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From "Nicolas Liochon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4689) freeze/seal a hdfs file
Date Mon, 15 Apr 2013 12:18:16 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4689?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13631679#comment-13631679

Nicolas Liochon commented on HDFS-4689:

It was an idea (a good one :-) ) from @umamahesowararao. It's not backported to branch 1.
If we use it it would have to be optional for backward compatibility. Will be meaning full
once HBase uses the 2.0.5 imho.
> freeze/seal a hdfs file
> -----------------------
>                 Key: HDFS-4689
>                 URL: https://issues.apache.org/jira/browse/HDFS-4689
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: datanode, hdfs-client, namenode
>    Affects Versions: 2.0.0-alpha
>            Reporter: Zesheng Wu
>              Labels: freeze, seal
> I would like to describe the problem scenario at first, that is in our hbase cluster:
> 1. rs1 loses its zookeeper lock, and hmaster realizes that
> 2. hmaster assigns the regions of rs1 to rs2
> 3. rs2 renames the hlog of rs1, and begins to replay the log
> 4. but at the meantime, rs1 is still running, and the client still writes data to rs1
> 5. in this scenario, the data written after rs2 renamed rs1's hlog will be lost
> The root cause of the problem is: 
> As we all know, when we open a hdfs file for write, the file meta is only updated when
a block is finished or when the file is closed. But the client thinks that the data is successfully
written when it receives ack from datanode. Under this premise, after a file is renamed, the
client is not required to update the meta immediately, so the client will not realize about
the renaming, and will keep writing to the block, and will write successfully until the block
is finished or the file is closed. The data written during this time will certainly be lost.
> The basic idea about how to solve this is to add a freeze/seal semantics for a file,
when a file is frozen/sealed, the client can't write any data to it, but it can be renamed
or deleted.
> If we can freeze/seal a file, the scenario at the beginning will like this:
> 1. rs1 loses its zookeeper lock, and hmaster realizes that
> 2. hmaster freezes/seals the hlog of rs1
> 3. hmaster assigns the regions of rs1 to rs2
> 4. rs2 renames the hlog of rs1, and begins to replay the log
> 5. after rs2 successfully replayed the log, the log file is deleted
> 6. in this scenario, after hmaster freezed/sealed the hlog file of rs1, rs1 can't write
any data to it even if it is still running, this can guarantee no data will be lost
> I hope I've described the problem clearly. Is there anyone has already worked on this
feature? And any idea about this will be very appreciated.

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