環境 mac 10.142.2 GHz Intel Core i7APPLE SSD AP0512M （底下有小伙伴測出相反的結果，跟硬盤有關系） 問題描述 看RocketMQ源碼的時候看到數據寫到MappedFile有兩種方式： 先寫入 writeBuffer， 再將writeBuffer 寫入到 FileChannel 再調用 force()刷盤； 數據直接寫入MappedByteBuffer， 調用force() 刷盤； 我的問題是為什么不直接采用第二種方法？于是我通過下面的代碼驗證兩種方式的寫性能。 相關代碼 import java.io.*; import java.nio.ByteBuffer; import java.nio.MappedByteBuffer; import java.nio.channels.FileChannel; public class MMapTest { static File file= new File("./test.txt"); static ByteBuffer buffer; static int fileSize = 8 * 1024 * 1024; static boolean del = true; public static void main(String[] args) { init(1); deleteFile(); int[] sizes = {128,256,512,4096,8192,1024*16,1024*32,1024*128,1024*512}; try { for (int size : sizes) { testDBChannel(size); testMappedByteBuffer(size); System.out.println(); } } catch (IOException e) { e.printStackTrace(); } } private static void init(int size) { buffer = ByteBuffer.allocateDirect(size); } private static void deleteFile() { file.delete(); } private static void testDBChannel(int size) throws IOException { init(size); RandomAccessFile rw = new RandomAccessFile(file, "rw"); FileChannel channel = rw.getChannel(); int writeSize = 0; Long start = System.currentTimeMillis(); while (writeSize < fileSize) { buffer.clear(); buffer.put(new byte[size]); buffer.flip(); channel.position(writeSize); channel.write(buffer); channel.force(false); writeSize += size; } //channel.force(false); System.out.println("DirectBuffer + FileChannel write " + size + " bytes every time cost: " + (System.currentTimeMillis() - start) + "ms"); if(del) deleteFile(); } private static void testMappedByteBuffer(int size) throws IOException { init(size); RandomAccessFile rw = new RandomAccessFile(file, "rw"); FileChannel channel = rw.getChannel(); MappedByteBuffer map = channel.map(FileChannel.MapMode.READ_WRITE, 0, fileSize); int writeSize = 0; Long start = System.currentTimeMillis(); while (writeSize < fileSize) { map.put(new byte[size]); map.force(); writeSize += size; } //map.force(); System.out.println("MappedByteBuffer write " + size + " bytes every time cost: " + (System.currentTimeMillis() - start) + "ms"); if(del) deleteFile(); } } 輸出： DirectBuffer + FileChannel write 128 bytes every time cost: 3577ms MappedByteBuffer write 128 bytes every time cost: 13518ms DirectBuffer + FileChannel write 256 bytes every time cost: 1968ms MappedByteBuffer write 256 bytes every time cost: 7044ms DirectBuffer + FileChannel write 512 bytes every time cost: 1001ms MappedByteBuffer write 512 bytes every time cost: 3037ms DirectBuffer + FileChannel write 1024 bytes every time cost: 659ms MappedByteBuffer write 1024 bytes every time cost: 1274ms DirectBuffer + FileChannel write 4096 bytes every time cost: 214ms MappedByteBuffer write 4096 bytes every time cost: 331ms DirectBuffer + FileChannel write 8192 bytes every time cost: 137ms MappedByteBuffer write 8192 bytes every time cost: 168ms DirectBuffer + FileChannel write 16384 bytes every time cost: 77ms MappedByteBuffer write 16384 bytes every time cost: 86ms DirectBuffer + FileChannel write 32768 bytes every time cost: 44ms MappedByteBuffer write 32768 bytes every time cost: 58ms DirectBuffer + FileChannel write 131072 bytes every time cost: 16ms MappedByteBuffer write 131072 bytes every time cost: 25ms DirectBuffer + FileChannel write 524288 bytes every time cost: 10ms MappedByteBuffer write 524288 bytes every time cost: 21ms 我的理解是兩種方式都都是將數據寫入到pageCache中再刷盤的，為什么耗時差這么多，具體兩種方式的實現原理是什么？ 一般情況下使用RocketMQ 都是異步刷盤，會利用OS的pageCache機制達到很高的性能；上面描述的這個問題是針對同步刷盤情況，按照 @Tyrael 第一種測試，SATA盤情況下，mbb 的性能 是要高于 db+fc 的，更加讓我懷疑為什么不直接用mbb。