Re: ext4: indirect block allocations not sequential in 3.4.67 and 3.11.7

[Benjamin LaHaise <bcrl@kvack.org>]

On Thu, Jan 16, 2014 at 02:12:27PM -0500, Theodore Ts'o wrote:
> An 8MB file will require two indirect blocks.  If you are using
> extents, almost certainly it will fit inside the inode, which means we
> don't need any external metadata blocks.  That massively speeds up
> fsck time, and unlink time, and it also speeds up the random read case
> since the best way to optimize a seek is to eliminate it.  :-)

> I understand that for your use case, it would be hard to move to using
> extents right away.  But I think you'd see so many improvements from
> going to ext4 and extents that it might be more efficient to optimize
> an indirect blocok scheme.

Unfortunately, the improvements from extents for our use-case are not 
enough to outweigh the other costs of deployment.  I think I've figured 
out a hack that results in the system doing most of what I want it to do: 
I've removed EXT4_MB_HINT_DATA in ext4_alloc_blocks().  With that change, 
the allocator is giving me mostly sequential allocations.  Hopefully that 
doesn't have any other negative side effects.

		-ben

> 						- Ted

-- 
"Thought is the essence of where you are now."