Add perf test to measure duration of blob store digest computation.

+ Move BlobStoreTestUtils from cts/ to frameworks/base/tests/.

Bug: 148898557
Test: atest ./apct-tests/perftests/blobstore/src/com/android/perftests/blob/BlobStorePerfTests.java
Change-Id: I2de155d0c0c1fb602c57353ba4819bdc9cda8c0a

apct-tests/perftests/blobstore/Android.bp

@@ -0,0 +1,28 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+android_test {
+  name: "BlobStorePerfTests",
+  srcs: ["src/**/*.java"],
+  static_libs: [
+    "BlobStoreTestUtils",
+    "androidx.test.rules",
+    "androidx.annotation_annotation",
+    "apct-perftests-utils",
+    "ub-uiautomator",
+  ],
+  platform_apis: true,
+  test_suites: ["device-tests"],
+  certificate: "platform",
+}

apct-tests/perftests/blobstore/AndroidManifest.xml

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2020 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+
+<manifest xmlns:android="http://schemas.android.com/apk/res/android"
+          package="com.android.perftests.blob">
+
+    <application>
+        <uses-library android:name="android.test.runner" />
+    </application>
+
+    <instrumentation android:name="androidx.test.runner.AndroidJUnitRunner"
+                     android:targetPackage="com.android.perftests.blob"/>
+
+</manifest>

apct-tests/perftests/blobstore/AndroidTest.xml

@@ -0,0 +1,28 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Copyright (C) 2020 The Android Open Source Project
+
+     Licensed under the Apache License, Version 2.0 (the "License");
+     you may not use this file except in compliance with the License.
+     You may obtain a copy of the License at
+
+          http://www.apache.org/licenses/LICENSE-2.0
+
+     Unless required by applicable law or agreed to in writing, software
+     distributed under the License is distributed on an "AS IS" BASIS,
+     WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+     See the License for the specific language governing permissions and
+     limitations under the License.
+-->
+<configuration description="Runs BlobStorePerfTests metric instrumentation.">
+    <option name="test-suite-tag" value="apct" />
+    <option name="test-suite-tag" value="apct-metric-instrumentation" />
+    <target_preparer class="com.android.tradefed.targetprep.suite.SuiteApkInstaller">
+        <option name="cleanup-apks" value="true" />
+        <option name="test-file-name" value="BlobStorePerfTests.apk" />
+    </target_preparer>
+
+    <test class="com.android.tradefed.testtype.AndroidJUnitTest" >
+        <option name="package" value="com.android.perftests.blob" />
+        <option name="hidden-api-checks" value="false"/>
+    </test>
+</configuration>

