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From "Steve Loughran (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6698) try to optimize DFSInputStream.getFileLength()
Date Tue, 04 Nov 2014 10:31:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6698?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14195981#comment-14195981
] 

Steve Loughran commented on HDFS-6698:
--------------------------------------

bq. it's just so easy to write incorrect code with volatile.

yes, but its very fast incorrect code ...

> try to optimize DFSInputStream.getFileLength()
> ----------------------------------------------
>
>                 Key: HDFS-6698
>                 URL: https://issues.apache.org/jira/browse/HDFS-6698
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: hdfs-client
>    Affects Versions: 3.0.0
>            Reporter: Liang Xie
>            Assignee: Liang Xie
>         Attachments: HDFS-6698.txt, HDFS-6698.txt, HDFS-6698v2.txt, HDFS-6698v2.txt,
HDFS-6698v3.txt
>
>
> HBase prefers to invoke read() serving scan request, and invoke pread() serving get reqeust.
Because pread() almost holds no lock.
> Let's image there's a read() running, because the definition is:
> {code}
> public synchronized int read
> {code}
> so no other read() request could run concurrently, this is known, but pread() also could
not run...  because:
> {code}
>   public int read(long position, byte[] buffer, int offset, int length)
>     throws IOException {
>     // sanity checks
>     dfsClient.checkOpen();
>     if (closed) {
>       throw new IOException("Stream closed");
>     }
>     failures = 0;
>     long filelen = getFileLength();
> {code}
> the getFileLength() also needs lock.  so we need to figure out a no lock impl for getFileLength()
before HBase multi stream feature done. [~saint.ack@gmail.com]



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