C# FileStream : Optimal buffer size for writing large files?

Suppose I'm writing a couple of files to disk, between 2MB and 5GB. What are sensible buffer values for the FileStream ?

Is it sensible to work with buffersizes of several megabytes, or should I stick to k开发者_JS百科ilobyte-buffers ?

Default buffer size is 4 KiB.

Also, take a look here: Sequential File Programming Patterns and Performance with .NET

Programming patterns for sequential file access in the .NET Framework are described and the performance is measured. The default behavior provides excellent performance on a single disk – 50 MBps both reading and writing. Using large request sizes and doing file pre-allocation when possible have quantifiable benefits. When one considers disk arrays, .NET unbuffered IO delivers 800 MBps on a 16-disk array, but buffered IO delivers about 12% of that performance. Consequently, high-performance file and database utilities are still forced to use unbuffered IO for maximum sequential performance. The report is accompanied by downloadable source code that demonstrates the concepts and code that was used to obtain these measurements.

A quick little benchmark based on the document referenced shows no increase in performance on my system greater than 128KB buffer size. Your mileage may vary, feel free to use the below.

        Stopwatch sw = new Stopwatch();
        Random rand = new Random();  // seed a random number generator
        int numberOfBytes = 2 << 22; //8,192KB File
        byte nextByte;
        for (int i = 1; i <= 28; i++) //Limited loop to 28 to prevent out of memory
        {
            sw.Start();
            using (FileStream fs = new FileStream(
                String.Format(@"C:\TEMP\TEST{0}.DAT", i),  // name of file
                FileMode.Create,    // create or overwrite existing file
                FileAccess.Write,   // write-only access
                FileShare.None,     // no sharing
                2 << i,             // block transfer of i=18 -> size = 256 KB
                FileOptions.None))  
            {
                for (int j = 0; j < numberOfBytes; j++)
                {
                    nextByte = (byte)(rand.Next() % 256); // generate a random byte
                    fs.WriteByte(nextByte);               // write it
                } 
            }
            sw.Stop();
            Console.WriteLine(String.Format("Buffer is 2 << {0} Elapsed: {1}", i, sw.Elapsed));
            sw.Reset();
        }

TLDR: Buffer size mostly doesn't matter. Micro-optimize something else. Just dont use a buffer below 128KiB with large files or if you set useAsync=true on the FileStream.

Was a little annoyed that there wasn't a definitive answer to this question. So here is the log from a test I created. Test was run on a secondary SSD to avoid OS and general usage from skewing results.

"File" and "Buffer" are in bytes.

useAsync = false:

