The hole in my falloaction

Sparse files save disk space by storing only non-zero data blocks, with zero-filled regions recorded as metadata. When using fallocate() to reserve disk space, the allocated regions are marked as 'unwritten' by the filesystem, which lseek(SEEK_HOLE) treats as holes even though space is physically allocated. This optimization avoids unnecessary I/O by returning zeros directly without reading from disk. The ioctl(FS_IOC_FIEMAP) system call can distinguish between true sparse holes and unwritten allocated regions.