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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1869) access times of HDFS files
Date Wed, 03 Sep 2008 17:55:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12628073#action_12628073
] 

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

> a file create (such as expansion of an archive and a distcp), [...]

During create is not enough even for these test cases. Say distcp copies 10GB file and set
Mod time at create time (to t - 1month), and the last block was written 1 min later.. then
the mod time after Distcp will be (t + 1min) rather than (t - 1month). -1 for extra options
to create, or close, etc.

Why not just provide utimes().. since we are using POSIX as a tie breaker? 

Another Konstantin's point is that FS should not allow setting future time.. which sounds
ok.. but it is just a file attribute to help users not something filesystem inherently depends
upon. I don't see need to police it that much .. and since POSIX is a tie breaker we could
just stick to it. For all the use case we need to be able to set modtime too. 

> 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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