Ability to switch off filesystem read-aheads

Summary:
Ability to switch off filesystem read-aheads. This change is
backward-compatible: the default setting is to allow file
system read-aheads.

Test Plan: run benchmarks

Reviewers: heyongqiang, adsharma

Reviewed By: heyongqiang

Differential Revision: https://reviews.facebook.net/D5391

db/db_bench.cc

@@ -165,6 +165,7 @@ static int FLAGS_readwritepercent = 90;
 static leveldb::Env* FLAGS_env = leveldb::Env::Default();
 
 extern bool useOsBuffer;
+extern bool useFsReadAhead;
 
 namespace leveldb {
 
@@ -1133,6 +1134,9 @@ int main(int argc, char** argv) {
     } else if (sscanf(argv[i], "--bufferedio=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       useOsBuffer = n;
+    } else if (sscanf(argv[i], "--readhead=%d%c", &n, &junk) == 1 &&
+               (n == 0 || n == 1)) {
+      useFsReadAhead = n;
     } else if (sscanf(argv[i], "--statistics=%d%c", &n, &junk) == 1 &&
                (n == 0 || n == 1)) {
       if (n == 1) {

util/env_posix.cc

@@ -27,6 +27,7 @@
 #include "util/posix_logger.h"
 
 bool useOsBuffer = 1;     // cache data in OS buffers 
+bool useFsReadAhead = 1;  // allow filesystem to do readaheads
 
 namespace leveldb {
 
@@ -81,7 +82,12 @@ class PosixRandomAccessFile: public RandomAccessFile {
 
  public:
   PosixRandomAccessFile(const std::string& fname, int fd)
-      : filename_(fname), fd_(fd) { }
+      : filename_(fname), fd_(fd) { 
+    if (!useFsReadAhead) {
+      // disable read-aheads
+      posix_fadvise(fd, 0, 0, POSIX_FADV_RANDOM);
+    }
+  }
   virtual ~PosixRandomAccessFile() { close(fd_); }
 
   virtual Status Read(uint64_t offset, size_t n, Slice* result,