Creating big file on Windows

问题:

I need to create big relatively big (1-8 GB) files. What is the fastest way to do so on Windows using C or C++ ? I need to create them on the fly and the speed is really an issue. File will be used for storage emulation i.e will be access randomly in different offsets and i need that all storage will be preallocate but not initialized, currently we are writing all storage with dummy data and it's taking too long.

Thanks.

回答1:

Use the Win32 API, CreateFile, SetFilePointerEx, SetEndOfFile, and CloseHandle. In that same order.

The trick is in the SetFilePointerEx function. From MSDN:

Note that it is not an error to set the file pointer to a position beyond the end of the file. The size of the file does not increase until you call the SetEndOfFile, WriteFile, or WriteFileEx function.

Windows explorer actually does this same thing when copying a file from one location to another. It does this so that the disk does not need to re-allocate the file for a fragmented disk.

回答2:

Check out memory mapped files.

They very much match the use case you describe, high performance and random access.

I believe they don't need to be created as large files. You just set a large max size on them and they will be expanded when you write to parts you haven't touched before.

回答3:

Use "fsutil" command:

E:\VirtualMachines>fsutil file createnew Usage : fsutil file createnew Eg : fsutil file createnew C:\testfile.txt 1000

Reagds

P.S. it is for Windows: 2000/XP/7

回答4:

Well this solution is not bad, but the thing you are looking for is SetFileValidData

As MSDN sais:

The SetFileValidData function allows you to avoid filling data with zeros when writing nonsequentially to a file.

So this always leave disk data as it is, SetFilePointerEx should set all data to zeros, so big allocation takes some time.

回答5:

If you're using NTFS then sparse files are the way to go:

A file in which much of the data is zeros is said to contain a sparse data set. Files like these are typically very large—for example, a file containing image data to be processed or a matrix within a high-speed database. The problem with files containing sparse data sets is that the majority of the file does not contain useful data and, because of this, they are an inefficient use of disk space.

The file compression in the NTFS file system is a partial solution to the problem. All data in the file that is not explicitly written is explicitly set to zero. File compression compacts these ranges of zeros. However, a drawback of file compression is that access time may increase due to data compression and decompression.

Support for sparse files is introduced in the NTFS file system as another way to make disk space usage more efficient. When sparse file functionality is enabled, the system does not allocate hard drive space to a file except in regions where it contains nonzero data. When a write operation is attempted where a large amount of the data in the buffer is zeros, the zeros are not written to the file. Instead, the file system creates an internal list containing the locations of the zeros in the file, and this list is consulted during all read operations. When a read operation is performed in areas of the file where zeros were located, the file system returns the appropriate number of zeros in the buffer allocated for the read operation. In this way, maintenance of the sparse file is transparent to all processes that access it, and is more efficient than compression for this particular scenario.

回答6:

I am aware that your question is tagged with Windows, and Brian R. Bondy gave you the best answer to your question if you know for certain you will not have to port your application to other platforms. However, if you might have to port your application to other platforms, you might want to do something more like what Adrian Cornish proposed as the answer for the question "How to create file of “x” size?" found at How to create file of "x" size?.

FILE *fp=fopen("myfile", "w");
fseek(fp, 1024*1024, SEEK_SET);
fputc('\n', fp);
fclose(fp);

Of course, there is an added twist. The answer proposed by Adrian Cornish makes use of the fseek function which has the following signature.

int fseek ( FILE * stream, long int offset, int origin );

The problem is that you want to create a very large file with a file size that is beyond the range of a 32-bit integer. You need to use the 64-bit equivalent of fseek. Unfortunately, on different platforms it has different names.

The header file LargeFileSupport.h found at http://mosaik-aligner.googlecode.com/svn-history/r2/trunk/src/CommonSource/Utilities/LargeFileSupport.h offers a solution to this problem.

This would allow you to write the following function.

#include "LargeFileSupport.h"
/* Include other headers. */

bool createLargeFile(const char * filename, off_type size)
{
    FILE *fp = fopen(filename, "w");
    if (!fp)
    {
        return false;
    }
    fseek64(fp, size, SEEK_SET);
    fputc('\n', fp);
    fclose(fp);
}

I thought I would add this just in case the information would be of use to you.