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https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623797#action_12623797
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dhruba borthakur commented on HADOOP-1869:
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Raghu, just to make sure I understand right: let's say the accuray setting in
24 hours. Now, suppose I read the contents of a file /tmp/foo now at 1PM. The
in-memory inode is updated with the accesstime of 1PM. But it is not recorded
in the transaction log. let's assue, that no other files are accessed in the
file system for the entire next day.
When it is 1 PM tomorrow, the system has to remember that /tmp/foo needs to be
flushed. How does this occur? How does hdfs find out that the inode /tmp/foo is
dirty and has to be flushed to the transaction log?
> 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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