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From "Himanshu Vashishtha (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-8741) Scope sequenceid to the region rather than regionserver (WAS: Mutations on Regions in recovery mode might have same sequenceIDs)
Date Fri, 03 Jan 2014 22:47:52 GMT

    [ https://issues.apache.org/jira/browse/HBASE-8741?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13861978#comment-13861978
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Himanshu Vashishtha commented on HBASE-8741:
--------------------------------------------

Yes, reopening a region is safe.

Re-opening a region involves closing and opening it again. On closing, the region is flushed.
On flushing, we update the oldestFlushingSeqNums and oldestUnFlushedSeqNums (basically, remove
its entry from these maps). Let's say latestSequenceNums still has two entries for that region.
There is no corresponding element in oldestUnflushedSeqNums and oldestFlushingSeqNums map
for the older entry. It will be ignored when considering that WAL file for archiving. https://github.com/apache/hbase/blob/trunk/hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/FSHLog.java#L676


> Scope sequenceid to the region rather than regionserver (WAS: Mutations on Regions in
recovery mode might have same sequenceIDs)
> --------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HBASE-8741
>                 URL: https://issues.apache.org/jira/browse/HBASE-8741
>             Project: HBase
>          Issue Type: Bug
>          Components: MTTR
>    Affects Versions: 0.95.1
>            Reporter: Himanshu Vashishtha
>            Assignee: Himanshu Vashishtha
>             Fix For: 0.98.0
>
>         Attachments: HBASE-8741-trunk-v6.1-rebased.patch, HBASE-8741-trunk-v6.2.1.patch,
HBASE-8741-trunk-v6.2.2.patch, HBASE-8741-trunk-v6.2.2.patch, HBASE-8741-trunk-v6.3.patch,
HBASE-8741-trunk-v6.4.patch, HBASE-8741-trunk-v6.patch, HBASE-8741-v0.patch, HBASE-8741-v2.patch,
HBASE-8741-v3.patch, HBASE-8741-v4-again.patch, HBASE-8741-v4-again.patch, HBASE-8741-v4.patch,
HBASE-8741-v5-again.patch, HBASE-8741-v5.patch
>
>
> Currently, when opening a region, we find the maximum sequence ID from all its HFiles
and then set the LogSequenceId of the log (in case the later is at a small value). This works
good in recovered.edits case as we are not writing to the region until we have replayed all
of its previous edits. 
> With distributed log replay, if we want to enable writes while a region is under recovery,
we need to make sure that the logSequenceId > maximum logSequenceId of the old regionserver.
Otherwise, we might have a situation where new edits have same (or smaller) sequenceIds. 
> We can store region level information in the WALTrailer, than this scenario could be
avoided by:
> a) reading the trailer of the "last completed" file, i.e., last wal file which has a
trailer and,
> b) completely reading the last wal file (this file would not have the trailer, so it
needs to be read completely).
> In future, if we switch to multi wal file, we could read the trailer for all completed
WAL files, and reading the remaining incomplete files.



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