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From "Yongjun Zhang (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-10587) Incorrect offset/length calculation in pipeline recovery causes block corruption
Date Thu, 14 Jul 2016 22:06:20 GMT

    [ https://issues.apache.org/jira/browse/HDFS-10587?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15378483#comment-15378483

Yongjun Zhang commented on HDFS-10587:

I think this looks why we had new data reaching the new DN after the init block transfer:
 after adding the new DN to the pipeline, doing the block transfer to this new DN, the client
resumed writing data. Then in the process, corruption is detected again, thus repeating the
pipeline recovery process. Even though from client side point of view, it keeps getting the
following exception

INFO org.apache.hadoop.hdfs.DFSClient: Exception in createBlockOutputStream
java.io.IOException: Bad connect ack with firstBadLink as
        at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.createBlockOutputStream(DFSOutputStream.java:1472)
        at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.setupPipelineForAppendOrRecovery(DFSOutputStream.java:1293)
        at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.processDatanodeError(DFSOutputStream.java:1016)
        at org.apache.hadoop.hdfs.DFSOutputStream$DataStreamer.run(DFSOutputStream.java:560)

Wei-Chiu and I discussed, and we think here is a more complete picture:

* 1. pipeline going on DN1 -> DN2 -> DN3
* 2. trouble at DN3, it's gone
* 3. pipeline recovery, new DN DN4 added
* 4. block transfer from DN1 to DN4, DN4's data is now a multiple of chunks.
* 5. DataStreamer resumed writing data to DN1 -> DN4 -> DN3 (this is where new data
gets in), the first chunk DN4 got is corrupt for some reason that we are searching for
* 6. DN3 detects corruption, quit; while new data has been written to DN1 and DN4
* 7. goes back to step 3, new pipeline recovery starts
DN1 ->DN4 -> DN5
DN1 -> DN4 -> DN6

At a corner case, Step 3 could be replaced with "DN3 restarted", in which case, another block
transfer would happen, and may cause corruption.

Since DN1's visibleLength in step 4 is not a multiple of chunks, this fact might be somehow
related to the corruption in step 5.

> Incorrect offset/length calculation in pipeline recovery causes block corruption
> --------------------------------------------------------------------------------
>                 Key: HDFS-10587
>                 URL: https://issues.apache.org/jira/browse/HDFS-10587
>             Project: Hadoop HDFS
>          Issue Type: Bug
>          Components: datanode
>            Reporter: Wei-Chiu Chuang
>            Assignee: Wei-Chiu Chuang
>         Attachments: HDFS-10587.001.patch
> We found incorrect offset and length calculation in pipeline recovery may cause block
corruption and results in missing blocks under a very unfortunate scenario. 
> (1) A client established pipeline and started writing data to the pipeline.
> (2) One of the data node in the pipeline restarted, closing the socket, and some written
data were unacknowledged.
> (3) Client replaced the failed data node with a new one, initiating block transfer to
copy existing data in the block to the new datanode.
> (4) The block is transferred to the new node. Crucially, the entire block, including
the unacknowledged data, was transferred.
> (5) The last chunk (512 bytes) was not a full chunk, but the destination still reserved
the whole chunk in its buffer, and wrote the entire buffer to disk, therefore some written
data is garbage.
> (6) When the transfer was done, the destination data node converted the replica from
temporary to rbw, which made its visible length as the length of bytes on disk. That is to
say, it thought whatever was transferred was acknowledged. However, the visible length of
the replica is different (round up to the next multiple of 512) than the source of transfer.
> (7) Client then truncated the block in the attempt to remove unacknowledged data. However,
because the visible length is equivalent of the bytes on disk, it did not truncate unacknowledged
> (8) When new data was appended to the destination, it skipped the bytes already on disk.
Therefore, whatever was written as garbage was not replaced.
> (9) the volume scanner detected corrupt replica, but due to HDFS-10512, it wouldn’t
tell NameNode to mark the replica as corrupt, so the client continued to form a pipeline using
the corrupt replica.
> (10) Finally the DN that had the only healthy replica was restarted. NameNode then update
the pipeline to only contain the corrupt replica.
> (11) Client continue to write to the corrupt replica, because neither client nor the
data node itself knows the replica is corrupt. When the restarted datanodes comes back, their
replica are stale, despite they are not corrupt. Therefore, none of the replica is good and
up to date.
> The sequence of events was reconstructed based on DataNode/NameNode log and my understanding
of code.
> Incidentally, we have observed the same sequence of events on two independent clusters.
> [1]
> The sender has the replica as follows:
> 2016-04-15 22:03:05,066 INFO org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl:
Recovering ReplicaBeingWritten, blk_1556997324_1100153495099, RBW
>   getNumBytes()     = 41381376
>   getBytesOnDisk()  = 41381376
>   getVisibleLength()= 41186444
>   getVolume()       = /hadoop-i/data/current
>   getBlockFile()    = /hadoop-i/data/current/BP-1043567091-
>   bytesAcked=41186444
>   bytesOnDisk=41381376
> while the receiver has the replica as follows:
> 2016-04-15 22:03:05,068 INFO org.apache.hadoop.hdfs.server.datanode.fsdataset.impl.FsDatasetImpl:
Recovering ReplicaBeingWritten, blk_1556997324_1100153495099, RBW
>   getNumBytes()     = 41186816
>   getBytesOnDisk()  = 41186816
>   getVisibleLength()= 41186816
>   getVolume()       = /hadoop-g/data/current
>   getBlockFile()    = /hadoop-g/data/current/BP-1043567091-
>   bytesAcked=41186816
>   bytesOnDisk=41186816

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