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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8340) WAL compression handling of seeks seems to be either inefficient or incorrect
Date Mon, 15 Apr 2013 19:02:16 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8340?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13632039#comment-13632039
] 

Jean-Daniel Cryans commented on HBASE-8340:
-------------------------------------------

The way I read it, emptyCompressionContext is set to false only when we were able to next()
all the way to the seek position.
                
> WAL compression handling of seeks seems to be either inefficient or incorrect
> -----------------------------------------------------------------------------
>
>                 Key: HBASE-8340
>                 URL: https://issues.apache.org/jira/browse/HBASE-8340
>             Project: HBase
>          Issue Type: Bug
>            Reporter: Sergey Shelukhin
>
> In next(...):
> {code}
>     if (compressionContext != null && emptyCompressionContext) {
>       emptyCompressionContext = false;
>     }
>     return ...
> {code}
>  
> In seek()
> {code}
>     if (compressionContext != null && emptyCompressionContext) {
>       while (next() != null) {
>         if (getPosition() == pos) {
>           emptyCompressionContext = false;
>           break;
>         }
>       }
> ...
> reader.seek(pos);
> {code}
> So, seek will seek the file directly if either any next, or any seek, has been called
before.
> I am not sure what this code is for, but my best guess is that it is to populate the
dictionary for compression.
> If it is so, it would seem that one next() call (or even one seek() call) would not be
enough, and seek must always use next(), otherwise it is incorrect.
> If we assume that one next() is enough to be able to use reader.seek, as the current
code would seem to imply, then there's no need for the first seek to call next() in a loop
- it can call next once and then do reader.seek.
> Note: even in case if all of this works fine because external usage creates the object
and does one seek before any next-s, and no seeks after (the only bug-free pattern currently
possible with both methods used if I'm not mistaken), then the code needs to be tightened
and bug potential removed.

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