apct-tests/perftests/blobstore/src/com/android/perftests/blob/AtraceUtils.java

@@ -0,0 +1,120 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.perftests.blob;
+
+import android.app.Instrumentation;
+import android.app.UiAutomation;
+import android.os.ParcelFileDescriptor;
+import android.perftests.utils.TraceMarkParser;
+import android.perftests.utils.TraceMarkParser.TraceMarkSlice;
+import android.support.test.uiautomator.UiDevice;
+import android.util.Log;
+
+import androidx.test.platform.app.InstrumentationRegistry;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.util.List;
+import java.util.function.BiConsumer;
+
+// Copy of com.android.frameworks.perftests.am.util.AtraceUtils. TODO: avoid this duplication.
+public class AtraceUtils {
+    private static final String TAG = "AtraceUtils";
+    private static final boolean VERBOSE = true;
+
+    private static final String ATRACE_START = "atrace --async_start -b %d -c %s";
+    private static final String ATRACE_DUMP = "atrace --async_dump";
+    private static final String ATRACE_STOP = "atrace --async_stop";
+    private static final int DEFAULT_ATRACE_BUF_SIZE = 1024;
+
+    private UiAutomation mAutomation;
+    private static AtraceUtils sUtils = null;
+    private boolean mStarted = false;
+
+    private AtraceUtils(Instrumentation instrumentation) {
+        mAutomation = instrumentation.getUiAutomation();
+    }
+
+    public static AtraceUtils getInstance(Instrumentation instrumentation) {
+        if (sUtils == null) {
+            sUtils = new AtraceUtils(instrumentation);
+        }
+        return sUtils;
+    }
+
+    /**
+     * @param categories The list of the categories to trace, separated with space.
+     */
+    public void startTrace(String categories) {
+        synchronized (this) {
+            if (mStarted) {
+                throw new IllegalStateException("atrace already started");
+            }
+            runShellCommand(String.format(
+                    ATRACE_START, DEFAULT_ATRACE_BUF_SIZE, categories));
+            mStarted = true;
+        }
+    }
+
+    public void stopTrace() {
+        synchronized (this) {
+            mStarted = false;
+            runShellCommand(ATRACE_STOP);
+        }
+    }
+
+    private String runShellCommand(String cmd) {
+        try {
+            return UiDevice.getInstance(
+                    InstrumentationRegistry.getInstrumentation()).executeShellCommand(cmd);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+
+    /**
+     * @param parser The function that can accept the buffer of atrace dump and parse it.
+     * @param handler The parse result handler
+     */
+    public void performDump(TraceMarkParser parser,
+            BiConsumer<String, List<TraceMarkSlice>> handler) {
+        parser.reset();
+        try {
+            if (VERBOSE) {
+                Log.i(TAG, "Collecting atrace dump...");
+            }
+            writeDataToBuf(mAutomation.executeShellCommand(ATRACE_DUMP), parser);
+        } catch (IOException e) {
+            Log.e(TAG, "Error in reading dump", e);
+        }
+        parser.forAllSlices(handler);
+    }
+
+    // The given file descriptor here will be closed by this function
+    private void writeDataToBuf(ParcelFileDescriptor pfDescriptor,
+            TraceMarkParser parser) throws IOException {
+        InputStream inputStream = new ParcelFileDescriptor.AutoCloseInputStream(pfDescriptor);
+        try (BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream))) {
+            String line;
+            while ((line = reader.readLine()) != null) {
+                parser.visit(line);
+            }
+        }
+    }
+}

apct-tests/perftests/blobstore/src/com/android/perftests/blob/BlobStorePerfTests.java

@@ -0,0 +1,146 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.perftests.blob;
+
+import android.app.blob.BlobStoreManager;
+import android.content.Context;
+import android.perftests.utils.ManualBenchmarkState;
+import android.perftests.utils.PerfManualStatusReporter;
+import android.perftests.utils.TraceMarkParser;
+import android.perftests.utils.TraceMarkParser.TraceMarkSlice;
+import android.support.test.uiautomator.UiDevice;
+
+import androidx.test.filters.LargeTest;
+import androidx.test.platform.app.InstrumentationRegistry;
+
+import com.android.utils.blob.DummyBlobData;
+
+import org.junit.After;
+import org.junit.Before;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
+
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.List;
+import java.util.concurrent.CompletableFuture;
+import java.util.concurrent.TimeUnit;
+
+@LargeTest
+@RunWith(Parameterized.class)
+public class BlobStorePerfTests {
+    // From frameworks/native/cmds/atrace/atrace.cpp
+    private static final String ATRACE_CATEGORY_SYSTEM_SERVER = "ss";
+    // From f/b/apex/blobstore/service/java/com/android/server/blob/BlobStoreSession.java
+    private static final String ATRACE_COMPUTE_DIGEST_PREFIX = "computeBlobDigest-";
+
+    private Context mContext;
+    private BlobStoreManager mBlobStoreManager;
+    private AtraceUtils mAtraceUtils;
+    private ManualBenchmarkState mState;
+
+    @Rule
+    public PerfManualStatusReporter mPerfManualStatusReporter = new PerfManualStatusReporter();
+
+    @Parameterized.Parameter(0)
+    public int fileSizeInMb;
+
+    @Parameterized.Parameters(name = "{0}MB")
+    public static Collection<Object[]> getParameters() {
+        return Arrays.asList(new Object[][] {
+                { 25 },
+                { 50 },
+                { 100 },
+                { 200 },
+        });
+    }
+
+    @Before
+    public void setUp() {
+        mContext = InstrumentationRegistry.getInstrumentation().getTargetContext();
+        mBlobStoreManager = (BlobStoreManager) mContext.getSystemService(
+                Context.BLOB_STORE_SERVICE);
+        mAtraceUtils = AtraceUtils.getInstance(InstrumentationRegistry.getInstrumentation());
+        mState = mPerfManualStatusReporter.getBenchmarkState();
+    }
+
+    @After
+    public void tearDown() {
+        // TODO: Add a blob_store shell command to trigger idle maintenance to avoid hardcoding
+        // job id like this.
+        // From BlobStoreConfig.IDLE_JOB_ID = 191934935.
+        runShellCommand("cmd jobscheduler run -f android 191934935");
+    }
+
+    @Test
+    public void testComputeDigest() throws Exception {
+        mAtraceUtils.startTrace(ATRACE_CATEGORY_SYSTEM_SERVER);
+        try {
+            final List<Long> durations = new ArrayList<>();
+            final DummyBlobData blobData = prepareDataBlob(fileSizeInMb);
+            final TraceMarkParser parser = new TraceMarkParser(
+                    line -> line.name.startsWith(ATRACE_COMPUTE_DIGEST_PREFIX));
+            while (mState.keepRunning(durations)) {
+                commitBlob(blobData);
+
+                durations.clear();
+                collectDigestDurationsFromTrace(parser, durations);
+                // get and delete blobId
+            }
+        } finally {
+            mAtraceUtils.stopTrace();
+        }
+    }
+
+    private void collectDigestDurationsFromTrace(TraceMarkParser parser, List<Long> durations) {
+        mAtraceUtils.performDump(parser, (key, slices) -> {
+            for (TraceMarkSlice slice : slices) {
+                durations.add(TimeUnit.MICROSECONDS.toNanos(slice.getDurationInMicroseconds()));
+            }
+        });
+    }
+
+    private DummyBlobData prepareDataBlob(int fileSizeInMb) throws Exception {
+        final DummyBlobData blobData = new DummyBlobData(mContext,
+                fileSizeInMb * 1024 * 1024 /* bytes */);
+        blobData.prepare();
+        return blobData;
+    }
+
+    private void commitBlob(DummyBlobData blobData) throws Exception {
+        final long sessionId = mBlobStoreManager.createSession(blobData.getBlobHandle());
+        try (BlobStoreManager.Session session = mBlobStoreManager.openSession(sessionId)) {
+            blobData.writeToSession(session);
+            final CompletableFuture<Integer> callback = new CompletableFuture<>();
+            session.commit(mContext.getMainExecutor(), callback::complete);
+            // Ignore commit callback result.
+            callback.get();
+        }
+    }
+
+    private String runShellCommand(String cmd) {
+        try {
+            return UiDevice.getInstance(
+                    InstrumentationRegistry.getInstrumentation()).executeShellCommand(cmd);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

