Add read_nanos to IOStatsContext.

Summary: MyRocks need a mechanism to track read outliers. We need to expose this
stat.

Test Plan: None

Reviewers: sdong

CC: leveldb

Task ID: #7152512

Blame Rev:

include/rocksdb/iostats_context.h

@@ -34,6 +34,8 @@ struct IOStatsContext {
   uint64_t allocate_nanos;
   // time spent in write() and pwrite().
   uint64_t write_nanos;
+  // time spent in read() and pread()
+  uint64_t read_nanos;
   // time spent in sync_file_range().
   uint64_t range_sync_nanos;
 

util/env_posix.cc

@@ -256,7 +256,11 @@ class PosixRandomAccessFile: public RandomAccessFile {
     size_t left = n;
     char* ptr = scratch;
     while (left > 0) {
-      r = pread(fd_, ptr, left, static_cast<off_t>(offset));
+      {
+        IOSTATS_TIMER_GUARD(read_nanos);
+        r = pread(fd_, ptr, left, static_cast<off_t>(offset));
+      }
+
       if (r <= 0) {
         if (errno == EINTR) {
           continue;
@@ -975,7 +979,11 @@ class PosixRandomRWFile : public RandomRWFile {
     size_t left = n;
     char* ptr = scratch;
     while (left > 0) {
-      r = pread(fd_, ptr, left, static_cast<off_t>(offset));
+      {
+        IOSTATS_TIMER_GUARD(read_nanos);
+        r = pread(fd_, ptr, left, static_cast<off_t>(offset));
+      }
+
       if (r <= 0) {
         if (errno == EINTR) {
           continue;

util/iostats_context.cc

@@ -20,6 +20,7 @@ void IOStatsContext::Reset() {
   open_nanos = 0;
   allocate_nanos = 0;
   write_nanos = 0;
+  read_nanos = 0;
   range_sync_nanos = 0;
   logger_nanos = 0;
 }
@@ -34,6 +35,7 @@ std::string IOStatsContext::ToString() const {
      << OUTPUT(open_nanos)
      << OUTPUT(allocate_nanos)
      << OUTPUT(write_nanos)
+     << OUTPUT(read_nanos)
      << OUTPUT(range_sync_nanos)
      << OUTPUT(logger_nanos);