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From "ryan rawson (JIRA)" <j...@apache.org>
Subject [jira] Created: (HBASE-1738) Scanner doesnt reset when a snapshot is created, could miss new updates into the 'kvset' (active part)
Date Mon, 03 Aug 2009 23:17:14 GMT
Scanner doesnt reset when a snapshot is created, could miss new updates into the 'kvset' (active
part)
------------------------------------------------------------------------------------------------------

                 Key: HBASE-1738
                 URL: https://issues.apache.org/jira/browse/HBASE-1738
             Project: Hadoop HBase
          Issue Type: Bug
    Affects Versions: 0.20.0
            Reporter: ryan rawson
             Fix For: 0.20.1


when a Scanner is created, it creates 2 MemStoreScanners on the kvset and the snapshot (internal
names of Memstore)... if the snapshot is originally empty, it only creates the 1, for kvset.
 When the snapshot is created, the outstanding Scanners now have a pointer to the tree that
is now the snapshot, but no pointer to the kvset.

When the flush completes, the scanner will reset the memstore scanners and 'see' the new values
again.

If there is a large delay between snapshot and finalization of the flush, there can be a large
period of time a scanner doesnt see 'new' values that are being inserted. the canonical 'bad'
case where this can do things is the META scanner, and we end up with double assignment.

The snapshot is really lightweight, it only takes out a small lock in memstore, so im not
sure there is an easy mechanism to hook to without building out a bit more code or restructuring
the memstore scanner.

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