diff --git a/db/db_bench.cc b/db/db_bench.cc
index 878a89b65..176927ba6 100644
--- a/db/db_bench.cc
+++ b/db/db_bench.cc
@@ -110,6 +110,8 @@ static int FLAGS_cache_numshardbits = -1;
 // Verify checksum for every block read from storage
 static bool FLAGS_verify_checksum = false;
 
+extern bool useOsBuffer;
+
 namespace leveldb {
 
 namespace {
@@ -782,7 +784,7 @@ class Benchmark {
   void ReadRandom(ThreadState* thread) {
     ReadOptions options(FLAGS_verify_checksum, true);
     std::string value;
-    int found = 0;
+    long found = 0;
     for (long i = 0; i < reads_; i++) {
       char key[100];
       const int k = thread->rand.Next() % FLAGS_num;
@@ -793,7 +795,7 @@ class Benchmark {
       thread->stats.FinishedSingleOp();
     }
     char msg[100];
-    snprintf(msg, sizeof(msg), "(%d of %d found)", found, num_);
+    snprintf(msg, sizeof(msg), "(%ld of %ld found)", found, num_);
     thread->stats.AddMessage(msg);
   }
 
@@ -825,7 +827,7 @@ class Benchmark {
   void SeekRandom(ThreadState* thread) {
     ReadOptions options(FLAGS_verify_checksum, true);
     std::string value;
-    int found = 0;
+    long found = 0;
     for (long i = 0; i < reads_; i++) {
       Iterator* iter = db_->NewIterator(options);
       char key[100];
@@ -837,7 +839,7 @@ class Benchmark {
       thread->stats.FinishedSingleOp();
     }
     char msg[100];
-    snprintf(msg, sizeof(msg), "(%d of %d found)", found, num_);
+    snprintf(msg, sizeof(msg), "(%ld of %ld found)", found, num_);
     thread->stats.AddMessage(msg);
   }
 
@@ -978,6 +980,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--verify_checksum=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       FLAGS_verify_checksum = n;
+    } else if (sscanf(argv[i], "--bufferedio=%d%c", &n, &junk) == 1 &&
+               (n == 0 || n == 1)) {
+      useOsBuffer = n;
     } else {
       fprintf(stderr, "Invalid flag '%s'\n", argv[i]);
       exit(1);
diff --git a/util/env_posix.cc b/util/env_posix.cc
index cb1f6fc05..95d324602 100644
--- a/util/env_posix.cc
+++ b/util/env_posix.cc
@@ -25,6 +25,8 @@
 #include "util/logging.h"
 #include "util/posix_logger.h"
 
+bool useOsBuffer = 1;     // cache data in OS buffers 
+
 namespace leveldb {
 
 namespace {
@@ -86,6 +88,9 @@ class PosixRandomAccessFile: public RandomAccessFile {
       // An error: return a non-ok status
       s = IOError(filename_, errno);
     }
+    if (!useOsBuffer) {
+      posix_fadvise(fd_, offset, n, POSIX_FADV_DONTNEED); // free OS pages
+    }
     return s;
   }
 };
@@ -329,7 +334,7 @@ class PosixEnv : public Env {
     int fd = open(fname.c_str(), O_RDONLY);
     if (fd < 0) {
       s = IOError(fname, errno);
-    } else if (sizeof(void*) >= 8) {
+    } else if (useOsBuffer && sizeof(void*) >= 8) {
       // Use mmap when virtual address-space is plentiful.
       uint64_t size;
       s = GetFileSize(fname, &size);