Warn/error if the created file exceeds limits

* if the created layer file is above the target size, warn
* if it is also above the S3 upload limit, don't error when
  uploading, but already error when writing the file.

pageserver/src/tenant/storage_layer/delta_layer.rs

@@ -764,6 +764,17 @@ impl DeltaLayerWriterInner {
             .metadata()
             .context("get file metadata to determine size")?;
 
+        // 5GB limit for objects without multipart upload (which we don't want to use)
+        // Make it a little bit below to account for differing GB units
+        // https://docs.aws.amazon.com/AmazonS3/latest/userguide/upload-objects.html
+        const S3_UPLOAD_LIMIT: u64 = 4_500_000_000;
+        ensure!(
+            metadata.len() <= S3_UPLOAD_LIMIT,
+            "Created delta layer file at {} of size {} above limit {S3_UPLOAD_LIMIT}!",
+            file.path.display(),
+            metadata.len()
+        );
+
         // Note: Because we opened the file in write-only mode, we cannot
         // reuse the same VirtualFile for reading later. That's why we don't
         // set inner.file here. The first read will have to re-open it.

pageserver/src/tenant/timeline.rs

@@ -3740,6 +3740,16 @@ impl Timeline {
 
         // Sync layers
         if !new_layers.is_empty() {
+            // Print a warning if the created layer is larger than double the target size
+            let warn_limit = target_file_size * 2;
+            for layer in new_layers.iter() {
+                if layer.desc.file_size > warn_limit {
+                    warn!(
+                        %layer,
+                        "created delta file of size {} larger than double of target of {target_file_size}", layer.desc.file_size
+                    );
+                }
+            }
             let mut layer_paths: Vec<PathBuf> = new_layers.iter().map(|l| l.path()).collect();
 
             // Fsync all the layer files and directory using multiple threads to