Test Name:  FileWritePerformance
Test Outcome:   Passed
Result StandardOutput:  
File: 1024, Buffer: 1, Elapsed: 00:00:00.0021467
File: 1024, Buffer: 2, Elapsed: 00:00:00.0011164
File: 1024, Buffer: 4, Elapsed: 00:00:00.0012392
File: 1024, Buffer: 8, Elapsed: 00:00:00.0010978
File: 1024, Buffer: 16, Elapsed: 00:00:00.0014263
File: 1024, Buffer: 32, Elapsed: 00:00:00.0010724
File: 1024, Buffer: 64, Elapsed: 00:00:00.0012182
File: 1024, Buffer: 128, Elapsed: 00:00:00.0013605
File: 1024, Buffer: 256, Elapsed: 00:00:00.0010234
File: 1024, Buffer: 512, Elapsed: 00:00:00.0011247
File: 1024, Buffer: 1024, Elapsed: 00:00:00.0013137
File: 1024, Buffer: 2048, Elapsed: 00:00:00.0013527
File: 1024, Buffer: 4096, Elapsed: 00:00:00.0010895
File: 1024, Buffer: 8192, Elapsed: 00:00:00.0017128
File: 1024, Buffer: 16384, Elapsed: 00:00:00.0011272
File: 1024, Buffer: 32768, Elapsed: 00:00:00.0010547
File: 1024, Buffer: 65536, Elapsed: 00:00:00.0011427
File: 1024, Buffer: 131072, Elapsed: 00:00:00.0011571
File: 1024, Buffer: 262144, Elapsed: 00:00:00.0010978
File: 1024, Buffer: 524288, Elapsed: 00:00:00.0010884
File: 1024, Buffer: 1048576, Elapsed: 00:00:00.0011504
File: 1024, Buffer: 2097152, Elapsed: 00:00:00.0011308
File: 1024, Buffer: 4194304, Elapsed: 00:00:00.0026100
File: 1024, Buffer: 8388608, Elapsed: 00:00:00.0015160
File: 1024, Buffer: 16777216, Elapsed: 00:00:00.0011654
File: 1024, Buffer: 33554432, Elapsed: 00:00:00.0013214
File: 1024, Buffer: 67108864, Elapsed: 00:00:00.0012531
File: 1024, Buffer: 134217728, Elapsed: 00:00:00.0031671
File: 1024, Buffer: 268435456, Elapsed: 00:00:00.0032789
File: 1024, Buffer: 536870912, Elapsed: 00:00:00.0049508
File: 1024, Buffer: 1073741824, Elapsed: 00:00:00.0079707
File: 1048576, Buffer: 1, Elapsed: 00:00:00.0175447
File: 1048576, Buffer: 2, Elapsed: 00:00:00.0213337
File: 1048576, Buffer: 4, Elapsed: 00:00:00.0139657
File: 1048576, Buffer: 8, Elapsed: 00:00:00.0135683
File: 1048576, Buffer: 16, Elapsed: 00:00:00.0124353
File: 1048576, Buffer: 32, Elapsed: 00:00:00.0226043
File: 1048576, Buffer: 64, Elapsed: 00:00:00.0123077
File: 1048576, Buffer: 128, Elapsed: 00:00:00.0130151
File: 1048576, Buffer: 256, Elapsed: 00:00:00.0130583
File: 1048576, Buffer: 512, Elapsed: 00:00:00.0130555
File: 1048576, Buffer: 1024, Elapsed: 00:00:00.0130760
File: 1048576, Buffer: 2048, Elapsed: 00:00:00.0111780
File: 1048576, Buffer: 4096, Elapsed: 00:00:00.0097345
File: 1048576, Buffer: 8192, Elapsed: 00:00:00.0096204
File: 1048576, Buffer: 16384, Elapsed: 00:00:00.0085635
File: 1048576, Buffer: 32768, Elapsed: 00:00:00.0081099
File: 1048576, Buffer: 65536, Elapsed: 00:00:00.0076785
File: 1048576, Buffer: 131072, Elapsed: 00:00:00.0069143
File: 1048576, Buffer: 262144, Elapsed: 00:00:00.0078431
File: 1048576, Buffer: 524288, Elapsed: 00:00:00.0065928
File: 1048576, Buffer: 1048576, Elapsed: 00:00:00.0096246
File: 1048576, Buffer: 2097152, Elapsed: 00:00:00.0094253
File: 1048576, Buffer: 4194304, Elapsed: 00:00:00.0089587
File: 1048576, Buffer: 8388608, Elapsed: 00:00:00.0073527
File: 1048576, Buffer: 16777216, Elapsed: 00:00:00.0075993
File: 1048576, Buffer: 33554432, Elapsed: 00:00:00.0069752
File: 1048576, Buffer: 67108864, Elapsed: 00:00:00.0087534
File: 1048576, Buffer: 134217728, Elapsed: 00:00:00.0089197
File: 1048576, Buffer: 268435456, Elapsed: 00:00:00.0072583
File: 1048576, Buffer: 536870912, Elapsed: 00:00:00.0077418
File: 1048576, Buffer: 1073741824, Elapsed: 00:00:00.0217475
File: 1073741824, Buffer: 1, Elapsed: 00:00:08.6674554
File: 1073741824, Buffer: 2, Elapsed: 00:00:08.3731788
File: 1073741824, Buffer: 4, Elapsed: 00:00:08.8015734
File: 1073741824, Buffer: 8, Elapsed: 00:00:08.4734106
File: 1073741824, Buffer: 16, Elapsed: 00:00:08.8242336
File: 1073741824, Buffer: 32, Elapsed: 00:00:08.3961399
File: 1073741824, Buffer: 64, Elapsed: 00:00:08.6481375
File: 1073741824, Buffer: 128, Elapsed: 00:00:08.4404953
File: 1073741824, Buffer: 256, Elapsed: 00:00:08.6811610
File: 1073741824, Buffer: 512, Elapsed: 00:00:08.3297239
File: 1073741824, Buffer: 1024, Elapsed: 00:00:08.9151381
File: 1073741824, Buffer: 2048, Elapsed: 00:00:06.7781371
File: 1073741824, Buffer: 4096, Elapsed: 00:00:05.7471923
File: 1073741824, Buffer: 8192, Elapsed: 00:00:04.6344858
File: 1073741824, Buffer: 16384, Elapsed: 00:00:04.4647610
File: 1073741824, Buffer: 32768, Elapsed: 00:00:04.2576851
File: 1073741824, Buffer: 65536, Elapsed: 00:00:04.3291236
File: 1073741824, Buffer: 131072, Elapsed: 00:00:04.1955776
File: 1073741824, Buffer: 262144, Elapsed: 00:00:04.5730469
File: 1073741824, Buffer: 524288, Elapsed: 00:00:05.1157704
File: 1073741824, Buffer: 1048576, Elapsed: 00:00:05.5262273
File: 1073741824, Buffer: 2097152, Elapsed: 00:00:05.6662534
File: 1073741824, Buffer: 4194304, Elapsed: 00:00:05.6285232
File: 1073741824, Buffer: 8388608, Elapsed: 00:00:05.5857300
File: 1073741824, Buffer: 16777216, Elapsed: 00:00:05.6386955
File: 1073741824, Buffer: 33554432, Elapsed: 00:00:05.6467390
File: 1073741824, Buffer: 67108864, Elapsed: 00:00:05.6969160
File: 1073741824, Buffer: 134217728, Elapsed: 00:00:05.7044284
File: 1073741824, Buffer: 268435456, Elapsed: 00:00:05.8279939
File: 1073741824, Buffer: 536870912, Elapsed: 00:00:05.9805622
File: 1073741824, Buffer: 1073741824, Elapsed: 00:00:06.1534688

