I am using VB6 and the Win32 API to write data to a file, this functionality is for the export of data, therefore write performance to the disk is the key factor in my considerations. As such I am using the FILE_FLAG_NO_BUFFERING and FILE_FLAG_WRITE_THROUGH options when opening the file with a call to CreateFile.
FILE_FLAG_NO_BUFFERING requires that I use my own buffer and write data to the file in multiples of the disk’s sector size, this is no problem generally, apart from the last part of data, which if it is not an exact multiple of the sector size will include character zero’s padding out the file, how do I set the file size once the last block is written to not include these character zero’s?
I can use SetEndOfFile however this requires me to close the file and re-open it without using FILE_FLAG_NO_BUFFERING. I have seen someone talk about NtSetInformationFile however I cannot find how to use and declare this in VB6. SetFileInformationByHandle can do exactly what I want however it is only available in Windows Vista, my application needs to be compatible with previous versions of Windows.
I’m not sure, but are YOU sure that setting
FILE_FLAG_NO_BUFFERINGandFILE_FLAG_WRITE_THROUGHgive you maximum performance?They’ll certainly result in your data hitting the disk as soon as possible, but that sort of thing doesn’t actually help performance – it just helps reliability for things like journal files that you want to be as complete as possible in the event of a crash.
For a data export routine like you describe, allowing the operating system to buffer your data will probably result in BETTER performance, since the writes will be scheduled in line with other disk activity, rather than forcing the disk to jump back to your file every write.
Why don’t you benchmark your code without those options? Leave in the zero-byte padding logic to make it a fair test.
If it turns out that skipping those options is faster, then you can remove the 0-padding logic, and your file size issue fixes itself.