experiment: for EphemeralFile write path, use StdFs io engine

together with previous commits, this brings us back down to
pre-regression

test_bulk_insert[neon-release-pg14-tokio-epoll-uring].wal_written: 345 MB
test_bulk_insert[neon-release-pg14-tokio-epoll-uring].wal_recovery: 9.991 s

pageserver/src/tenant/ephemeral_file.rs

@@ -5,7 +5,8 @@ use crate::config::PageServerConf;
 use crate::context::RequestContext;
 use crate::page_cache::{self, PAGE_SZ};
 use crate::tenant::block_io::{BlockCursor, BlockLease, BlockReader};
-use crate::virtual_file::{self, io_engine, VirtualFile};
+use crate::virtual_file::io_engine::IoEngine;
+use crate::virtual_file::{self, VirtualFile};
 use bytes::BytesMut;
 use camino::Utf8PathBuf;
 use pageserver_api::shard::TenantShardId;
@@ -160,7 +161,11 @@ impl EphemeralFile {
                         let (mutable_tail, res) = self
                             .ephemeral_file
                             .file
-                            .write_all_at(mutable_tail, self.blknum as u64 * PAGE_SZ as u64, io_engine::get() /* TODO: experiment with StdFs here, did it in an earlier commit in this branch */)
+                            .write_all_at(
+                                mutable_tail,
+                                self.blknum as u64 * PAGE_SZ as u64,
+                                IoEngine::StdFs,
+                            )
                             .await;
                         // TODO: If we panic before we can put the mutable_tail back, subsequent calls will fail.
                         // I.e., the IO isn't retryable if we panic.