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https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13532781#comment-13532781
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stack commented on HBASE-5979:
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The nice thing about the original Todd suggestion is that we can be preading
while we are opening the scanner's own file. Once a successful open, we can
then switch to read from the scanner's own reader.
Scanner will more often than not have to open multiple files -- as many as
there are under the store/column family.
Things get interesting around flush and compaction. Scanner could listen for
changes and adjust accordingly. Might have to slow to pread speeds for a while
it did new opens.
> 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.
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