SetFileValidData Function (Windows)

There seems to be two options on Win32 for preallocating disk space to files.

Basically, I want a equivilent to posix_fallocate or the ever wonderful xfsctl XFS_IOC_RESVSP64 call.

The idea being to (quickly) create a large file on disk that is stored efficiently (i.e. isn’t fragmented).

From SQL, you’d do something like “CREATE LOGFILE GROUP lg1 ADD UNDOFILE ‘uf1′ INITIAL_SIZE 1G;” and expect a 1GB file on disk. One way of getting this is calling write() (or WriteFile() on Win32) repeatedly until you’ve written a 1GB file full of zeros. This means you’re generating approximately 1GB of IO.

Except it’s worse than that: every time you extend the file, you’re going to be changing the metadata (file and free space information). If you’re lucky, you won’t be …

The first point I’d like to make is that you’re using a Microsoft Windows API, so you have already lost. You are just not quite aware of how much you have lost.

A quick look around and you say “Ahh… GetFileSize, that’s what I want to do!” Except, of course, you’re wrong. You don’t want to use GetFileSize at all. It has the following signature:

DWORD WINAPI GetFileSize(  __in       HANDLE hFile,

__out_opt  LPDWORD lpFileSizeHigh

);

Yes, it supports larger than 4GB files! How? A pointer to the high-order doubleword is passed in! So how do you know if this errored? Return -1? WRONG! Because the high word could have been set and your file length could legitimately be 0×1ffffffff. So to find out if you actually had an error, you must call GetLastError! Instead of one call, you now have two.

The Microsoft documentation even acknowledges that this is stupid: “Because …