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https://issues.apache.org/jira/browse/HBASE-810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12624163#action_12624163
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Jim Kellerman commented on HBASE-810:
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Ok, now I see the problem (duh!). HRegion.close acquires the splitsAndCloses
write lock preventing further updates. But then it waits until all the scanners
exit. But the process doing the scanning also wants to do updates, so it is
blocked on the splitsAndCloses write lock. Thus all the scanners cannot exit so
close is blocked, and the process cannot do updates so it is blocked.
I will post a new lock sequence diagram that indicates the changes needed. All
changes will be marked with RED.
> Prevent temporary deadlocks when, during a scan with write operations, the
> region splits
> ----------------------------------------------------------------------------------------
>
> Key: HBASE-810
> URL: https://issues.apache.org/jira/browse/HBASE-810
> Project: Hadoop HBase
> Issue Type: Bug
> Affects Versions: 0.2.0
> Reporter: Jean-Daniel Cryans
> Priority: Blocker
> Fix For: 0.2.1, 0.3.0
>
> Attachments: lock-sequencing.jpg, locking-compatibility.jpg
>
>
> HBASE-804 was not about the good problem, this one is. Anyone that iterates
> through the results of a scanner and that rewrites data back into the row at
> each iteration will hit a UnknownScannerException if a split occurs. See the
> stack in the referred jira. Timeline :
> Split occurs, acquires a write lock and waits for scanners to finish
> The scanner in the custom code iterates and writes data until the write is
> blocked by the lock
> deadlock
> The scanner timeouts thus the region splits but the USE will be thrown when
> next() is called
> Inside a Map, the task will simply be retried when the first one fails.
> Elsewhere, it becomes more complicated.
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