More realistic feature request would be support for sparse files.
The empty areas could be used for appending new data efficiently, normal operations on RAF should ignore then.
Support for sparse files is available in Win32 via DeviceIoControl(FSCTL_QUERY_ALLOCATED_RANGES).
I have vague feeling Linux has something like that too.
ZFS on Solaris 10 has an extension to the the Posix lseek() flags, SEEK_DATA and SEEK_HOLE. It makes it easy to find the holes in your files. I may be wrong here but I don't think Linux supports this.
Sparce file handling was described in Kernighan book more then 20 years ago. I think that most UNIX-systems produces "holes" by default (Linux do this on ext2/ext3 and XFS for sure). It is not a problem to RAF. It just ask a system to allocate space and it is not important whether the data allocated or "hole" created. when writing in the "hole" system will allocate blocks for a file. There may be a problem that there is no more space on device, exactly such case is descried in MSDN (using memory-maped file on sparce or comprressed file). We will just check for an error in such cases. -- Svyatoslav Trukhanov, Oleksii Ursulenko