pageserver: add direct io config to virtual file (#9214)

## Problem
We need a way to incrementally switch to direct IO. During the rollout
we might want to switch to O_DIRECT on image and delta layer read path
first before others.

## Summary of changes
- Revisited and simplified direct io config in `PageserverConf`. 
- We could add a fallback mode for open, but for read there isn't a
reasonable alternative (without creating another buffered virtual file).
- Added a wrapper around `VirtualFile`, current implementation become
`VirtualFileInner`
- Use `open_v2`, `create_v2`, `open_with_options_v2` when we want to use
the IO mode specified in PS config.
- Once we onboard all IO through VirtualFile using this new API, we will
delete the old code path.
- Make io mode live configurable for benchmarking.
- Only guaranteed for files opened after the config change, so do it
before the experiment.

As an example, we are using `open_v2` with
`virtual_file::IoMode::Direct` in
https://github.com/neondatabase/neon/pull/9169

We also remove `io_buffer_alignment` config in
a04cfd754b and use it as a compile time
constant. This way we don't have to carry the alignment around or make
frequent call to retrieve this information from the static variable.

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

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -573,7 +573,7 @@ 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()
         );
 
@@ -791,7 +791,7 @@ 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")?;
 
@@ -1022,7 +1022,7 @@ impl DeltaLayerInner {
                             blob_meta.key,
                             PageReconstructError::Other(anyhow!(
                                 "Failed to read blobs from virtual file {}: {}",
-                                self.file.path,
+                                self.file.path(),
                                 kind
                             )),
                         );
@@ -1048,7 +1048,7 @@ impl DeltaLayerInner {
                             meta.meta.key,
                             PageReconstructError::Other(anyhow!(e).context(format!(
                                 "Failed to decompress blob from virtual file {}",
-                                self.file.path,
+                                self.file.path(),
                             ))),
                         );
 
@@ -1066,7 +1066,7 @@ impl DeltaLayerInner {
                             meta.meta.key,
                             PageReconstructError::Other(anyhow!(e).context(format!(
                                 "Failed to deserialize blob from virtual file {}",
-                                self.file.path,
+                                self.file.path(),
                             ))),
                         );
 
@@ -1198,7 +1198,6 @@ 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
@@ -1247,7 +1246,6 @@ impl DeltaLayerInner {
                         offsets.end.pos(),
                         meta,
                         max_read_size,
-                        align,
                     ))
                 }
             } else {