fallocate() goes thru standard blocks allocator, which try to behave very good for life allocation cases: block placement and future file size prediction, delayed blocks allocation, etc. But it almost impossible to allocate blocks from specified place for our specific case. The only ext4 block allocator option possible to use is that the allocator firstly tries to allocate blocks from the same block group, that inode is related to. But this is not enough for effective files compaction. This patchset implements an extension of fallocate(): fallocate2(int fd, int mode, loff_t offset, loff_t len, unsigned long long physical) The new argument is @physical offset from start of device, which is must for block allocation. In case of [@physical, @physical + len] block range is available for allocation, the syscall assigns the corresponding extent/ extents to inode. In case of the range or its part is occupied, the syscall returns with error (maybe, smaller range will be allocated. The behavior is the same as when fallocate() meets no space in the middle).
Doesn't this interface kills the whole philosophy of letting filesystems to decide which block it sees as most fit for allocation. IMHO user passing over actual physical location from where the FS should allocate, does not sound like a good interface. -ritesh
