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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, 06 Apr 2015 20:55:13 GMT

    [ https://issues.apache.org/jira/browse/HBASE-13389?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14481916#comment-14481916
] 

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

We do need to revisit the 6 days, right? Would 3 days be enough? 

Lemme try to understand the cases:
# when we replay data due to recovery we want it to fall into the right place w.r.t to existing
data. Why do we need more then the maximum time to roll a log (1h)?
# replication... Yeah, that's important. I'd say if you have a replication lag of more than
a few hours you have a larger problem anyway.
# This too... Although I do not actually agree that this is an advantage. Mutations (including
deletes) being idempotent in HBase is a feature and not a problem.

So with all this I do need any reason to keep these for more than a few hours. It's very possible
that I am missing something.

> [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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