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From "dhruba borthakur (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1869) access times of HDFS files
Date Mon, 18 Aug 2008 22:43:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623481#action_12623481
] 

dhruba borthakur commented on HADOOP-1869:
------------------------------------------

I agree that HADOOP-3336 does this from an auditing perspective.

I am interested in making some form of archival store in HDFS. Files that are not used for
a long time can automatically be moved to slower and/or denser storage. Given the rate at
which a cluster  size increases, and given the fact that the cost to store data for infinitely
long time is very low, it makes sense for the file system to make intelligent storage decisions
based on how/when data was accessed. This argues for "access time" to be stored in the file
system itself.

HADOOP-3336 can be used to accomplish this to some extent... the separate log that it generates
can be periodically merged with the file system image. But, I feel that design is a little
awkward and not too elegant.


> 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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