apex/blobstore/service/java/com/android/server/blob/BlobStoreSession.java

@@ -21,6 +21,7 @@ import static android.app.blob.XmlTags.ATTR_PACKAGE;
 import static android.app.blob.XmlTags.ATTR_UID;
 import static android.app.blob.XmlTags.TAG_ACCESS_MODE;
 import static android.app.blob.XmlTags.TAG_BLOB_HANDLE;
+import static android.os.Trace.TRACE_TAG_SYSTEM_SERVER;
 import static android.system.OsConstants.O_CREAT;
 import static android.system.OsConstants.O_RDONLY;
 import static android.system.OsConstants.O_RDWR;
@@ -41,6 +42,7 @@ import android.os.FileUtils;
 import android.os.ParcelFileDescriptor;
 import android.os.RemoteException;
 import android.os.RevocableFileDescriptor;
+import android.os.Trace;
 import android.os.storage.StorageManager;
 import android.system.ErrnoException;
 import android.system.Os;
@@ -382,9 +384,13 @@ class BlobStoreSession extends IBlobStoreSession.Stub {
     void verifyBlobData() {
         byte[] actualDigest = null;
         try {
+            Trace.traceBegin(TRACE_TAG_SYSTEM_SERVER,
+                    "computeBlobDigest-i" + mSessionId + "-l" + getSessionFile().length());
             actualDigest = FileUtils.digest(getSessionFile(), mBlobHandle.algorithm);
         } catch (IOException | NoSuchAlgorithmException e) {
             Slog.e(TAG, "Error computing the digest", e);
+        } finally {
+            Trace.traceEnd(TRACE_TAG_SYSTEM_SERVER);
         }
         synchronized (mSessionLock) {
             if (actualDigest != null && Arrays.equals(actualDigest, mBlobHandle.digest)) {

tests/BlobStoreTestUtils/Android.bp

@@ -0,0 +1,20 @@
+// Copyright (C) 2020 The Android Open Source Project
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+java_library {
+  name: "BlobStoreTestUtils",
+  srcs: ["src/**/*.java"],
+  static_libs: ["truth-prebuilt"],
+  platform_apis: true
+}

