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From "Liyin Tang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-5987) HFileBlockIndex improvement
Date Fri, 11 May 2012 01:21:48 GMT
Liyin Tang created HBASE-5987:
---------------------------------

             Summary: HFileBlockIndex improvement
                 Key: HBASE-5987
                 URL: https://issues.apache.org/jira/browse/HBASE-5987
             Project: HBase
          Issue Type: Improvement
            Reporter: Liyin Tang
            Assignee: Liyin Tang


Recently we find out a performance problem that it is quite slow when multiple requests are
reading the same block of data or index. 

>From the profiling, one of the causes is the IdLock contention which has been addressed
in HBASE-5898. 

Another issue is that the HFileScanner will keep asking the HFileBlockIndex about the data
block location for each target key value during the scan process(reSeekTo), even though the
target key value has already been in the current data block. This issue will cause certain
index block very HOT, especially when it is a sequential scan.

To solve this issue, we propose the following solutions:

First, we propose to lookahead for one more block index so that the HFileScanner would know
the start key value of next data block. So if the target key value for the scan(reSeekTo)
is "smaller" than that start kv of next data block, it means the target key value has a very
high possibility in the current data block (if not in current data block, then the start kv
of next data block should be returned. +Indexing on the start key has some defects here+)
and it shall NOT query the HFileBlockIndex in this case. On the contrary, if the target key
value is "bigger", then it shall query the HFileBlockIndex. This improvement shall help to
reduce the hotness of HFileBlockIndex and avoid some unnecessary IdLock Contention or Index
Block Cache lookup.

Secondary, we propose to push this idea a little further that the HFileBlockIndex shall index
on the last key value of each data block instead of indexing on the start key value. The motivation
is to solve the HBASE-4443 issue (avoid seeking to "previous" block when key you are interested
in is the first one of a block) as well as +the defects mentioned above+.

For example, if the target key value is "smaller" than the start key value of the data block
N. There is no way for sure the target key value is in the data block N or N-1. So it has
to seek from data block N-1. However, if the block index is based on the last key value for
each data block and the target key value is beween the last key value of data block N-1 and
data block N, then the target key value is supposed be data block N for sure. 


As long as HBase only supports the forward scan, the last key value makes more sense to be
indexed on than the start key value.  

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