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From "stack (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2513) [hbase] HStore#get and HStore#getFull may not return expected values by timestamp when there is more than one MapFile
Date Thu, 17 Jan 2008 00:20:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2513?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12559757#action_12559757

stack commented on HADOOP-2513:

Patch looks good but its going to kill performance so I want to run basic PE test before committing
just to see how much its going to cost us.

Chatting w/ Bryan, scanners likely have same issue.  This patch doesn't address that.

> [hbase] HStore#get and HStore#getFull may not return expected values by timestamp when
there is more than one MapFile
> ---------------------------------------------------------------------------------------------------------------------
>                 Key: HADOOP-2513
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2513
>             Project: Hadoop
>          Issue Type: Bug
>          Components: contrib/hbase
>            Reporter: Bryan Duxbury
>            Assignee: stack
>             Fix For: 0.16.0
>         Attachments: 2512-v2.patch, 2513.patch
> Ok, this one is a little tricky. Let's say that you write a row with some value without
a timestamp, thus meaning right now. Then, the memcache gets flushed out to a MapFile. Then,
you write another value to the same row, this time with a timestamp that is in the past, ie,
before the "now" timestamp of the first put. 
> Some time later, but before there is a compaction, if you do a get for this row, and
only ask for a single version, you will logically be expecting the latest version of the cell,
which you would assume would be the one written at "now" time. Instead, you will get the value
written into the "past" cell, because even though it is tagged as having happened in the past,
it actually *was written* after the "now" cell, and thus when #get searches for satisfying
values, it runs into the one most recently written first. 
> The result of this problem is inconsistent data results. Note that this problem only
ever exists when there's an uncompacted HStore, because during compaction, these cells will
all get sorted into the correct order by timestamp and such. In a way, this actually makes
the problem worse, because then you could easily get inconsistent results from HBase about
the same (unchanged) row depending on whether there's been a flush/compaction.
> The only solution I can think of for this problem at the moment is to scan all the MapFiles
and Memcache for possible results, sort them, and then select the desired number of versions
off of the top. This is unfortunate because it means you never get the snazzy shortcircuit
logic except within a single mapfile or memcache. 

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