Re: Why is network reading slow?

Hector Santos <>
Thu, 14 Jan 2010 03:10:59 -0500
Peter Olcott wrote:

"Hector Santos" <> wrote in message

Peter Olcott wrote:

By File Copy, you mean DOS copy command or the CopyFile()


I am using the DOS command prompt's copy command. This is

The problem is the contradiction formed by the fact that
reading and writng the file is fast, while reading and not
wrting this same file is slow.
I am currently using fopen() and fread(); I am using
Windows XP.

True, if the DOS copy command is fast,then I believe the code you are
using is not optimal. The DOS Copy is using the same CreateFile() API
which fopen() also finally uses in the RTL. So you should be able to
match the same performance of the DOS Copy command.

Have you tried using setvbuf to set a buffer cache?

Here is a small test code that opens a 50 meg file:

// File: V:\wc7beta\testbufsize.cpp
// Compile with: cl testbufsize.cpp

#include <stdio.h>
#include <windows.h>

void main(char argc, char *argv[])
    char _cache[1024*16] = {0}; // 16K cache
    BYTE buf[1024*1] = {0}; // 1K buffer

    FILE *fv = fopen("largefile.dat","rb");
    if (fv) {
        int res = setvbuf(fv, _cache, _IOFBF, sizeof(_cache));
        DWORD nTotal = 0;
        DWORD nDisks = 0;
        DWORD nLoops = 0;
        DWORD nStart = GetTickCount();
        while (!feof(fv)) {
             int nRead = fread(buf,1,sizeof(buf),fv);
             nTotal +=nRead;
             if (nRead > 0 && !fv->_cnt) nDisks++;
        printf("Time: %d | Size: %d | Reads: %d | Disks: %d\n",

What this basically shows is the number of disk hits it makes
by checking the fv->_cnt value. It shows that as long as the cache
size is larger than the read buffer size, you get the same number of
disk hits. I also spit out the milliseconds. Subsequent runs, of
course, is faster since the OS API CreateFile() is used by the RTL in
buffer mode.

Also do you know what protocol you have Samba using?


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