virtual_file metrics: distinguish first and subsequent open() syscalls (#6066)

This helps with identifying thrashing.

I don't love the name, but, there is already "close-by-replace".

While reading the code, I also found a case where we waste
work in a cache pressure situation:
https://github.com/neondatabase/neon/issues/6065

refs https://github.com/neondatabase/cloud/issues/8351

pageserver/src/metrics.rs

@@ -774,6 +774,7 @@ const STORAGE_IO_TIME_BUCKETS: &[f64] = &[
 )]
 pub(crate) enum StorageIoOperation {
     Open,
+    OpenAfterReplace,
     Close,
     CloseByReplace,
     Read,
@@ -787,6 +788,7 @@ impl StorageIoOperation {
     pub fn as_str(&self) -> &'static str {
         match self {
             StorageIoOperation::Open => "open",
+            StorageIoOperation::OpenAfterReplace => "open-after-replace",
             StorageIoOperation::Close => "close",
             StorageIoOperation::CloseByReplace => "close-by-replace",
             StorageIoOperation::Read => "read",
@@ -840,7 +842,6 @@ pub(crate) static STORAGE_IO_SIZE: Lazy<IntGaugeVec> = Lazy::new(|| {
     )
     .expect("failed to define a metric")
 });
-
 #[derive(Debug)]
 struct GlobalAndPerTimelineHistogram {
     global: Histogram,

pageserver/src/virtual_file.rs

@@ -288,6 +288,9 @@ impl VirtualFile {
         }
         let (handle, mut slot_guard) = get_open_files().find_victim_slot();
 
+        // NB: there is also StorageIoOperation::OpenAfterReplace which is for the case
+        // where our caller doesn't get to use the returned VirtualFile before its
+        // slot gets re-used by someone else.
         let file = STORAGE_IO_TIME_METRIC
             .get(StorageIoOperation::Open)
             .observe_closure_duration(|| open_options.open(path))?;
@@ -311,6 +314,9 @@ impl VirtualFile {
             timeline_id,
         };
 
+        // TODO: Under pressure, it's likely the slot will get re-used and
+        // the underlying file closed before they get around to using it.
+        // => https://github.com/neondatabase/neon/issues/6065
         slot_guard.file.replace(file);
 
         Ok(vfile)
@@ -421,9 +427,12 @@ impl VirtualFile {
         // now locked in write-mode. Find a free slot to put it in.
         let (handle, mut slot_guard) = open_files.find_victim_slot();
 
-        // Open the physical file
+        // Re-open the physical file.
+        // NB: we use StorageIoOperation::OpenAferReplace for this to distinguish this
+        // case from StorageIoOperation::Open. This helps with identifying thrashing
+        // of the virtual file descriptor cache.
         let file = STORAGE_IO_TIME_METRIC
-            .get(StorageIoOperation::Open)
+            .get(StorageIoOperation::OpenAfterReplace)
             .observe_closure_duration(|| self.open_options.open(&self.path))?;
 
         // Perform the requested operation on it