use O_DIRECT as preferred

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

libs/pageserver_api/src/models.rs

@@ -1005,7 +1005,11 @@ pub mod virtual_file {
 
     impl IoMode {
         pub const fn preferred() -> Self {
-            Self::Buffered
+            if cfg!(target_os = "linux") {
+                Self::Direct
+            } else {
+                Self::Buffered
+            }
         }
     }