What is the fastest way to create a checksum for large files in C

Question

I have to sync large files across some machines  The files can be up to 6GB in size  The sync will be done manually every few weeks  I cant take the filename into consideration because they can change anytime    My plan is to create checksums on the destination PC and on the source PC and then copy all files with a checksum  which are not already in the destination  to the destination  My first attempt was something like this   using System IO  using System Security Cryptography   private static string GetChecksum string file        using  FileStream stream   File OpenRead file                 SHA256Managed sha   new SHA256Managed            byte   checksum   sha ComputeHash stream           return BitConverter ToString checksum  Replace  -   String Empty             The Problem was the runtime  - with SHA256 with a 1 6 GB File -  20 minutes - with MD5 with a 1 6 GB File -  6 15 minutes  Is there a better - faster - way to get the checksum  maybe with a better hash function

User · Answer

As Anton Gogolev noted  FileStream reads 4096 bytes at a time by default  But you can specify any other value using the FileStream constructor   new FileStream file  FileMode Open  FileAccess Read  FileShare ReadWrite  16   1024   1024    Note that Brad Abrams from Microsoft wrote in 2004      there is zero benefit from wrapping a BufferedStream around a   FileStream   We copied BufferedStream   s buffering logic into   FileStream about 4 years ago to encourage better default performance   source

User · Answer

You re doing something wrong  probably too small read buffer   On a machine of undecent age  Athlon 2x1800MP from 2002  that has DMA on disk probably out of whack  6 6M s is damn slow when doing sequential reads    Create a 1G file with  random  data     dd if  dev sdb of temp dat bs 1M count 1024     1073741824 bytes  1 1 GB  copied  161 698 s  6 6 MB s    time sha1sum -b temp dat abb88a0081f5db999d0701de2117d2cb21d192a2  temp dat   1m5 299s    time md5sum -b temp dat 9995e1c1a704f9c1eb6ca11e7ecb7276  temp dat   1m58 832s  This is also weird  md5 is consistently slower than sha1 for me  reran several times

User · Answer

Ok - thanks to all of you - let me wrap this up    using a  native  exe to do the hashing took time from 6 Minutes to 10 Seconds which is huge  Increasing the buffer was even faster - 1 6GB file took 5 2 seconds using MD5 in  Net  so I will go with this solution - thanks again

User · Answer

You can have a look to XxHash Net   https   github com wilhelmliao xxHash NET   The xxHash algorythm seems to be faster than all other  Some benchmark on the xxHash site   https   github com Cyan4973 xxHash  PS  I ve not yet used it

User · Answer

Invoke the windows port of md5sum exe  It s about two times as fast as the  NET implementation  at least on my machine using a 1 2 GB file   public static string Md5SumByProcess string file        var p   new Process         p StartInfo FileName    md5sum exe       p StartInfo Arguments   file                  p StartInfo UseShellExecute   false      p StartInfo RedirectStandardOutput   true      p Start        p WaitForExit                   string output   p StandardOutput ReadToEnd        return output Split      0  Substring 1  ToUpper

User · Answer

I know that I am late to party but performed test before actually implement the solution   I did perform test against inbuilt MD5 class and also md5sum exe  In my case inbuilt class took 13 second where md5sum exe too around 16-18 seconds in every run       DateTime current   DateTime Now      string file     C  text iso    It s 2 5 Gb file     string output      using  var md5   MD5 Create                  using  var stream   File OpenRead file                         byte   checksum   md5 ComputeHash stream               output   BitConverter ToString checksum  Replace  -   String Empty  ToLower                Console WriteLine  Total seconds        DateTime Now - current  TotalSeconds ToString           output

User · Answer

Don t checksum the entire file  create checksums every 100mb or so  so each file has a collection of checksums   Then when comparing checksums  you can stop comparing after the first different checksum  getting out early  and saving you from processing the entire file   It ll still take the full time for identical files

User · Answer

The problem here is that SHA256Managed reads 4096 bytes at a time  inherit from FileStream and override Read byte    int  int  to see how much it reads from the filestream   which is too small a buffer for disk IO   To speed things up  2 minutes for hashing 2 Gb file on my machine with SHA256  1 minute for MD5  wrap FileStream in BufferedStream and set reasonably-sized buffer size  I tried with  1 Mb buffer       Not sure if BufferedStream should be wrapped in using block using var stream   new BufferedStream File OpenRead filePath   1200000            The rest remains the same

User · Answer

I did tests with buffer size  running this code  using  var stream   new BufferedStream File OpenRead file   bufferSize         SHA256Managed sha   new SHA256Managed        byte   checksum   sha ComputeHash stream       return BitConverter ToString checksum  Replace  -   String Empty  ToLower        And I tested with a file of 29   GB in size  the results were   10 000  369 24s 100 000  362 55s 1 000 000  361 53s 10 000 000  434 15s 100 000 000  435 15s 1 000 000 000  434 31s And 376 22s when using the original  none buffered code    I am running an i5 2500K CPU  12 GB ram and a OCZ Vertex 4 256 GB SSD drive   So I thought  what about a standard 2TB harddrive  And the results were like this   10 000  368 52s 100 000  364 15s 1 000 000  363 06s 10 000 000  678 96s 100 000 000  617 89s 1 000 000 000  626 86s And for none buffered 368 24   So I would recommend either no buffer or a buffer of max 1 mill

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