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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:
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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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