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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1869) access times of HDFS files
Date Wed, 03 Sep 2008 21:33:44 GMT

     [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

dhruba borthakur updated HADOOP-1869:
-------------------------------------

    Attachment: accessTime6.patch

Given that Raghu, Owen and Allen commented that it is better to follow the POSIX semantics
of allowing an user to set either access time or modification time to any arbitrary value
he/she likes, I change my earlier patch sightly to add the following API:

{quote}
 /**
 * Set access time of a file
 * @param p The path
 * @param mtime Set the modification time of this file.
 *              The number of milliseconds since Jan 1, 1970. 
 *              A value of -1 means that this call should not set modification time.
  * @param atime Set the access time of this file.
  *              The number of milliseconds since Jan 1, 1970. 
  *              A value of -1 means that this call should not set access time.
  */
public void setTimes(Path p, long mtime, long atime
     ) throws IOException;

{quote}

This is precisely similar to the POSIX utimes call, but follows the Hadoop naming pattern
for method names. This allows setting access time or modification time or both.


> access times of HDFS files
> --------------------------
>
>                 Key: HADOOP-1869
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1869
>             Project: Hadoop Core
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: dhruba borthakur
>            Assignee: dhruba borthakur
>             Fix For: 0.19.0
>
>         Attachments: accessTime1.patch, accessTime4.patch, accessTime5.patch, accessTime6.patch
>
>
> HDFS should support some type of statistics that allows an administrator to determine
when a file was last accessed. 
> Since HDFS does not have quotas yet, it is likely that users keep on accumulating files
in their home directories without much regard to the amount of space they are occupying. This
causes memory-related problems with the namenode.
> Access times are costly to maintain. AFS does not maintain access times. I thind DCE-DFS
does maintain access times with a coarse granularity.
> One proposal for HDFS would be to implement something like an "access bit". 
> 1. This access-bit is set when a file is accessed. If the access bit is already set,
then this call does not result in a transaction.
> 2. A FileSystem.clearAccessBits() indicates that the access bits of all files need to
be cleared.
> An administrator can effectively use the above mechanism (maybe a daily cron job) to
determine files that are recently used.

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