useAsync = true:

Test Name:  FileWritePerformance
Test Outcome:   Passed
Result StandardOutput:  
File: 1024, Buffer: 1, Elapsed: 00:00:00.0034314
File: 1024, Buffer: 2, Elapsed: 00:00:00.0026302
File: 1024, Buffer: 4, Elapsed: 00:00:00.0024232
File: 1024, Buffer: 8, Elapsed: 00:00:00.0009797
File: 1024, Buffer: 16, Elapsed: 00:00:00.0047272
File: 1024, Buffer: 32, Elapsed: 00:00:00.0010624
File: 1024, Buffer: 64, Elapsed: 00:00:00.0010657
File: 1024, Buffer: 128, Elapsed: 00:00:00.0039249
File: 1024, Buffer: 256, Elapsed: 00:00:00.0011540
File: 1024, Buffer: 512, Elapsed: 00:00:00.0011479
File: 1024, Buffer: 1024, Elapsed: 00:00:00.0010245
File: 1024, Buffer: 2048, Elapsed: 00:00:00.0011083
File: 1024, Buffer: 4096, Elapsed: 00:00:00.0011263
File: 1024, Buffer: 8192, Elapsed: 00:00:00.0009716
File: 1024, Buffer: 16384, Elapsed: 00:00:00.0011319
File: 1024, Buffer: 32768, Elapsed: 00:00:00.0035012
File: 1024, Buffer: 65536, Elapsed: 00:00:00.0010170
File: 1024, Buffer: 131072, Elapsed: 00:00:00.0010995
File: 1024, Buffer: 262144, Elapsed: 00:00:00.0010381
File: 1024, Buffer: 524288, Elapsed: 00:00:00.0010018
File: 1024, Buffer: 1048576, Elapsed: 00:00:00.0011629
File: 1024, Buffer: 2097152, Elapsed: 00:00:00.0011969
File: 1024, Buffer: 4194304, Elapsed: 00:00:00.0021307
File: 1024, Buffer: 8388608, Elapsed: 00:00:00.0015213
File: 1024, Buffer: 16777216, Elapsed: 00:00:00.0012500
File: 1024, Buffer: 33554432, Elapsed: 00:00:00.0012102
File: 1024, Buffer: 67108864, Elapsed: 00:00:00.0013962
File: 1024, Buffer: 134217728, Elapsed: 00:00:00.0036005
File: 1024, Buffer: 268435456, Elapsed: 00:00:00.0034270
File: 1024, Buffer: 536870912, Elapsed: 00:00:00.0070516
File: 1024, Buffer: 1073741824, Elapsed: 00:00:00.0094566
File: 1048576, Buffer: 1, Elapsed: 00:00:00.1161675
File: 1048576, Buffer: 2, Elapsed: 00:00:00.0867371
File: 1048576, Buffer: 4, Elapsed: 00:00:00.0938809
File: 1048576, Buffer: 8, Elapsed: 00:00:00.0891819
File: 1048576, Buffer: 16, Elapsed: 00:00:00.0856688
File: 1048576, Buffer: 32, Elapsed: 00:00:00.0829121
File: 1048576, Buffer: 64, Elapsed: 00:00:00.1021716
File: 1048576, Buffer: 128, Elapsed: 00:00:00.0819636
File: 1048576, Buffer: 256, Elapsed: 00:00:00.0870802
File: 1048576, Buffer: 512, Elapsed: 00:00:00.0938601
File: 1048576, Buffer: 1024, Elapsed: 00:00:00.0906152
File: 1048576, Buffer: 2048, Elapsed: 00:00:00.0203416
File: 1048576, Buffer: 4096, Elapsed: 00:00:00.0156412
File: 1048576, Buffer: 8192, Elapsed: 00:00:00.0188322
File: 1048576, Buffer: 16384, Elapsed: 00:00:00.0090570
File: 1048576, Buffer: 32768, Elapsed: 00:00:00.0089590
File: 1048576, Buffer: 65536, Elapsed: 00:00:00.0111345
