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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2073) Datanode corruption if machine dies while writing VERSION file
Date Fri, 19 Oct 2007 21:54:50 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2073?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#action_12536354
] 

Konstantin Shvachko commented on HADOOP-2073:
---------------------------------------------

> Does windows allow changing length when the file open?
Yes.

> It still leaves the problem with multiple data directories?
What is that problem? This is not intended to solve all reliability problems, just one.

> We exit if different VERSION files are inconsistent right?
For data-nodes inconsistent file values will cause an exception, for the name-node we choose
the most recently updated directory.

> datanode could die while the file is being rewritten, or before the file is resized.
Yes, the perfect solution would be if we could write and resize in memory and then flush and
close at once.
Here our problem is that Properties.store() writes data and flushes. So if data-node dies
at this point version file will have extra data
if the new data size is less then the old one. The only extra data we write in version file
now is related to distributed upgrades.
Even if the upgrade fields remain in the version file the data-node will restart and detect
the upgrade has been completed.
So you never end up with the empty version file and that we always have either the new or
the old data in it, and never the mixture of the two.
The point is that although this approach does not work for arbitrary file modifications, it
works for what we do with the version file.


> Datanode corruption if machine dies while writing VERSION file
> --------------------------------------------------------------
>
>                 Key: HADOOP-2073
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2073
>             Project: Hadoop
>          Issue Type: Bug
>          Components: dfs
>    Affects Versions: 0.14.0
>            Reporter: Michael Bieniosek
>            Assignee: Raghu Angadi
>         Attachments: versionFileSize.patch
>
>
> Yesterday, due to a bad mapreduce job, some of my machines went on OOM killing sprees
and killed a bunch of datanodes, among other processes.  Since my monitoring software kept
trying to bring up the datanodes, only to have the kernel kill them off again, each machine's
datanode was probably killed many times.  A large percentage of these datanodes will not come
up now, and write this message to the logs:
> 2007-10-18 00:23:28,076 ERROR org.apache.hadoop.dfs.DataNode: org.apache.hadoop.dfs.InconsistentFSStateException:
Directory /hadoop/dfs/data is in an inconsistent state: file VERSION is invalid.
> When I check, /hadoop/dfs/data/current/VERSION is an empty file.  Consequently, I have
to delete all the blocks on the datanode and start over.  Since the OOM killing sprees happened
simultaneously on several datanodes in my DFS cluster, this could have crippled my dfs cluster.
> I checked the hadoop code, and in org.apache.hadoop.dfs.Storage, I see this:
> {{{
>     /**
>      * Write version file.
>      * 
>      * @throws IOException
>      */
>     void write() throws IOException {
>       corruptPreUpgradeStorage(root);
>       write(getVersionFile());
>     }
>     void write(File to) throws IOException {
>       Properties props = new Properties();
>       setFields(props, this);
>       RandomAccessFile file = new RandomAccessFile(to, "rws");
>       FileOutputStream out = null;
>       try {
>         file.setLength(0);
>         file.seek(0);
>         out = new FileOutputStream(file.getFD());
>         props.store(out, null);
>       } finally {
>         if (out != null) {
>           out.close();
>         }
>         file.close();
>       }
>     }
> }}}
> So if the datanode dies after file.setLength(0), but before props.store(out, null), the
VERSION file will get trashed in the corrupted state I see.  Maybe it would be better if this
method created a temporary file VERSION.tmp, and then copied it to VERSION, then deleted VERSION.tmp?
 That way, if VERSION was detected to be corrupt, the datanode could look at VERSION.tmp to
recover the data.

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