HP c-Class Performance Tuning manual Using direct I/O on Windows ++ code sample

memset(buf, 0xaa, ps*256);

if( (fd = open(FILENAME, O_WRONLY O_DIRECT) ) < 0 ) { perror("Open failed");

exit(ret);

}

bytes_written = 0;

while( (ret = pwrite(fd, buf, ps*256, bytes_written)) == ps*256) { bytes_written += ret;

}

printf("Wrote %lld GB\n", bytes_written/1000/1000/1000); close(fd);

free(buf);

}

Using direct I/O on Windows

Direct I/O on Windows operating systems is set up through the CreateFile() call, using the FILE_FLAG_NO_BUFFERING flag. For more information, see the "CreateFile Function (http://msdn.microsoft.com/en-us/library/aa363858(VS.85).aspx)" topic on MSDN.

When using direct IO, HP recommends that you keep read, writes, and buffers used for read/write sector size aligned. For a discussion of alignment requirements, see the “File Buffering (http://msdn.microsoft.com/en-us/library/cc644950(VS.85).aspx)” topic on MSDN. The HP IO Accelerator sector size defaults to 512 bytes.

C++ code sample

The following code sample writes to the block device using O_DIRECT. The code sample has a class that encapsulates the operating system file functions that support O_DIRECT.

#include <unistd.h> #include <iostream> #include <errno.h> #include <fcntl.h> using namespace std;

//Short demonstration of a container class that writes using O_DIRECT.

//Compiled using GNU C++.

#define FILENAME "/dev/fioc" class DirectFile

{

public: DirectFile() {}; ~DirectFile() { ::close(fd); };

Programming using direct I/O 26