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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2638) Add close of idle connection to DFSClient and to DataNode DataXceiveServer
Date Thu, 17 Jan 2008 21:54:34 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2638?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12560095#action_12560095
] 

Raghu Angadi commented on HADOOP-2638:
--------------------------------------

bq. Are you suggesting that MapFile#Reader change to use read(pos, buf, off, len), aka pread,
exclusively?  [...]

Especially if suites a particular application of MapFile. It was not clear from brief reading
of MapFile if the index could store size of an entry. If we had a MapFile which does not need
any scanning of data, then it could use pread exclusively (just get the location and size
from the index file and fetch just that much data). I am not very familiar with MapFile yet.
HADOOP-2604 looks like it wants to implement something like this.

> Add close of idle connection to DFSClient and to DataNode DataXceiveServer
> --------------------------------------------------------------------------
>
>                 Key: HADOOP-2638
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2638
>             Project: Hadoop
>          Issue Type: Improvement
>          Components: dfs
>            Reporter: stack
>
> This issue is for adding timeout and shutdown of idle DFSClient <-> DataNode connections.
> Applications can have DFS usage patterns than deviate from that of MR 'norm' where files
are generally opened, sucked down as fast as is possible, and then closed.  For example, at
the other extreme, hbase wants to support fast random reading of key values over a sometimes
relatively large set of MapFiles or MapFile equivalents.  To avoid paying startup costs on
every random read -- opening the file and reading in the index each time -- hbase just keeps
all of its MapFiles open all the time.
> In an hbase cluster of any significant size, this can add up to lots of file handles
per process: See HADOOP-2577, " [hbase] Scaling: Too many open file handles to datanodes"
for an accounting.
> Given how DFSClient and DataXceiveServer interact when random reading, and given past
observations that have the client-side file handles mostly stuck in CLOSE_WAIT (See HADOOP-2341,
'Datanode active connections never returns to 0'), a suggestion made up on the list today,
that idle connections should be timedout and closed, would help applications that have hbase-like
access patterns conserve file handles and allow them scale.
> Below is context that comes of the mailing list under the subject: 'Re: Multiplexing
sockets in DFSClient/datanodes?'
> {code}
> stack wrote:
> > Doug Cutting wrote:
> >> RPC also tears down idle connections, which HDFS does not.  I wonder how much
doing that alone might help your case?  That would probably be much simpler to implement.
 Both client and server must already handle connection failures, so it shouldn't be too great
of a change to have one or both sides actively close things down if they're idle for more
than a few seconds.
> >
> > If we added tear down of idle sockets, that'd work for us and, as you suggest, should
be easier to do than rewriting the client to use async i/o.   Currently, random reading, its
probably rare that the currently opened HDFS block has the wanted offset and so a tear down
of the current socket and an open of a new one is being done anyways.
> HADOOP-2346 helps with the Datanode side of the problem. We still need DFSClient to clean
up idle connections (otherwise these sockets will stay in CLOSE_WAIT state on the client).
This would require an extra thread on client to clean up these connections. You could file
a jira for it.
> Raghu. 
> {code}

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