tests/BlobStoreTestUtils/src/com/android/utils/blob/DummyBlobData.java

@@ -0,0 +1,227 @@
+/*
+ * Copyright 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.utils.blob;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import android.app.blob.BlobHandle;
+import android.app.blob.BlobStoreManager;
+import android.content.Context;
+import android.os.FileUtils;
+import android.os.ParcelFileDescriptor;
+
+import java.io.BufferedInputStream;
+import java.io.File;
+import java.io.FileDescriptor;
+import java.io.FileInputStream;
+import java.io.FileOutputStream;
+import java.io.InputStream;
+import java.io.OutputStream;
+import java.io.RandomAccessFile;
+import java.nio.file.Files;
+import java.security.MessageDigest;
+import java.util.Random;
+import java.util.concurrent.TimeUnit;
+
+public class DummyBlobData {
+    private static final long DEFAULT_SIZE_BYTES = 10 * 1024L * 1024L;
+    private static final int BUFFER_SIZE_BYTES = 16 * 1024;
+
+    private final Context mContext;
+    private final Random mRandom;
+    private final File mFile;
+    private final long mFileSize;
+    private final String mLabel;
+
+    byte[] mFileDigest;
+    long mExpiryTimeMs;
+
+    public DummyBlobData(Context context) {
+        this(context, new Random(0), "blob_" + System.nanoTime());
+    }
+
+    public DummyBlobData(Context context, long fileSize) {
+        this(context, fileSize, new Random(0), "blob_" + System.nanoTime(), "Test label");
+    }
+
+    public DummyBlobData(Context context, Random random, String fileName) {
+        this(context, DEFAULT_SIZE_BYTES, random, fileName, "Test label");
+    }
+
+    public DummyBlobData(Context context, Random random, String fileName, String label) {
+        this(context, DEFAULT_SIZE_BYTES, random, fileName, label);
+    }
+
+    public DummyBlobData(Context context, long fileSize, Random random, String fileName,
+            String label) {
+        mContext = context;
+        mRandom = random;
+        mFile = new File(mContext.getFilesDir(), fileName);
+        mFileSize = fileSize;
+        mLabel = label;
+    }
+
+    public void prepare() throws Exception {
+        try (RandomAccessFile file = new RandomAccessFile(mFile, "rw")) {
+            writeRandomData(file, mFileSize);
+        }
+        mFileDigest = FileUtils.digest(mFile, "SHA-256");
+        mExpiryTimeMs = System.currentTimeMillis() + TimeUnit.DAYS.toMillis(1);
+    }
+
+    public BlobHandle getBlobHandle() throws Exception {
+        return BlobHandle.createWithSha256(createSha256Digest(mFile), mLabel,
+                mExpiryTimeMs, "test_tag");
+    }
+
+    public long getFileSize() throws Exception {
+        return mFileSize;
+    }
+
+    public long getExpiryTimeMillis() {
+        return mExpiryTimeMs;
+    }
+
+    public void delete() {
+        mFile.delete();
+    }
+
+    public void writeToSession(BlobStoreManager.Session session) throws Exception {
+        writeToSession(session, 0, mFileSize);
+    }
+
+    public void writeToSession(BlobStoreManager.Session session,
+            long offsetBytes, long lengthBytes) throws Exception {
+        try (FileInputStream in = new FileInputStream(mFile)) {
+            in.getChannel().position(offsetBytes);
+            try (FileOutputStream out = new ParcelFileDescriptor.AutoCloseOutputStream(
+                    session.openWrite(offsetBytes, lengthBytes))) {
+                copy(in, out, lengthBytes);
+            }
+        }
+    }
+
+    public void writeToFd(FileDescriptor fd, long offsetBytes, long lengthBytes) throws Exception {
+        try (FileInputStream in = new FileInputStream(mFile)) {
+            in.getChannel().position(offsetBytes);
+            try (FileOutputStream out = new FileOutputStream(fd)) {
+                copy(in, out, lengthBytes);
+            }
+        }
+    }
+
+    private void copy(InputStream in, OutputStream out, long lengthBytes) throws Exception {
+        final byte[] buffer = new byte[BUFFER_SIZE_BYTES];
+        long bytesWrittern = 0;
