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https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623756#action_12623756
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dhruba borthakur commented on HADOOP-1869:
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Hi Raghu, the idea you propose "update access times olce every 24 hours" sounds
good. However, how will the namenode remember which inodes are dirty and need
to be flushed? It can keep a data structure to record all inodes that have
"dirty" access times, but it needs memory. Another option would be to "walk"
the entire tree looking for "dirty" inodes. Both approaches are not very
appealing. Do you have any other options?
> 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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