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From "Aaron Cordova (Commented) (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-473) Support binary search within RFile blocks
Date Tue, 20 Mar 2012 12:57:38 GMT


Aaron Cordova commented on ACCUMULO-473:

Keith, when are you proposing doing the binary search? On the block after the block has been
read into memory or before? 

I would advocate keeping block sizes small enough to be read into memory and if a binary search
is done, do it on the block in memory. This is in order to restrict lookups and scans to one
disk seek.
> Support binary search within RFile blocks
> -----------------------------------------
>                 Key: ACCUMULO-473
>                 URL:
>             Project: Accumulo
>          Issue Type: Improvement
>          Components: tserver
>            Reporter: Keith Turner
>            Assignee: Keith Turner
>             Fix For: 1.5.0
> RFiles store blocks of key values using relative encoding.  By default these blocks are
small (100k).  To find a key in an RFile the index is used to find the block, then the block
is scanned for the key.  It would be nice for iterators that do alot of seeking if a binary
search were done instead of a scan.  
> Relative encoding is a form of compression that serializes a key relative to the last
key.  For example if the row in a key is the same as the previous key, then the row is not
stored again.  This works well with sorted data.   The current on disk format does not support
random access within a block, because to read entry N all previous entries must be read. 
 One option would be to deserialize the block into a form that supports random access and
then do binary search.  However this will consume memory and CPU.  If the relative encoding
format could be modified to support random access, then binary search could be supported in
a memory and CPU efficient way.

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