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From "Lars Hofhansl (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-13389) [REGRESSION] HBASE-12600 undoes skip-mvcc parse optimizations
Date Mon, 20 Apr 2015 01:02:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14502201#comment-14502201
] 

Lars Hofhansl commented on HBASE-13389:
---------------------------------------

Correctness > Performance :)  We need to have this correct in all cases.

OK. So lemme file a sub issue to apply the patch I attache here. Not as good as the original
patch, but better.

Here we need to have the discussion about how long to keep the Cells, it seems we want this
less than the minimum time between major compactions (which is 3.5 days currently - 1 week
+- 1/2 week) for performance (but again correctness is more important).
Might also want to change the detection code for whether a major compaction is needed, if
we can rid of MVCC stamp, we should major compact.


> [REGRESSION] HBASE-12600 undoes skip-mvcc parse optimizations
> -------------------------------------------------------------
>
>                 Key: HBASE-13389
>                 URL: https://issues.apache.org/jira/browse/HBASE-13389
>             Project: HBase
>          Issue Type: Sub-task
>          Components: Performance
>            Reporter: stack
>         Attachments: 13389.txt
>
>
> HBASE-12600 moved the edit sequenceid from tags to instead exploit the mvcc/sequenceid
slot in a key. Now Cells near-always have an associated mvcc/sequenceid where previous it
was rare or the mvcc was kept up at the file level. This is sort of how it should be many
of us would argue but as a side-effect of this change, read-time optimizations that helped
speed scans were undone by this change.
> In this issue, lets see if we can get the optimizations back -- or just remove the optimizations
altogether.
> The parse of mvcc/sequenceid is expensive. It was noticed over in HBASE-13291.
> The optimizations undone by this changes are (to quote the optimizer himself, Mr [~lhofhansl]):
> {quote}
> Looks like this undoes all of HBASE-9751, HBASE-8151, and HBASE-8166.
> We're always storing the mvcc readpoints, and we never compare them against the actual
smallestReadpoint, and hence we're always performing all the checks, tests, and comparisons
that these jiras removed in addition to actually storing the data - which with up to 8 bytes
per Cell is not trivial.
> {quote}
> This is the 'breaking' change: https://github.com/apache/hbase/commit/2c280e62530777ee43e6148fd6fcf6dac62881c0#diff-07c7ac0a9179cedff02112489a20157fR96



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