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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-5979) Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers
Date Wed, 02 Jul 2014 19:22:24 GMT

    [ https://issues.apache.org/jira/browse/HBASE-5979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14050591#comment-14050591

Todd Lipcon commented on HBASE-5979:

bq. DFSInputStream extends the above class and does not override the default implementation.
You have no parallelism even for simple gets.

That's not true -- DFSInputStream definitely overrides the positional read method and provides

  public int read(long position, byte[] buffer, int offset, int length)
    throws IOException {
    // sanity checks

(and it has since at least 5 years ago when I started working on Hadoop, as far as I can remember)

> 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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