HAMMER Update 13-June-2008 - HEADS UP: MEDIA CHANGED AGAIN

[Erik Wikström]

On 2008-06-16 23:37, Matthew Dillon wrote:
> :<dillon at apollo.backplane.com> wrote:
> :>    I am not too familiar with Reiser so I can't really come to that
> :>    conclusion, but it doesn't seem likely that the reasons are similar.
> :
> :They used B+ in earlier versions. In Reiser4 they decided to use
> :
> :http://en.wikipedia.org/wiki/B*-tree
> :
> :and
> :
> :http://en.wikipedia.org/wiki/Dancing_trees
> :
> :-- 
> :Dan
> 
>     I can never keep all the variations straight, I need a Wiki reference
>     every time to get the name right :-).
> 
>     These are all minor variations of B+Trees.  The B* variation keeps
>     nodes 2/3rds full instead of half full.  It is an optimization which
>     is intended to maintain higher fill ratios to make better use of system
>     caches.
> 
>     A Dancing tree ... hehehehe.  Looks like Hans invented that one.  I'm
>     not sure it even counts as a variation of a B+Tree.  It doesn't apply
>     to HAMMER at all because HAMMER makes no modifications to the B-Tree
>     whatsoever on the front-end.  When you create, delete, rename, write,
>     etc...  when you do those operations HAMMER caches them in a
>     virtualization layer in memory and doesn't make any modifications to
>     its on-media data structures (or their in-memory representations) at
>     all until the meta-data is synced to disk.
> 
>     HAMMER doesn't have to make real-time on-media modifications for any
>     meta-data change for ANY operation.  Not a single one.  Not rename, not
>     hard or soft links, not truncations, nothing.
> 
>     HAMMER uses a modified B+Tree for its on-disk representation, which is
>     a B-Tree with only keys at internal nodes and only records at the leafs.
>     This was done to reduce structural bloat, allow for a leaf->leaf
>     linking optimization in the future, and for other reasons.  The Wiki
>     definition isn't quite right.  The leaf->leaf links are optional (and
>     HAMMER doesn't implement them, though I've left room to do so).  It's
>     still a B+Tree if you don't have them.
> 
>     HAMMER's custom modification is to formalize both a left AND a right
>     bounding element.  So a HAMMER B+Tree looks like this:
> 
> 	B B B B B B	<---- internal node  (e.g. 6 elements w/ 5 leaf ptrs)
> 	 L L L L L	<---- leaf nodes
> 
>     Whereas a normal B+Tree looks like this (note the missing right bound):
> 
> 	B B B B B  	<---- internal node
> 	 L L L L L	<---- leaf nodes
> 
>     Note that HAMMER uses a radix of 64, so e.g. 64 internal elements but one
>     is a right-bound so only 63 pointers to the next level.  The leafs then
>     have 64 elements.  So HAMMER's B+Tree recursion is 63x63x63....x64.
> 
>     HAMMER's internal nodes have a left and right bounding element.  A 
>     standard B+Tree only has a left bounding element.  By adding a right
>     bounding element HAMMER can cache pointers into its B+Tree and 'pick up'
>     searches, insertions, and deletions relative to the cached pointers instead
>     of having to start at the root of the tree.  More importantly, it can 
>     pickup searches, insertions, and deletions at internal nodes, not just
>     leaf nodes.  So I can cache a proximity pointer and if I do a good job
>     I never have to traverse the B+Tree above that point.
> 
>     Without the right bound a search cannot be picked up from mid-tree
>     without potentially going down the wrong branch and then having to 
>     backtrack.

But the case when you have to backtrack should be easily detected (when
the sought element is greater than the right-most bound), right? And
with a radix of 64 that would on average only happen on every 64'th
lookup. I'm not arguing the effectiveness of your implementation, just
checking my understanding of B+trees.

-- 
Erik Wikström