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https://issues.apache.org/jira/browse/HDFS-3828?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13439972#comment-13439972
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Andy Isaacson commented on HDFS-3828:
-------------------------------------
bq. If the scanner scans exactly once shouldn't scansLastRun be 0 after this
first run? Ie getBlocksScannedInLastRun shouldn't always return 1 right?
Empirically it is always 1 after a block has been scanned. This is because
when we call scanBlockPoolSlice but there is nothing to scan we're doing a
bunch of useless work:
# creating a new HashMap {{processedBlocks}}
# parsing the verificationLogs and putting the results in the new
{{processedBlocks}}
# calling scan() which returns immediately
# setting totalBlocksScannedInLastRun to the resulting size of
{{processedBlocks}}
bq. Like the new approach better.
I also like the new code better, but the fact that we can't shortcircuit all
the nonsense enumerated above in {{scanBlockPoolSlice}} is a bummer. The
previous approach avoided doing all of this extra work.
As an alternative, we could propagate a "please wake me up at time T" up from
BlockPoolSliceScanner to DataBlockScanner#run and adjust the sleep time there,
accordingly. If all threadpools continue to have work to do, then preserve the
existing 5-second sleep; if all threadpools are done working then
DataBlockScanner could go to sleep for much longer.
> Block Scanner rescans blocks too frequently
> -------------------------------------------
>
> Key: HDFS-3828
> URL: https://issues.apache.org/jira/browse/HDFS-3828
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 0.23.0, 2.0.0-alpha
> Reporter: Andy Isaacson
> Assignee: Andy Isaacson
> Attachments: hdfs-3828-1.txt, hdfs3828.txt
>
>
> {{BlockPoolSliceScanner#scan}} calls cleanUp every time it's invoked from
> {{DataBlockScanner#run}} via {{scanBlockPoolSlice}}. But cleanUp
> unconditionally roll()s the verificationLogs, so after two iterations we have
> lost the first iteration of block verification times. As a result a cluster
> with just one block repeatedly rescans it every 10 seconds:
> {noformat}
> 2012-08-16 15:59:57,884 INFO datanode.BlockPoolSliceScanner
> (BlockPoolSliceScanner.java:verifyBlock(391)) - Verification succeeded for
> BP-2101131164-172.29.122.91-1337906886255:blk_7919273167187535506_4915
> 2012-08-16 16:00:07,904 INFO datanode.BlockPoolSliceScanner
> (BlockPoolSliceScanner.java:verifyBlock(391)) - Verification succeeded for
> BP-2101131164-172.29.122.91-1337906886255:blk_7919273167187535506_4915
> 2012-08-16 16:00:17,925 INFO datanode.BlockPoolSliceScanner
> (BlockPoolSliceScanner.java:verifyBlock(391)) - Verification succeeded for
> BP-2101131164-172.29.122.91-1337906886255:blk_7919273167187535506_4915
> {noformat}
> {quote}
> To fix this, we need to avoid roll()ing the logs multiple times per period.
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