Only writes to disk, but iotop also reads

Question

I would like to understand the following problem:

The process only executes a sys write call, in an infinite loop. When I raise iotop, I expect to see non-zero write speed and zero read speed associated with this process. But iotop says that reading and writing can be equal (depending on the size of one record). Take a look at the C code:

#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdio.h>
#define BUFSIZE 1000000

char buf[BUFSIZE];
const int write_size = 4000;

int main(){
  int fd;
  if ((fd = open("filename", O_RDWR | O_CREAT, 0666)) < 0){
    return -1;
  }
  ssize_t ret;
  while (1){
    ret = write(fd, buf, write_size);
    if (ret != write_size){
      return -1;
    }
  }
  return 0;
}

If you assign different values ​​to "write_size", you will see different read speeds in iotop. If the value is indicated in the code above, iotop shows that reading and writing are equal.

Important:
The problem only occurs under certain conditions:
- The file must be created and filled with data (say, at least 8 GB) before running the code

conf:
Debian lenny, 2TB ( xfs ext4), uname -a Linux g-6 2.6.26-bpo.1-xen-amd64 # 1 SMP Mon Jan 12 14:32:40 UTC 2009 x86_64 GNU/Linux

,
.

Answer 1

. ( O_APPEND). , X (= ), , .

Answer 2

, , (4000 4096). , , , .

O_TRUNC, . , , 8GB .

Answer 3

O_WRONLY | O_CREAT, . - , , open().