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Lars Hofhansl commented on HBASE-5979: -------------------------------------- Can we use a scan's cacheBlocks flag as an indicator for whether it would benefit from its own input stream? Presumably only large scans would have caching disabled. HBASE-7336 has a simple adhoc fix for the truly terrible read performance when two long running scanners scan the same store file. > Non-pread DFSInputStreams should be associated with scanners, not > HFile.Readers > ------------------------------------------------------------------------------- > > Key: HBASE-5979 > URL: https://issues.apache.org/jira/browse/HBASE-5979 > Project: HBase > Issue Type: Improvement > Components: Performance, regionserver > Reporter: Todd Lipcon > > Currently, every HFile.Reader has a single DFSInputStream, which it uses to > service all gets and scans. For gets, we use the positional read API (aka > "pread") and for scans we use a synchronized block to seek, then read. The > advantage of pread is that it doesn't hold any locks, so multiple gets can > proceed at the same time. The advantage of seek+read for scans is that the > datanode starts to send the entire rest of the HDFS block, rather than just > the single hfile block necessary. So, in a single thread, pread is faster for > gets, and seek+read is faster for scans since you get a strong pipelining > effect. > However, in a multi-threaded case where there are multiple scans (including > scans which are actually part of compactions), the seek+read strategy falls > apart, since only one scanner may be reading at a time. Additionally, a large > amount of wasted IO is generated on the datanode side, and we get none of the > earlier-mentioned advantages. > In one test, I switched scans to always use pread, and saw a 5x improvement > in throughput of the YCSB scan-only workload, since it previously was > completely blocked by contention on the DFSIS lock. -- This message is automatically generated by JIRA. If you think it was sent incorrectly, please contact your JIRA administrators For more information on JIRA, see: http://www.atlassian.com/software/jira