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Date: Sun, 2 Oct 2011 21:35:34 +0000 (UTC)
From: "Lars Hofhansl (Assigned) (JIRA)" <jira@apache.org>
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Subject: [jira] [Assigned] (HBASE-4496) HFile V2 does not honor
 setCacheBlocks when scanning.
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     [ https://issues.apache.org/jira/browse/HBASE-4496?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Lars Hofhansl reassigned HBASE-4496:
------------------------------------

    Assignee: Mikhail Bautin  (was: Lars Hofhansl)

Mikhail did the work, he deserves the credit :)
                
> HFile V2 does not honor setCacheBlocks when scanning.
> -----------------------------------------------------
>
>                 Key: HBASE-4496
>                 URL: https://issues.apache.org/jira/browse/HBASE-4496
>             Project: HBase
>          Issue Type: Bug
>          Components: regionserver
>    Affects Versions: 0.92.0, 0.94.0
>            Reporter: Lars Hofhansl
>            Assignee: Mikhail Bautin
>             Fix For: 0.92.0, 0.94.0
>
>         Attachments: 4496.final, 4496.txt
>
>
> While testing the LRU cache during the scanning I noticed quite some churn in the cache even when Scan.cacheBlocks is set to false. After debugging this, I found that HFile V2 always caches blocks in the LRU cache regardless of the cacheBlocks setting.
> Here's a trace (from Eclipse) showing the problem:
> HFileReaderV2.readBlock(long, int, boolean, boolean, boolean) line: 279	
> HFileReaderV2.readBlockData(long, long, int, boolean) line: 219	
> HFileBlockIndex$BlockIndexReader.seekToDataBlock(byte[], int, int, HFileBlock) line: 191	
> HFileReaderV2$ScannerV2.seekTo(byte[], int, int, boolean) line: 502	
> HFileReaderV2$ScannerV2.reseekTo(byte[], int, int) line: 539	
> StoreFileScanner.reseekAtOrAfter(HFileScanner, KeyValue) line: 151	
> StoreFileScanner.reseek(KeyValue) line: 110	
> KeyValueHeap.reseek(KeyValue) line: 255	
> StoreScanner.reseek(KeyValue) line: 409	
> StoreScanner.next(List<KeyValue>, int) line: 304	
> KeyValueHeap.next(List<KeyValue>, int) line: 114	
> KeyValueHeap.next(List<KeyValue>) line: 143	
> HRegion$RegionScannerImpl.nextRow(byte[]) line: 2774	
> HRegion$RegionScannerImpl.nextInternal(int) line: 2722	
> HRegion$RegionScannerImpl.next(List<KeyValue>, int) line: 2682	
> HRegion$RegionScannerImpl.next(List<KeyValue>) line: 2699	
> HRegionServer.next(long, int) line: 2092	
> Every scanner.next causes a reseek, which eventually causes a call to HFileBlockIndex$BlockIndexReader.seekToDataBlock(...) at which point the cacheBlocks information is lost. HFileReaderV2.readBlockData calls HFileReaderV2.readBlock with cacheBlocks set unconditionally to true.
> The fix is not immediately clear, unless we want to pass cacheBlocks to HFileBlockIndex$BlockIndexReader.seekToDataBlock and then on to HFileBlock.BasicReader.readBlockData and all its implementers, which is ugly as readBlockData should not care about caching.
> Avoiding caching during scans is somewhat important for us.

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