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From "jiraposter@reviews.apache.org (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-4496) HFile V2 does not honor setCacheBlocks when scanning.
Date Sun, 02 Oct 2011 21:50:35 GMT

    [ https://issues.apache.org/jira/browse/HBASE-4496?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13119081#comment-13119081
] 

jiraposter@reviews.apache.org commented on HBASE-4496:
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This is an automatically generated e-mail. To reply, visit:
https://reviews.apache.org/r/2136/
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(Updated 2011-10-02 21:49:56.235130)


Review request for hbase, Jonathan Gray and Lars Hofhansl.


Changes
-------

Addressing Ted's comment and fixing TestBlocksRead, since we are now reading fewer blocks
in a couple of cases. Re-running test suite just in case.


Summary
-------

This fixes a couple of long-existing code issues in HFile v2:
- Making seekBefore cache the previous block it has to read when the scanner happens to be
at the first key of a block (this was a performance regression introduced in HFile v2).
- Fixing the accounting of the number of blocks read for the one-level index case in HFileBlockIndex.seekToDataBlock
if the current block is the same as the requested block.
- Getting rid of HFileBlock.BasicReader, which was used both by FSReaderV2 and HFileReaderV2,
but the former did not cache blocks (a source of confusion).
- Adding a new interface HFile.CachingBlockReader instead, which is implemented by HFile readers
and passed to HFileBlockIndex.


This addresses bug HBASE-4496.
    https://issues.apache.org/jira/browse/HBASE-4496


Diffs (updated)
-----

  src/test/java/org/apache/hadoop/hbase/regionserver/TestBlocksRead.java 019fddd 

Diff: https://reviews.apache.org/r/2136/diff


Testing
-------

This is in production in Facebook's hbase-89 branch. 

Still testing this open-source patch -- please don't commit yet.


Thanks,

Mikhail


                
> 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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