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Asked: 2026-05-15T12:25:02+00:00

I have a program that opens a large binary file, appends a small amount

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I have a program that opens a large binary file, appends a small amount of data to it, and closes the file. 

FileStream fs = File.Open( "\\\\s1\\temp\\test.tmp", FileMode.Append, FileAccess.Write, FileShare.None );
fs.Write( data, 0, data.Length );
fs.Close();

If test.tmp is 5MB before this program is run and the data array is 100 bytes, this program will cause over 5MB of data to be transmitted across the network. I would have expected that the data already in the file would not be transmitted across the network since I’m not reading it or writing it. Is there any way to avoid this behavior? This makes it agonizingly slow to append to very large files.

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1 Answer

  1. Editorial Team

    0xA3 provided the answer in a commment above. The poor performance was due to an on-access virus scan. Each time my program opened the file, the virus scanner read the entire contents of the file to check for viruses even though my program didn’t read any of the existing content. Disabling the on-access virus scan eliminated the excessive network I/O and the poor performance.

    Thanks to everyone for your suggestions.

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