[jira] [Commented] (HBASE-4811) Support reverse Scan

["Liang Xie (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-4811?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13604860#comment-13604860 ] 

Liang Xie commented on HBASE-4811:
----------------------------------

Here is the perf number from leveldb's site: https://code.google.com/p/leveldb/
{quote}
We list the performance of reading sequentially in both the forward and reverse direction, and also the performance of a random lookup. Note that the database created by the benchmark is quite small. Therefore the report characterizes the performance of leveldb when the working set fits in memory. The cost of reading a piece of data that is not present in the operating system buffer cache will be dominated by the one or two disk seeks needed to fetch the data from disk. Write performance will be mostly unaffected by whether or not the working set fits in memory.

   readrandom   :      16.677 micros/op;  (approximately 60,000 reads per second)
   readseq      :       0.476 micros/op;  232.3 MB/s    
   readreverse  :       0.724 micros/op;  152.9 MB/s    
 

LevelDB compacts its underlying storage data in the background to improve read performance. The results listed above were done immediately after a lot of random writes. The results after compactions (which are usually triggered automatically) are better.

   readrandom   :      11.602 micros/op;  (approximately 85,000 reads per second)   
   readseq      :       0.423 micros/op;  261.8 MB/s    
   readreverse  :       0.663 micros/op;  166.9 MB/s    
{quote}

                
> Support reverse Scan
> --------------------
>
>                 Key: HBASE-4811
>                 URL: https://issues.apache.org/jira/browse/HBASE-4811
>             Project: HBase
>          Issue Type: Improvement
>          Components: Client
>    Affects Versions: 0.20.6
>            Reporter: John Carrino
>
> All the documentation I find about HBase says that if you want forward and reverse scans you should just build 2 tables and one be ascending and one descending.  Is there a fundamental reason that HBase only supports forward Scan?  It seems like a lot of extra space overhead and coding overhead (to keep them in sync) to support 2 tables.  
> I am assuming this has been discussed before, but I can't find the discussions anywhere about it or why it would be infeasible.

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