[jira] [Resolved] (HBASE-5979) Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers

["Andrew Kyle Purtell (Jira)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Andrew Kyle Purtell resolved HBASE-5979.
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    Resolution: Incomplete

> 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
>            Priority: Critical
>
> 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.



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