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From "Raghu Angadi (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-1869) access times of HDFS files
Date Tue, 19 Aug 2008 21:01:45 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12623805#action_12623805
] 

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

> The in-memory inode is updated with the accesstime of 1PM. But it is not recorded in
the transaction log. 
 
It _is_ recorded in the transaction log at this time (assuming it was not accessed in 24 hours
prior to that). 

> When it is 1 PM tomorrow, the system has to remember that /tmp/foo needs to be flushed.
How does this occur? [...]

It does not need to remember, since the transaction was written at 1 PM previous day.

I am trying to see if I am missing something here. Note that effect of not sync-ing the editslog
file for each access is same as before.

> 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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