win32: improve performance of disk write - performance

Question

In order to write to HD at max. performance I'm using overlapped I/O.
It works.
Upon acquiring 4MB of data (from sensor) I'm writing it to disk.
Then, upon getting the next 4MB I first ask if the previous writing completed.
How can I know what is the optimal block size (4MB ?) that is best for my disk ?
// AsyncFile.cpp : Defines the exported functions for the DLL application.
//
#include "stdafx.h"
#include "AsyncFile.h"
/****************************************************************************/
CAsyncFile::CAsyncFile()
{
}
/****************************************************************************/
CAsyncFile::~CAsyncFile()
{
}
/****************************************************************************/
int CAsyncFile::OpenFile(char *pcFileName,
bool bAsync, // Whether async read/write is required
bool bWrite) // True is file is used for writing to
{
DWORD dwAsyncMask = bAsync ? (FILE_FLAG_OVERLAPPED | FILE_FLAG_NO_BUFFERING) : 0;
DWORD dwCreation = bWrite ? CREATE_ALWAYS : OPEN_EXISTING;
DWORD dwAccess = bWrite ? GENERIC_WRITE : GENERIC_READ;
DWORD dwShareMode = bWrite ? FILE_SHARE_READ : FILE_SHARE_WRITE;
if (strlen(pcFileName) < sizeof(m_cFileName))
strcpy_s(m_cFileName, 256, pcFileName);
else
m_cFileName[0] = 0; // NULL (error - file name is too long)
// Calling openFile() sets a valid value to the file handle
m_hFileHandle = INVALID_HANDLE_VALUE;
// Auto reset (manual reset=false), init state = false, no name
m_hIoCompleted = CreateEvent(NULL, FALSE, FALSE, NULL);
// Init OVERLAPPED structure, for async read
m_tOverlapped.Offset = 0;
m_tOverlapped.OffsetHigh = 0;
m_tOverlapped.hEvent = m_hIoCompleted;
m_Event = m_tOverlapped.hEvent;
if (m_hFileHandle != INVALID_HANDLE_VALUE)
{
// File is already openned; check open mode
if ((bAsync == m_bAsync) && (bWrite == m_bWrite))
return (ASYNCFILE_OK);
// File is already openned, but in other mode; Should close file
// before using it again
return ASYNCFILE_FILE_IS_NOT_IN_WRITE_MODE;
}
m_hFileHandle =
CreateFile((LPCTSTR)m_cFileName,
dwAccess, // Open for read or write
dwShareMode, //
NULL, // No SECURITY_ATTRBUTES
dwCreation, // Open exisiting file (if read) \ create new (if write)
dwAsyncMask, // For asynchronous operations, for maximum asynchronous performence
0);
if (m_hFileHandle == INVALID_HANDLE_VALUE)
{
DWORD dwError = GetLastError();
return ASYNCFILE_FAILED_TO_OPEN_FILE;
}
//In case file opened for reading, get its size
if (bWrite == false)
{
GetFileSizeEx(m_hFileHandle, &m_FileSize);
}
// Save open mode
m_bAsync = bAsync;
m_bWrite = bWrite;
return ASYNCFILE_OK;
}
/****************************************************************************/
int CAsyncFile::CloseFile()
{
//BOOL Status;
if (!CloseHandle(m_hFileHandle))
return ASYNCFILE_FAILED_TO_CLOSE_FILE;
if (!CloseHandle(m_hIoCompleted))
return ASYNCFILE_FAILED_TO_CLOSE_FILE;
return ASYNCFILE_OK;
}
/****************************************************************************/
int CAsyncFile::StartAsyncRead(void* pBuffer,
DWORD dwReadSize,
bool* pbEof)
{
*pbEof = false; // By default, EOF is false
int iError;
if (m_hFileHandle == INVALID_HANDLE_VALUE)
return (false);
if (!ReadFile(m_hFileHandle,
pBuffer,
dwReadSize,
NULL, // actual bytes read is not valid now
&m_tOverlapped))
{
if ((iError = GetLastError()) == ERROR_HANDLE_EOF)
{
*pbEof = true;
return ASYNCFILE_OK;
}
else if (!(m_bAsync && (iError == ERROR_IO_PENDING)))
{
return ASYNCFILE_START_READ_FAILED;
}
}
return ASYNCFILE_OK;
}
/****************************************************************************/
int CAsyncFile::WaitAsyncOperationEnd(DWORD* pdwActualBytesTransferred)
{
if (m_hFileHandle == INVALID_HANDLE_VALUE)
return ASYNCFILE_WAIT_FOR_COMPLETION_FAILED;
// Wait for read operation to complete
if (!GetOverlappedResult(m_hFileHandle,
&m_tOverlapped,
pdwActualBytesTransferred,
true))
return ASYNCFILE_WAIT_FOR_COMPLETION_FAILED;
return ASYNCFILE_OK;
}
/****************************************************************************/
int CAsyncFile::StartAsyncWrite(void* pSrcBuf,
DWORD dwSize) // In bytes
{
int iError;
if (!WriteFile(m_hFileHandle,
pSrcBuf,
dwSize,
NULL, // actual bytes written is not valid now
&m_tOverlapped))
{
iError = GetLastError();
if (iError != ERROR_IO_PENDING)
return ASYNCFILE_START_WRITE_FAILED;
}
return ASYNCFILE_OK;
}
/****************************************************************************/
void CAsyncFile::SetFilePosition(UINT64 Position)
{
m_tOverlapped.Offset = Position & 0xFFFFFFFF;
m_tOverlapped.OffsetHigh = Position >> 32;
}
/****************************************************************************/
UINT64 CAsyncFile::GetFilePosition()
{
UINT64 Position;
Position = (m_tOverlapped.Offset) | ((UINT64)m_tOverlapped.OffsetHigh << 32);
return (Position);
}
/****************************************************************************/
UINT64 CAsyncFile::GetFileSize()
{
return (m_FileSize.QuadPart);
}

Answer 1

Well it depends on the average size of your files as well. If your file sizes are constantly ranging near 7 to 8 MB, then there would be some benefit in increasing the buffer size to 8192KB. How old is the drive? How often have you used it, many other factors come into play. We are able to learn more about this topic from an excellent piece of software called FastCopy. Hope this helps.