Btrfs devel - Oct 2008 - Question about the extent root

Hello,

I''m having problems wrapping my head around the purpose of the extent
root.  It
seems we can allocate space from the main root, and the extent objects are
inserted into that tree, but we can also make allocations out of the extent
root, and the extent objects are inserted into that tree.  What is the purpose
of the extent tree?  The inserts/deletions from the extent tree only happen via
del_pending_extents and finish_current_insert, but it seems I end up with
extents that are being deleted from the extent_tree and inserted onto the normal
root (I''m killing alloc_mutex so I''m getting weird problems). 
I figure instead
of screwing around with the locking until it works its probably best I
understand what exactly we''re doing with allocations on both trees, and
hopefully then I can fix this weirdness.  Thank you,

Josef
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On Tue, Oct 14, 2008 at 08:50:18AM -0400, Josef Bacik
wrote:
> Hello,
> 
> I''m having problems wrapping my head around the purpose of the
extent root.  It
> seems we can allocate space from the main root, and the extent objects are
> inserted into that tree, but we can also make allocations out of the extent
> root, and the extent objects are inserted into that tree.  What is the
purpose
> of the extent tree?  The inserts/deletions from the extent tree only happen
via
> del_pending_extents and finish_current_insert, but it seems I end up with
> extents that are being deleted from the extent_tree and inserted onto the
normal
> root (I''m killing alloc_mutex so I''m getting weird
problems).  I figure instead
> of screwing around with the locking until it works its probably best I
> understand what exactly we''re doing with allocations on both
trees, and
> hopefully then I can fix this weirdness.  Thank you,
>
nm, now that i''ve thought about it i get it.  all extent keys go into
the extent
root, but extent tree allocations only happen when we COW a block in the extent
tree or allocate a new block, which is the root == extent_root case, and all
normal allocations will just be from the normal root.  Thanks,

Josef 
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