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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1869) access times of HDFS files
Date Tue, 19 Aug 2008 18:05:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623750#action_12623750
] 

Raghu Angadi commented on HADOOP-1869:
--------------------------------------

If and only if increase in EditsLog data noticeably affects performance (which I suspect it
will) : Couple of options :

* Provide an accuracy setting : For e.g. if an admin is interested only in accuracy of days,
this setting could be 24 hours and an access to a file with 24hours will be recorded only
once.

* Maintain only the in-memory access times. Since NameNode is not restarted very often, admins
could always run a script every week or so to get a snap shot of access times. When the NameNode
restarts similar script could update the access times from prev snap-shot.


> 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
>
> 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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