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From "Jean-Daniel Cryans (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-10958) [dataloss] Bulk loading with seqids can prevent some log entries from being replayed
Date Thu, 10 Apr 2014 18:08:22 GMT
Jean-Daniel Cryans created HBASE-10958:
------------------------------------------

             Summary: [dataloss] Bulk loading with seqids can prevent some log entries from
being replayed
                 Key: HBASE-10958
                 URL: https://issues.apache.org/jira/browse/HBASE-10958
             Project: HBase
          Issue Type: Bug
    Affects Versions: 0.94.18, 0.98.1, 0.96.2
            Reporter: Jean-Daniel Cryans
            Priority: Blocker
             Fix For: 0.99.0, 0.94.19, 0.98.2, 0.96.3


We found an issue with bulk loads causing data loss when assigning sequence ids (HBASE-6630)
that is triggered when replaying recovered edits. We're nicknaming this issue *Blindspot*.

The problem is that the sequence id given to a bulk loaded file is higher than those of the
edits in the region's memstore. When replaying recovered edits, the rule to skip some of them
is that they have to be _lower than the highest sequence id_. In other words, the edits that
have a sequence id lower than the highest one in the store files *should* have also been flushed.
This is not the case with bulk loaded files since we now have an HFile with a sequence id
higher than unflushed edits.

The log recovery code takes this into account by simply skipping the bulk loaded files, but
this "bulk loaded status" is *lost* on compaction. The edits in the logs that have a sequence
id lower than the bulk loaded file that got compacted are put in a blind spot and are skipped
during replay.

Here's the easiest way to recreate this issue:
 - Create an empty table
 - Put one row in it (let's say it gets seqid 1)
 - Bulk load one file (it gets seqid 2). I used ImporTsv and set hbase.mapreduce.bulkload.assign.sequenceNumbers.
 - Bulk load a second file the same way (it gets seqid 3).
 - Major compact the table (the new file has seqid 3 and isn't considered bulk loaded).
 - Kill the region server that holds the table's region.
 - Scan the table once the region is made available again. The first row, at seqid 1, will
be missing since the HFile with seqid 3 makes us believe that everything that came before
it was flushed.



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