[jira] [Created] (HBASE-8741) Mutations on Regions in recovery mode might have same sequenceIDs

["Himanshu Vashishtha (JIRA)" <ji...@apache.org>]

Himanshu Vashishtha created HBASE-8741:
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             Summary: 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


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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