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

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 } } } } } 

Short answer: For large file set buffer size >81920 and preallocate all file size before writing.

Long answer:

1. There are penalty for buffers that are smaller then physical sector size and buffers smaller then 4096 bytes must not be used, because since 2011 disk are using 4096 bytes in sector (aka 4K native, 4Kn), and now some SSD have sectors larger than 4096 bytes.
2. For large file you must preallocate it size (set Length of FileStream) before writing. This will speed up copy process, because file system will increase file size only once.
3. If you look in .NET Source FileStream use 4096 as default buffer size. Pipilines use 81920 as default buffer size and 2M as maximum buffer size for copy processes that involves async operations.