metrics: rename prefetch_mr_count to prefetch_requests_count

Signed-off-by: Changwei Ge <gechangwei@bytedance.com>

storage/src/cache/worker.rs

@@ -356,7 +356,7 @@ impl AsyncWorkerMgr {
         // So the average backend merged request size will be prefetch_data_amount/prefetch_mr_count.
         // We can measure merging possibility by this.
         let metrics = mgr.metrics.clone();
-        metrics.prefetch_mr_count.inc();
+        metrics.prefetch_requests_count.inc();
         metrics.prefetch_data_amount.add(size);
 
         if let Some(obj) = cache.get_blob_object() {
@@ -414,7 +414,7 @@ impl AsyncWorkerMgr {
         // Record how much prefetch data is requested from storage backend.
         // So the average backend merged request size will be prefetch_data_amount/prefetch_mr_count.
         // We can measure merging possibility by this.
-        mgr.metrics.prefetch_mr_count.inc();
+        mgr.metrics.prefetch_requests_count.inc();
         mgr.metrics.prefetch_data_amount.add(blob_size);
 
         if let Some(obj) = cache.get_blob_object() {

utils/src/metrics.rs

@@ -739,13 +739,24 @@ pub struct BlobcacheMetrics {
     // to estimate the possibility to merge backend IOs.
     // In unit of Bytes
     pub prefetch_data_amount: BasicMetric,
-    pub prefetch_mr_count: BasicMetric,
+    // Total prefetch requests issued from storage/blobs or rafs filesystem layer for each file that needs prefetch
+    pub prefetch_requests_count: BasicMetric,
     pub prefetch_workers: AtomicUsize,
     pub prefetch_unmerged_chunks: BasicMetric,
+    // Cumulative time latencies of each prefetch request which can be handled in parallel.
+    // It starts when the request is born including nydusd processing and schedule and end when the chunk is downloaded and stored.
+    // Then the average prefetch latency can be calculated by
+    // `prefetch_cumulative_time_millis / prefetch_requests_count`
     pub prefetch_cumulative_time_millis: BasicMetric,
+    // The time seconds part when nydusd begins to prefetch
+    // We can calculate prefetch average bandwidth by
+    // `prefetch_data_amount / (prefetch_end_time_secs - prefetch_begin_time_secs)`. Note, it does not take milliseconds into account yet.s
     pub prefetch_begin_time_secs: BasicMetric,
+    // The time milliseconds part when nydusd begins to prefetch
     pub prefetch_begin_time_millis: BasicMetric,
+    // The time seconds part when nydusd ends prefetching
     pub prefetch_end_time_secs: BasicMetric,
+    // The time milliseconds part when nydusd ends prefetching
     pub prefetch_end_time_millis: BasicMetric,
     pub buffered_backend_size: BasicMetric,
     pub data_all_ready: AtomicBool,