[HBASE-4496] HFile V2 does not honor setCacheBlocks when scanning.

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 0.92.0, 0.94.0
• Fix Version/s: 0.92.0, 0.94.0
• Component/s: regionserver
• Hadoop Flags: Reviewed
• Assignee: Mikhail Gryzykhin
• Reporter: Lars Hofhansl
• Created: 27/Sep/11 07:22
• Updated: 12/Oct/12 05:35
• Resolved: 02/Oct/11 21:30

Description

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.

Issue Links

• is related to: HBASE-4422 Move block cache parameters and references into single CacheConf class