+
+    /// Drops the all the contents of the buffer, setting its length to `0`.
+    #[inline]
+    pub fn clear(&mut self) {
+        self.len = 0;
+    }
+
+    /// Reserves capacity for at least `additional` more bytes to be inserted
+    /// in the given `IoBufferMut`. The collection may reserve more space to
+    /// speculatively avoid frequent reallocations. After calling `reserve`,
+    /// capacity will be greater than or equal to `self.len() + additional`.
+    /// Does nothing if capacity is already sufficient.
+    ///
+    /// # Panics
+    ///
+    /// Panics if the new capacity exceeds `isize::MAX` _bytes_.
+    pub fn reserve(&mut self, additional: usize) {
+        if additional > self.capacity() - self.len() {
+            self.reserve_inner(additional);
+        }
+    }
+
+    fn reserve_inner(&mut self, additional: usize) {
+        let Some(required_cap) = self.len().checked_add(additional) else {
+            capacity_overflow()
+        };
+
+        let old_capacity = self.capacity();
+        let align = self.align();
+        // This guarantees exponential growth. The doubling cannot overflow
+        // because `cap <= isize::MAX` and the type of `cap` is `usize`.
+        let cap = cmp::max(old_capacity * 2, required_cap);
+
+        if !is_valid_alloc(cap) {
+            capacity_overflow()
+        }
+        let new_layout = Layout::from_size_align(cap, self.align()).expect("Invalid layout");
+
+        let old_ptr = self.as_mut_ptr();
+
+        // SAFETY: old allocation was allocated with std::alloc::alloc with the same layout,
+        // and we panics on null pointer.
+        let (ptr, cap) = unsafe {
+            let old_layout = Layout::from_size_align_unchecked(old_capacity, align);
+            let ptr = alloc::realloc(old_ptr, old_layout, new_layout.size());
+            if ptr.is_null() {
+                alloc::handle_alloc_error(new_layout);
+            }
+            (IoBufferPtr(ptr), cap)
+        };
+
+        self.ptr = ptr;
+        self.capacity = cap;
+    }
+
+
+    /// Consumes and leaks the `IoBufferMut`, returning a mutable reference to the contents, &'a mut [u8].
+    pub fn leak<'a>(self) -> &'a mut [u8] {
+        let mut buf = ManuallyDrop::new(self);
+        // SAFETY: leaking the buffer as intended.
+        unsafe { slice::from_raw_parts_mut(buf.as_mut_ptr(), buf.len) }
+    }
+}
+
+fn capacity_overflow() -> ! {
+    panic!("capacity overflow")
+}
+
+// We need to guarantee the following:
+// * We don't ever allocate `> isize::MAX` byte-size objects.
+// * We don't overflow `usize::MAX` and actually allocate too little.
+//
+// On 64-bit we just need to check for overflow since trying to allocate
+// `> isize::MAX` bytes will surely fail. On 32-bit and 16-bit we need to add
+// an extra guard for this in case we're running on a platform which can use
+// all 4GB in user-space, e.g., PAE or x32.
+#[inline]
+fn is_valid_alloc(alloc_size: usize) -> bool {
+    !(usize::BITS < 64 && alloc_size > isize::MAX as usize)
+}
+
+impl Drop for IoBufferMut {
+    fn drop(&mut self) {
+        // SAFETY: memory was allocated with std::alloc::alloc with the same layout.
+        unsafe {
+            alloc::dealloc(
+                self.as_mut_ptr(),
+                Layout::from_size_align_unchecked(self.capacity, self.align),
+            )
+        }
+    }
+}
+
+impl Deref for IoBufferMut {
+    type Target = [u8];
+
+    fn deref(&self) -> &Self::Target {
+        self.as_slice()
+    }
+}
+
+impl DerefMut for IoBufferMut {
+    fn deref_mut(&mut self) -> &mut Self::Target {
+        self.as_mut_slice()
+    }
+}
+
+/// SAFETY: When advancing the internal cursor, the caller needs to make sure the bytes advcanced past have been initialized.
+unsafe impl bytes::BufMut for IoBufferMut {
+    #[inline]
+    fn remaining_mut(&self) -> usize {
+        // Although a `Vec` can have at most isize::MAX bytes, we never want to grow `IoBufferMut`.
+        // Thus, it can have at most `self.capacity` bytes.
+        self.capacity() - self.len()
+    }
+
+    // SAFETY: Caller needs to make sure the bytes being advanced past have been initialized.
+    #[inline]
+    unsafe fn advance_mut(&mut self, cnt: usize) {
+        let len: usize = self.len();
+        let remaining = self.remaining_mut();
+
+        if remaining < cnt {
+            panic_advance(cnt, remaining);
+        }
+
+        // Addition will not overflow since the sum is at most the capacity.
+        self.set_len(len + cnt);
+    }
+
+    #[inline]
+    fn chunk_mut(&mut self) -> &mut bytes::buf::UninitSlice {