File: 1048576, Buffer: 131072, Elapsed: 00:00:00.0074629
File: 1048576, Buffer: 262144, Elapsed: 00:00:00.0087473
File: 1048576, Buffer: 524288, Elapsed: 00:00:00.0090963
File: 1048576, Buffer: 1048576, Elapsed: 00:00:00.0106754
File: 1048576, Buffer: 2097152, Elapsed: 00:00:00.0072628
File: 1048576, Buffer: 4194304, Elapsed: 00:00:00.0072143
File: 1048576, Buffer: 8388608, Elapsed: 00:00:00.0108595
File: 1048576, Buffer: 16777216, Elapsed: 00:00:00.0078417
File: 1048576, Buffer: 33554432, Elapsed: 00:00:00.0078852
File: 1048576, Buffer: 67108864, Elapsed: 00:00:00.0101042
File: 1048576, Buffer: 134217728, Elapsed: 00:00:00.0107911
File: 1048576, Buffer: 268435456, Elapsed: 00:00:00.0099841
File: 1048576, Buffer: 536870912, Elapsed: 00:00:00.0135941
File: 1048576, Buffer: 1073741824, Elapsed: 00:00:00.0199248
File: 1073741824, Buffer: 1, Elapsed: 00:01:24.9154625
File: 1073741824, Buffer: 2, Elapsed: 00:01:13.0649384
File: 1073741824, Buffer: 4, Elapsed: 00:01:20.7892031
File: 1073741824, Buffer: 8, Elapsed: 00:01:14.3207983
File: 1073741824, Buffer: 16, Elapsed: 00:01:19.1491590
File: 1073741824, Buffer: 32, Elapsed: 00:01:13.1612342
File: 1073741824, Buffer: 64, Elapsed: 00:01:21.2668008
File: 1073741824, Buffer: 128, Elapsed: 00:01:12.1101598
File: 1073741824, Buffer: 256, Elapsed: 00:01:17.3565432
File: 1073741824, Buffer: 512, Elapsed: 00:01:14.2698611
File: 1073741824, Buffer: 1024, Elapsed: 00:01:28.4650820
File: 1073741824, Buffer: 2048, Elapsed: 00:00:20.3608743
File: 1073741824, Buffer: 4096, Elapsed: 00:00:13.4953693
File: 1073741824, Buffer: 8192, Elapsed: 00:00:09.7105073
File: 1073741824, Buffer: 16384, Elapsed: 00:00:07.8587447
File: 1073741824, Buffer: 32768, Elapsed: 00:00:06.9201750
File: 1073741824, Buffer: 65536, Elapsed: 00:00:06.4229520
File: 1073741824, Buffer: 131072, Elapsed: 00:00:05.9591664
File: 1073741824, Buffer: 262144, Elapsed: 00:00:05.7492599
File: 1073741824, Buffer: 524288, Elapsed: 00:00:05.6921116
File: 1073741824, Buffer: 1048576, Elapsed: 00:00:05.6765633
File: 1073741824, Buffer: 2097152, Elapsed: 00:00:05.7068171
File: 1073741824, Buffer: 4194304, Elapsed: 00:00:05.6758360
File: 1073741824, Buffer: 8388608, Elapsed: 00:00:05.5947740
File: 1073741824, Buffer: 16777216, Elapsed: 00:00:05.5885432
File: 1073741824, Buffer: 33554432, Elapsed: 00:00:06.0881748
File: 1073741824, Buffer: 67108864, Elapsed: 00:00:06.2771976
File: 1073741824, Buffer: 134217728, Elapsed: 00:00:06.2455592
File: 1073741824, Buffer: 268435456, Elapsed: 00:00:06.4810064
File: 1073741824, Buffer: 536870912, Elapsed: 00:00:06.4810521
File: 1073741824, Buffer: 1073741824, Elapsed: 00:00:06.5314156

