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From "Anu Engineer (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4015) Safemode should count and report orphaned blocks
Date Mon, 21 Nov 2016 18:28:58 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4015?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15684329#comment-15684329

Anu Engineer commented on HDFS-4015:

bq. Should this be marked as incompatible change? 
I think we can argue both sides, so I am fine with either call you make after reading through
both points of view.

bq. Earlier "dfsadmin -safemode leave" was leaving the safemode.
The behaviour in 99.99999% of cases is exactly same. So it is a rare case of incompatibility,
even if we end up defining this as an incompatibility.

bq. Now expects "-forceExit" also if there are any future bytes detected.
Future bytes -- is an error condition that should have been flagged by HDFS. This was a missing
error check, if we call this incompatibility, it would mean that copying old fsImage or old
NN metadata was a supported operation. I would argue that it never was a supported operation
in the sense that NN metadata is sacrosanct and you are not supposed to roll it back.

So from that point of view, it just confirms what we always knew and avoids booting up with
incorrect metadata, but both of us very well know that the reason why this JIRA is fixed is
because people do this and lose data. 

With this change copy/restoring NN metadata it has become a supported operation (that is,
HDFS is aware users are going to do this), and we explicitly warn the user of harm that this
action can cause. If we were to argue that old behavior was a feature, then we are saying
that changing NN metadata and losing data was a supported feature.

While it is still possible to copy an older version of NN metadata, now HDFS is going warn
the end user about data loss. The question you are asking is should we classify that as a
incompatibility or as enforcement of the core axioms of HDFS? 

My *personal* view is that is it not an incompatible change, since HDFS has never officially
encouraged people to copy *older* versions of NN metadata. If you agree with that, then this
change merely formalizes that assumption that NN metadata is sacrosanct and if you roll it
back, we are in an error state that needs explicit user intervention.

But I also see that from an end users point of view (especially someone with lot of HDFS experience),
this enforcement of a NN metadata integrity takes way some of the old dangerous behavior.
Now we have added detection of an error condition which requires explicit action from user,
you can syntactically argue that it is an incompatible change, though semantically I would
suppose that it is obvious to any HDFS user that copying old versions of NN metadata is a
bad idea.

> Safemode should count and report orphaned blocks
> ------------------------------------------------
>                 Key: HDFS-4015
>                 URL: https://issues.apache.org/jira/browse/HDFS-4015
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: namenode
>    Affects Versions: 3.0.0-alpha1
>            Reporter: Todd Lipcon
>            Assignee: Anu Engineer
>             Fix For: 2.8.0, 3.0.0-alpha1
>         Attachments: HDFS-4015.001.patch, HDFS-4015.002.patch, HDFS-4015.003.patch, HDFS-4015.004.patch,
HDFS-4015.005.patch, HDFS-4015.006.patch, HDFS-4015.007.patch
> The safemode status currently reports the number of unique reported blocks compared to
the total number of blocks referenced by the namespace. However, it does not report the inverse:
blocks which are reported by datanodes but not referenced by the namespace.
> In the case that an admin accidentally starts up from an old image, this can be confusing:
safemode and fsck will show "corrupt files", which are the files which actually have been
deleted but got resurrected by restarting from the old image. This will convince them that
they can safely force leave safemode and remove these files -- after all, they know that those
files should really have been deleted. However, they're not aware that leaving safemode will
also unrecoverably delete a bunch of other block files which have been orphaned due to the
namespace rollback.
> I'd like to consider reporting something like: "900000 of expected 1000000 blocks have
been reported. Additionally, 10000 blocks have been reported which do not correspond to any
file in the namespace. Forcing exit of safemode will unrecoverably remove those data blocks"
> Whether this statistic is also used for some kind of "inverse safe mode" is the logical
next step, but just reporting it as a warning seems easy enough to accomplish and worth doing.

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