+        let cap = self.capacity();
+        let len = self.len();
+
+        // SAFETY: Since `self.ptr` is valid for `cap` bytes, `self.ptr.add(len)` must be
+        // valid for `cap - len` bytes. The subtraction will not underflow since
+        // `len <= cap`.
+        unsafe { UninitSlice::from_raw_parts_mut(self.as_mut_ptr().add(len), cap - len) }
+    }
+}
+
+/// Panic with a nice error message.
+#[cold]
+fn panic_advance(idx: usize, len: usize) -> ! {
+    panic!(
+        "advance out of bounds: the len is {} but advancing by {}",
+        len, idx
+    );
+}
+
+/// Safety: [`IoBufferMut`] has exclusive ownership of the io buffer,
+/// and the location remains stable even if [`Self`] is moved.
+unsafe impl tokio_epoll_uring::IoBuf for IoBufferMut {
+    fn stable_ptr(&self) -> *const u8 {
+        self.as_ptr()
+    }
+
+    fn bytes_init(&self) -> usize {
+        self.len()
+    }
+
+    fn bytes_total(&self) -> usize {
+        self.capacity()
+    }
+}
+
+// SAFETY: See above.
+unsafe impl tokio_epoll_uring::IoBufMut for IoBufferMut {
+    fn stable_mut_ptr(&mut self) -> *mut u8 {
+        self.as_mut_ptr()
+    }
+
+    unsafe fn set_init(&mut self, init_len: usize) {
+        if self.len() < init_len {
+            self.set_len(init_len);
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+
+    use super::*;
+
+    #[test]
+    fn test_with_capacity_aligned() {
+        const ALIGN: usize = 4 * 1024;
+        let v = IoBufferMut::with_capacity_aligned(ALIGN * 4, ALIGN);
+        assert_eq!(v.len(), 0);
+        assert_eq!(v.capacity(), ALIGN * 4);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+
+        let v = IoBufferMut::with_capacity_aligned(ALIGN / 2, ALIGN);
+        assert_eq!(v.len(), 0);
+        assert_eq!(v.capacity(), ALIGN / 2);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+    }
+
+    #[test]
+    fn test_with_capacity_aligned_zeroed() {
+        const ALIGN: usize = 4 * 1024;
+        let v = IoBufferMut::with_capacity_aligned_zeroed(ALIGN, ALIGN);
+        assert_eq!(v.len(), ALIGN);
+        assert_eq!(v.capacity(), ALIGN);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+        assert_eq!(&v[..], &[0; ALIGN])
+    }
+
+    #[test]
+    fn test_reserve() {
+        use bytes::BufMut;
+        const ALIGN: usize = 4 * 1024;
+        let mut v = IoBufferMut::with_capacity_aligned(ALIGN, ALIGN);
+        let capacity = v.capacity();
+        v.reserve(capacity);
+        assert_eq!(v.capacity(), capacity);
+        let data = [b'a'; ALIGN];
+        v.put(&data[..]);
+        v.reserve(capacity);
+        assert!(v.capacity() >= capacity * 2);
+        assert_eq!(&v[..], &data[..]);
+        let capacity = v.capacity();
+        v.clear();
+        v.reserve(capacity);
+        assert_eq!(capacity, v.capacity());
+    }
+
+    #[test]
+    fn test_bytes_put() {
+        use bytes::BufMut;
+        const ALIGN: usize = 4 * 1024;
+        let mut v = IoBufferMut::with_capacity_aligned(ALIGN * 4, ALIGN);
+        let x = [b'a'; ALIGN];
+
+        for _ in 0..2 {
+            for _ in 0..4 {
+                v.put(&x[..]);
+            }
+            assert_eq!(v.len(), ALIGN * 4);
+            assert_eq!(v.capacity(), ALIGN * 4);
+            assert_eq!(v.align(), ALIGN);
+            assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+            v.clear()
+        }
+        assert_eq!(v.len(), 0);
+        assert_eq!(v.capacity(), ALIGN * 4);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+    }
+
+    #[test]
+    #[should_panic]
+    fn test_bytes_put_panic() {
+        use bytes::BufMut;
+        const ALIGN: usize = 4 * 1024;
+        let mut v = IoBufferMut::with_capacity_aligned(ALIGN * 4, ALIGN);
+        let x = [b'a'; ALIGN];
+        for _ in 0..5 {
+            v.put_slice(&x[..]);
+        }
+    }
+
+    #[test]
+    fn test_io_buf_put_slice() {
+        use tokio_epoll_uring::BoundedBufMut;
+        const ALIGN: usize = 4 * 1024;
+        let mut v = IoBufferMut::with_capacity_aligned(ALIGN, ALIGN);
+        let x = [b'a'; ALIGN];
+
+        for _ in 0..2 {
+            v.put_slice(&x[..]);
+            assert_eq!(v.len(), ALIGN);
+            assert_eq!(v.capacity(), ALIGN);
+            assert_eq!(v.align(), ALIGN);
+            assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+            v.clear()
+        }
+        assert_eq!(v.len(), 0);
+        assert_eq!(v.capacity(), ALIGN);
+        assert_eq!(v.align(), ALIGN);
+        assert_eq!(v.as_ptr().align_offset(ALIGN), 0);
+    }
+}

