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From "Chris Nauroth (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-9038) DFS reserved space is erroneously counted towards non-DFS used.
Date Tue, 15 Dec 2015 20:45:46 GMT

    [ https://issues.apache.org/jira/browse/HDFS-9038?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15058764#comment-15058764

Chris Nauroth commented on HDFS-9038:

Thanks everyone for sticking with this.  This has turned out to be much trickier than I anticipated
when I filed the issue.  I'd like to summarize current status.

Arpit and I are in agreement about my analysis of how the calculation changed after HDFS-5215.
 However, we are not yet in agreement about which calculation is truly correct.  I believe
the pre-HDFS-5215 calculation (subtracting {{dfs.datanode.du.reserved}}) is correct, because
it allowed me to monitor for unexpected non-zero non-DFS usage and react.  Since this was
an established operations workflow (at least for me), I argue that we have a responsibility
to restore that behavior.  Arpit believes that it's correct to cancel out {{dfs.datanode.du.reserved}},
because then non-DFS used would report space used for non-HDFS purposes more accurately. 
Essentially, it's a question of whether this metric means "Raw Non-DFS Used" or "Unplanned
Non-DFS Used".

We also discovered an interesting side issue about {{File#getUsableSpace}} vs. {{File#getFreeSpace}}.
 Pre-HDFS-5215, it could be considered a bug that we did not account for system reserved space.
 Interestingly, it seems in our testing that ext holds back 5% by default, but xfs does not.

I pushed pretty hard for restoring the pre-HDFS-5215 behavior in my earlier comments, but
I'm just one voice.  I suggest that we leave this issue open for a while for others to comment.
 I could be swayed if others think I'm approaching this incorrectly.  Meanwhile, [~brahmareddy],
would you please hold off on posting more patches?  Let's wait for the discussion to settle
a little more first.  Thanks for your patience.

> DFS reserved space is erroneously counted towards non-DFS used.
> ---------------------------------------------------------------
>                 Key: HDFS-9038
>                 URL: https://issues.apache.org/jira/browse/HDFS-9038
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>    Affects Versions: 2.7.1
>            Reporter: Chris Nauroth
>            Assignee: Brahma Reddy Battula
>         Attachments: HDFS-9038-002.patch, HDFS-9038-003.patch, HDFS-9038-004.patch, HDFS-9038-005.patch,
HDFS-9038-006.patch, HDFS-9038-007.patch, HDFS-9038.patch
> HDFS-5215 changed the DataNode volume available space calculation to consider the reserved
space held by the {{dfs.datanode.du.reserved}} configuration property.  As a side effect,
reserved space is now counted towards non-DFS used.  I don't believe it was intentional to
change the definition of non-DFS used.  This issue proposes restoring the prior behavior:
do not count reserved space towards non-DFS used.

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