+        while (bytesWrittern < lengthBytes) {
+            final int toWrite = (bytesWrittern + buffer.length <= lengthBytes)
+                    ? buffer.length : (int) (lengthBytes - bytesWrittern);
+            in.read(buffer, 0, toWrite);
+            out.write(buffer, 0, toWrite);
+            bytesWrittern += toWrite;
+        }
+    }
+
+    public void readFromSessionAndVerifyBytes(BlobStoreManager.Session session,
+            long offsetBytes, int lengthBytes) throws Exception {
+        final byte[] expectedBytes = new byte[lengthBytes];
+        try (FileInputStream in = new FileInputStream(mFile)) {
+            read(in, expectedBytes, offsetBytes, lengthBytes);
+        }
+
+        final byte[] actualBytes = new byte[lengthBytes];
+        try (FileInputStream in = new ParcelFileDescriptor.AutoCloseInputStream(
+                session.openWrite(0L, 0L))) {
+            read(in, actualBytes, offsetBytes, lengthBytes);
+        }
+
+        assertThat(actualBytes).isEqualTo(expectedBytes);
+
+    }
+
+    private void read(FileInputStream in, byte[] buffer,
+            long offsetBytes, int lengthBytes) throws Exception {
+        in.getChannel().position(offsetBytes);
+        in.read(buffer, 0, lengthBytes);
+    }
+
+    public void readFromSessionAndVerifyDigest(BlobStoreManager.Session session)
+            throws Exception {
+        readFromSessionAndVerifyDigest(session, 0, mFile.length());
+    }
+
+    public void readFromSessionAndVerifyDigest(BlobStoreManager.Session session,
+            long offsetBytes, long lengthBytes) throws Exception {
+        final byte[] actualDigest;
+        try (FileInputStream in = new ParcelFileDescriptor.AutoCloseInputStream(
+                session.openWrite(0L, 0L))) {
+            actualDigest = createSha256Digest(in, offsetBytes, lengthBytes);
+        }
+
+        assertThat(actualDigest).isEqualTo(mFileDigest);
+    }
+
+    public void verifyBlob(ParcelFileDescriptor pfd) throws Exception {
+        final byte[] actualDigest;
+        try (FileInputStream in = new ParcelFileDescriptor.AutoCloseInputStream(pfd)) {
+            actualDigest = FileUtils.digest(in, "SHA-256");
+        }
+        assertThat(actualDigest).isEqualTo(mFileDigest);
+    }
+
+    private byte[] createSha256Digest(FileInputStream in, long offsetBytes, long lengthBytes)
+            throws Exception {
+        final MessageDigest digest = MessageDigest.getInstance("SHA-256");
+        in.getChannel().position(offsetBytes);
+        final byte[] buffer = new byte[BUFFER_SIZE_BYTES];
+        long bytesRead = 0;
+        while (bytesRead < lengthBytes) {
+            int toRead = (bytesRead + buffer.length <= lengthBytes)
+                    ? buffer.length : (int) (lengthBytes - bytesRead);
+            toRead = in.read(buffer, 0, toRead);
+            digest.update(buffer, 0, toRead);
+            bytesRead += toRead;
+        }
+        return digest.digest();
+    }
+
+    private byte[] createSha256Digest(File file) throws Exception {
+        final MessageDigest digest = MessageDigest.getInstance("SHA-256");
+        try (BufferedInputStream in = new BufferedInputStream(
+                Files.newInputStream(file.toPath()))) {
+            final byte[] buffer = new byte[BUFFER_SIZE_BYTES];
+            int bytesRead;
+            while ((bytesRead = in.read(buffer)) > 0) {
+                digest.update(buffer, 0, bytesRead);
+            }
+        }
+        return digest.digest();
+    }
+
+    private void writeRandomData(RandomAccessFile file, long fileSize)
+            throws Exception {
+        long bytesWritten = 0;
+        final byte[] buffer = new byte[BUFFER_SIZE_BYTES];
+        while (bytesWritten < fileSize) {
+            mRandom.nextBytes(buffer);
+            final int toWrite = (bytesWritten + buffer.length <= fileSize)
+                    ? buffer.length : (int) (fileSize - bytesWritten);
+            file.seek(bytesWritten);
+            file.write(buffer, 0, toWrite);
+            bytesWritten += toWrite;
+        }
+    }
+}