pageserver/src/virtual_file/owned_buffers_io/io_buf_aligned.rs

@@ -0,0 +1,9 @@
+#![allow(unused)]
+
+use tokio_epoll_uring::IoBufMut;
+
+use crate::virtual_file::dio::IoBufferMut;
+
+pub(crate) trait IoBufAlignedMut: IoBufMut {}
+
+impl IoBufAlignedMut for IoBufferMut {}

pageserver/src/virtual_file/owned_buffers_io/io_buf_ext.rs

@@ -1,5 +1,6 @@
 //! See [`FullSlice`].
 
+use crate::virtual_file::dio::IoBufferMut;
 use bytes::{Bytes, BytesMut};
 use std::ops::{Deref, Range};
 use tokio_epoll_uring::{BoundedBuf, IoBuf, Slice};
@@ -76,3 +77,4 @@ macro_rules! impl_io_buf_ext {
 impl_io_buf_ext!(Bytes);
 impl_io_buf_ext!(BytesMut);
 impl_io_buf_ext!(Vec<u8>);
+impl_io_buf_ext!(IoBufferMut);

test_runner/fixtures/neon_fixtures.py

@@ -401,6 +401,7 @@ class NeonEnvBuilder:
         safekeeper_extra_opts: Optional[list[str]] = None,
         storage_controller_port_override: Optional[int] = None,
         pageserver_io_buffer_alignment: Optional[int] = None,
+        pageserver_virtual_file_io_mode: Optional[str] = None,
     ):
         self.repo_dir = repo_dir
         self.rust_log_override = rust_log_override
@@ -455,6 +456,7 @@ class NeonEnvBuilder:
         self.storage_controller_port_override = storage_controller_port_override
 
