On Mon, Mar 05, 2007 at 12:02:59PM -0800, Mingming Cao wrote:

Yep, I think it makes sense to use preallocation for defragmentation.
After all both preallocation and defragmentation shall call underlying 
filesystem multiple block allocator to try to allocate a chunk of 
contiguous blocks on disk. ext4 online defrag implementation by Takashi 
already support to choose a "goal" allocation block to guide the ext4 
block allocator to place the defraged file is a specific location.

Passing a little bit more hint to sys_fallocate() (i.e, goal block, 
and/or whether the goal block is important over the size of prealloc 
extent), might make it more useful for the orginial goal (get contigous 
and guranteed blocks) and for defragmentation.

fallocate with the whence argument and flags is already quite complicated,
I'd rather have another call for placement decisions, that would
be called on an fd to do placement decissions for any further allocations
(prealloc, write, etc)