Code for the test:

    [TestClass]
    public class WritePerfTests
    {
        [TestMethod]
        public async Task FileWritePerformance()
        {
            var fileSize = 1L;
            var useAsync = true;
            using (Loggers.Register(ConsoleLogger.CreateDefault()))
            using (var logger = Loggers.Register(FileLogger.CreateDefault(typeof(WritePerfTests))))
            {
                logger.ClearLog();
                var bytes = Rng.Pseudo.GetBytes(1024);
                for (var i = 0; i < 3; i++)
                {
                    fileSize *= 1024;
                    for (var p = 0; p < 31; p++)
                    {
                        var buffer = (int)Math.Pow(2, p);
                        var file = new FileInfo($"d:/{fileSize}.{p}.test");
                        using (Disposable.Action(() => file.Delete()))
                        using (var stream = new FileStream(file.FullName, FileMode.Create, FileAccess.Write, FileShare.None, buffer, useAsync))
                        {
                            var w = Stopwatch.StartNew();
                            while (stream.Length < fileSize)
                                await stream.WriteAsync(bytes, 0, bytes.Length);
                            await stream.FlushAsync();
                            Loggers.Info($"File: {fileSize}, Buffer: {buffer}, Elapsed: {w.Elapsed}");
                        }
                        await Task.Delay(1000); //give file system chance to cleanup so next test wont be skewed
                    }
                }
            }
        }
    }