         self.pageserver_io_buffer_alignment = pageserver_io_buffer_alignment
+        self.pageserver_virtual_file_io_mode = pageserver_virtual_file_io_mode
 
         assert test_name.startswith(
             "test_"
@@ -1028,6 +1030,7 @@ class NeonEnv:
         self.pageserver_virtual_file_io_engine = config.pageserver_virtual_file_io_engine
         self.pageserver_aux_file_policy = config.pageserver_aux_file_policy
         self.pageserver_io_buffer_alignment = config.pageserver_io_buffer_alignment
+        self.pageserver_virtual_file_io_mode = config.pageserver_virtual_file_io_mode
 
         # Create the neon_local's `NeonLocalInitConf`
         cfg: Dict[str, Any] = {
@@ -1091,7 +1094,10 @@ class NeonEnv:
                         for key, value in override.items():
                             ps_cfg[key] = value
 
-            ps_cfg["io_buffer_alignment"] = self.pageserver_io_buffer_alignment
+            if self.pageserver_io_buffer_alignment is not None:
+                ps_cfg["io_buffer_alignment"] = self.pageserver_io_buffer_alignment
+            if self.pageserver_virtual_file_io_mode is not None:
+                ps_cfg["virtual_file_io_mode"] = self.pageserver_virtual_file_io_mode
 
             # Create a corresponding NeonPageserver object
             self.pageservers.append(
@@ -1330,6 +1336,7 @@ def neon_simple_env(
     pageserver_aux_file_policy: Optional[AuxFileStore],
     pageserver_default_tenant_config_compaction_algorithm: Optional[Dict[str, Any]],
     pageserver_io_buffer_alignment: Optional[int],
+    pageserver_virtual_file_io_mode: Optional[str],
 ) -> Iterator[NeonEnv]:
     """
     Simple Neon environment, with no authentication and no safekeepers.
@@ -1356,6 +1363,7 @@ def neon_simple_env(
         pageserver_aux_file_policy=pageserver_aux_file_policy,
         pageserver_default_tenant_config_compaction_algorithm=pageserver_default_tenant_config_compaction_algorithm,
         pageserver_io_buffer_alignment=pageserver_io_buffer_alignment,
+        pageserver_virtual_file_io_mode=pageserver_virtual_file_io_mode,
     ) as builder:
         env = builder.init_start()
 
@@ -1380,6 +1388,7 @@ def neon_env_builder(
     pageserver_aux_file_policy: Optional[AuxFileStore],
     record_property: Callable[[str, object], None],
     pageserver_io_buffer_alignment: Optional[int],
+    pageserver_virtual_file_io_mode: Optional[str],
 ) -> Iterator[NeonEnvBuilder]:
     """
     Fixture to create a Neon environment for test.
@@ -1415,6 +1424,7 @@ def neon_env_builder(
         pageserver_aux_file_policy=pageserver_aux_file_policy,
         pageserver_default_tenant_config_compaction_algorithm=pageserver_default_tenant_config_compaction_algorithm,
         pageserver_io_buffer_alignment=pageserver_io_buffer_alignment,
+        pageserver_virtual_file_io_mode=pageserver_virtual_file_io_mode,
     ) as builder:
         yield builder
         # Propogate `preserve_database_files` to make it possible to use in other fixtures,

test_runner/fixtures/parametrize.py

@@ -39,6 +39,11 @@ def pageserver_io_buffer_alignment() -> Optional[int]:
     return None
 
 
+@pytest.fixture(scope="function", autouse=True)
+def pageserver_virtual_file_io_mode() -> Optional[str]:
+    return os.getenv("PAGESERVER_VIRTUAL_FILE_IO_MODE")
+
+
 @pytest.fixture(scope="function", autouse=True)
 def pageserver_aux_file_policy() -> Optional[AuxFileStore]:
     return None