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https://issues.apache.org/jira/browse/HADOOP-1869?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12625935#action_12625935
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Konstantin Shvachko commented on HADOOP-1869:
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# Test data -1-2% looks good.
# FSConstants
#- {{accessTimePrecision}} is not a constant and is not used anywhere.
#- white space added.
# FileSystem
#- I strongly disagree with introduction of {{FileSystem.setAccessTime()}}.
File systems usually do not provide such an interface.
Access time is changed to *current* when you access a file (call
{{getBlockLocations()}} in our case), but the clients should not be able to set
atime. It is not secure. Users can mistakenly set it to some time in the past
or maliciously to a future value and screw the system up.
#- white space added.
# FSNamesystem. It is better to replace boolean member {{supportAccessTime}} by
a method
{code}
boolean isAccessTimeSupported() {
return accessTimePrecision > 0;
}
{code}
# INode. Please revert changes to this line
{code}
+ /** * Get last modification time of inode.
{code}
# FSDirectory. The name-node will print the message below every time you access
the file.
{code}
NameNode.stateChangeLog.info("File " + src +
" already has an precise access time."
{code}
This is going to be a problem for log watchers, and will affect the performance.
Is it just a debug message? I'd remove it.
# I thought that ls should print aTime. Did not find any references to
FileStatus.getAccessTime() in shell files.
> 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
> Attachments: accessTime1.patch, accessTime4.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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