Detect seek penalties on Linux/ChromeOS.

chrome/browser/metrics/drive_metrics_provider_linux.cc

+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "chrome/browser/metrics/drive_metrics_provider.h"
+
+#include <linux/kdev_t.h>  // For MAJOR()/MINOR().
+#include <sys/stat.h>
+#include <string>
+
+#include "base/basictypes.h"
+#include "base/files/file.h"
+#include "base/files/file_path.h"
+#include "base/files/file_util.h"
+#include "base/strings/stringprintf.h"
+
+#if defined(OS_CHROMEOS)
+#include "base/sys_info.h"
+#endif
+
+namespace {
+
+// See http://www.kernel.org/doc/Documentation/devices.txt for more info.
+const int kScsiMajorNumber = 8;
+const char kRotationalFormat[] = "/sys/block/sd%s/queue/rotational";
+
+}  // namespace
+
+// static
+bool DriveMetricsProvider::HasSeekPenalty(const base::FilePath& path,
+                                          bool* has_seek_penalty) {
+#if defined(OS_CHROMEOS)
+  std::string board = base::SysInfo::GetLsbReleaseBoard();
+  if (board != "unknown" && board != "parrot") {
+    // All ChromeOS devices have SSDs. Except some parrots.
+    *has_seek_penalty = false;
+    return true;
+  }
+#endif
+
+  base::File file(path, base::File::FLAG_OPEN | base::File::FLAG_READ);
+  if (!file.IsValid())
+    return false;
+
+  struct stat path_stat;
+  int error = fstat(file.GetPlatformFile(), &path_stat);
+  if (error < 0 || MAJOR(path_stat.st_dev) != kScsiMajorNumber) {
+    // TODO(dbeam): support non-SCSI drives (e.g., LVM)?
+    return false;
+  }
+
+  uint16 minor = MINOR(path_stat.st_dev) / 16;

[Lei Zhang, 2015/04/03 22:45:57]

Why is this divided by 16?

[Dan Beam, 2015/04/03 22:47:09]

16 partitions per drive

[Lei Zhang, 2015/04/03 22:51:10]

Oh, I forgot about that. That's why I was thinking device 17 is sdaa, but if the
max minor value is 255, and we have to divide by 16... how do we get to sdaa?
.... http://www.tldp.org/HOWTO/SCSI-2.4-HOWTO/dnames.html

[Dan Beam, 2015/04/03 23:01:40]

ignoring /dev/sdq+ for now

+  std::string sdXX;
+  if (minor >= 26)
+    sdXX += 'a' + (minor / 26) - 1;

[Dan Beam, 2015/04/03 22:27:05]

maybe this will

+  sdXX += 'a' + minor % 26;
+
+  std::string rotational = base::StringPrintf(kRotationalFormat, sdXX.c_str());
+  std::string rotates;
+  if (!base::ReadFileToString(base::FilePath(rotational), &rotates))
+    return false;
+
+  *has_seek_penalty = rotates.substr(0, 1) == "1";
+  return true;
+}