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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Updated: (HADOOP-1869) access times of HDFS files
Date Wed, 27 Aug 2008 21:39:44 GMT

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

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

    Attachment: accessTime5.patch

Incorporated all of Konstantin's review comments other than the one that said that setAccessTime()
call should be removed.

The setAccessTime() call is a utility method that allows an application to set the access
time of a file without having to "open" the file. The permission-access-checks are precisely
the same as that for "opening" a file, so there isn't any security concern IMO. Most file
systems support setting access times/modification times on a file, see http://linux.die.net/man/2/utimes.

I purposely did not add a command to the FsShell to display access times. An application can
fetch the accessTime using a programmatic API FileSystem.getFileStatus().



> 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
>
>
> 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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