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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-3504) HLog performance improvement
Date Fri, 04 Feb 2011 19:10:30 GMT

    [ https://issues.apache.org/jira/browse/HBASE-3504?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12990683#comment-12990683

Jonathan Gray commented on HBASE-3504:

bq. Out of order edits though could be a problem in case where we got a new value and a delete
of that same value coming in at around the same time. Out of order could change result seen
on other side of the log split.

This would only be an issue if it is the _exact_ same time (same timestamp).  The same potential
issue would exist with two Put operations of the same row/col/ts (we expect the later one
to win).  However, in this case, it's not actually an issue because the row lock ensures that
these two things (even if at the same ms timestamp) will happen serially from POV of HLog...
even without the update lock, the writes will happen in order because they are done under
the row lock.

So the guarantee is that for a given row, it is always increasing seqids.  Across rows, it
mostly will be but there is no guarantee.  In my code inspection, it doesn't seem like this
is a problem

> HLog performance improvement
> ----------------------------
>                 Key: HBASE-3504
>                 URL: https://issues.apache.org/jira/browse/HBASE-3504
>             Project: HBase
>          Issue Type: Improvement
>          Components: regionserver
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
> The HLog.updateLock protects the rolling of logs with concurrent writes to the HDFS log
file. This is a scalability bottleneck for a workload that comprises mostly of counter-increments.

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