Re: About strange behaviour of ext4 allocation algorithm

[tytso@mit.edu]

On Wed, Dec 23, 2009 at 01:52:48PM +0300, Vyacheslav Dubeyko wrote:
>  
> I use kernel: 2.6.29.4-167.fc11.i686.PAE #1 SMP Wed May 27 17:28:22
> EDT 2009 i686 i686 i386 GNU/Linux.

Yeah, that was before a massive number of changes to the ext4
allocator.  The changes to the allocators which speed up fsck
described here:

http://thunk.org/tytso/blog/2009/02/26/fast-ext4-fsck-times-revisited/

All went in *after* 2.6.29.  That is, how the block and inode
allocators worked change significantly between 2.6.29 and 2.6.31.

> > If you delete your file, without reformatting the filesystem, and
> > then re-run the test, does it produce the same results?  If not,
> > then it is likely you are seeing the problem with uninitialized
> > groups that was fixed a month or two ago.
> 
> After deletion of the file and re-run test (without reformatting the
> filesystem) I have slightly different extents' tree. Index block
> (depth of the tree = 1) has changed place and several extents has
> another sizes. But nature of the extents' sequence is the same.

The change which Andreas was referring to --- taking out the bias
against opening up uninitialized block groups for allocations until
absolutely necessary, which had a tendency to cause unnecessary
fragmentation --- was merged into mainline between 2.6.30 and 2.6.31.

Best regards,

						- Ted