[jira] Commented: (HADOOP-1869) access times of HDFS files

["Doug Cutting (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12526230 ] 

Doug Cutting commented on HADOOP-1869:
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Another approach might be to process the namenode logs (if they're kept) to find which files have been accessed when.

Also, in HDFS, would access times really be that expensive?  We have relatively few files and relatively many blocks.  So increasing the data structure size of a file shouldn't be that costly.  The larger expense might be logging each time a file is opened.  How bad would that be?  Perhaps we could make it optional?

I'm just brainstorming...


> access times of HDFS files
> --------------------------
>
>                 Key: HADOOP-1869
>                 URL: https://issues.apache.org/jira/browse/HADOOP-1869
>             Project: Hadoop
>          Issue Type: New Feature
>          Components: dfs
>            Reporter: 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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