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https://issues.apache.org/jira/browse/HBASE-8346?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13632263#comment-13632263
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Himanshu Vashishtha commented on HBASE-8346:
--------------------------------------------
Ah, yes.
> Prefetchiing .META. rows in case only when useCache is set to true
> ------------------------------------------------------------------
>
> Key: HBASE-8346
> URL: https://issues.apache.org/jira/browse/HBASE-8346
> Project: HBase
> Issue Type: Bug
> Components: Client
> Affects Versions: 0.95.0
> Reporter: Himanshu Vashishtha
> Priority: Minor
> Attachments: HBase-8346-v1.patch
>
>
> While doing a .META. lookup (HCM#locateRegionInMeta), we also prefetch some
> other region's info for that table. The usual call to the meta lookup has
> useCache variable set to true.
> Currently, it calls preFetch irrespective of the value useCache flag:
> {code}
> if (Bytes.equals(parentTable, HConstants.META_TABLE_NAME) &&
> (getRegionCachePrefetch(tableName))) {
> prefetchRegionCache(tableName, row);
> }
> {code}
> Later on, if useCache flag is set to false, it deletes the entry for that row
> from the cache with a forceDeleteCachedLocation() call. This always results
> in two calls to the .META. table in this case. The useCache variable is set
> to false in case we are retrying to find a region (regionserver failover).
> It can be verified from the log statements of a client while having a
> regionserver failover. In the below example, the client was connected to
> a1217, when a1217 got killed. The region in question is moved to a1215.
> Client got this info from META scan, where as client cache this info from
> META, but then delete it from cache as it want the latest info.
> The result is even the meta provides the latest info, it is still deleted
> This causes even the latest info to be deleted. Thus, client deletes
> a1215.abc.com even though it is correct info.
> {code}
> 13/04/15 09:49:12 DEBUG client.HConnectionManager$HConnectionImplementation:
> Cached location for
> t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is
> a1217.abc.com:40020
> 13/04/15 09:49:12 WARN client.ServerCallable: Received exception, tries=1,
> numRetries=30 message=Connection refused
> 13/04/15 09:49:12 DEBUG client.HConnectionManager$HConnectionImplementation:
> Removed all cached region locations that map to
> a1217.abc.com,40020,1365621947381
> 13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results =
> {NAME =>
> 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.',
> STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED =>
> 40382355b8c45e1338d620c018f8ff6c,}
> 13/04/15 09:49:13 DEBUG client.MetaScanner: Scanning .META. starting at
> row=t,user7225973201630273569,00000000000000 for max=10 rows using
> hconnection-0x7786df0f
> 13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results =
> {NAME =>
> 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.',
> STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED =>
> 40382355b8c45e1338d620c018f8ff6c,}
> 13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation:
> Cached location for
> t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is
> a1215.abc.com:40020
> 13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation:
> Removed a1215.abc.com:40020 as a location of
> t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. for
> tableName=t from cache
> 13/04/15 09:49:13 DEBUG client.MetaScanner: Current INFO from scan results =
> {NAME =>
> 't,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c.',
> STARTKEY => 'user7225973201630273569', ENDKEY => '', ENCODED =>
> 40382355b8c45e1338d620c018f8ff6c,}
> 13/04/15 09:49:13 DEBUG client.HConnectionManager$HConnectionImplementation:
> Cached location for
> t,user7225973201630273569,1365536809331.40382355b8c45e1338d620c018f8ff6c. is
> a1215.abc.com:40020
> 13/04/15 09:49:13 WARN client.ServerCallable: Received exception, tries=2,
> numRetries=30
> message=org.apache.hadoop.hbase.exceptions.UnknownScannerException: Name:
> -6313340536390503703, already closed?
> 13/04/15 09:49:13 DEBUG client.ClientScanner: Advancing internal scanner to
> startKey at 'user760712450403198900'
> {code}
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