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From Uma Maheswara Rao G <mahesw...@huawei.com>
Subject RE: data loss after cluster wide power loss
Date Tue, 02 Jul 2013 07:31:22 GMT
Hi Dave,

 Looks like your analysis is correct. I have faced similar issue some time back. 
See the discussion link: http://markmail.org/message/ruev3aa4x5zh2l4w#query:+page:1+mid:33gcdcu3coodkks3+state:results
On sudden restarts, it can lost the OS filesystem edits. Similar thing happened in our case,
i.e, after restart blocks were moved back to BeingWritten directory even though they were
finalized.
After restart they were marked as corrupt. You could set dfs.datanode.synconclose to true
to avoid this sort of things, but that will degrade performance.

Regards,
Uma

-----Original Message-----
From: ddlatham@gmail.com [mailto:ddlatham@gmail.com] On Behalf Of Dave Latham
Sent: 01 July 2013 16:08
To: hdfs-user@hadoop.apache.org
Cc: hdfs-dev@hadoop.apache.org
Subject: Re: data loss after cluster wide power loss

Much appreciated, Suresh.  Let me know if I can provide any more information or if you'd like
me to open a JIRA.

Dave


On Mon, Jul 1, 2013 at 8:48 PM, Suresh Srinivas <suresh@hortonworks.com>wrote:

> Dave,
>
> Thanks for the detailed email. Sorry I did not read all the details 
> you had sent earlier completely (on my phone). As you said, this is 
> not related to data loss related to HBase log and hsync. I think you 
> are right; the rename operation itself might not have hit the disk. I 
> think we should either ensure metadata operation is synced on the 
> datanode or handle it being reported as blockBeingWritten. Let me 
> spend sometime to debug this issue.
>
> One surprising thing is, all the replicas were reported as 
> blockBeingWritten.
>
> Regards,
> Suresh
>
>
> On Mon, Jul 1, 2013 at 6:03 PM, Dave Latham <latham@davelink.net> wrote:
>
>> (Removing hbase list and adding hdfs-dev list as this is pretty 
>> internal stuff).
>>
>> Reading through the code a bit:
>>
>> FSDataOutputStream.close calls
>> DFSOutputStream.close calls
>> DFSOutputStream.closeInternal
>>  - sets currentPacket.lastPacketInBlock = true
>>  - then calls
>> DFSOutputStream.flushInternal
>>  - enqueues current packet
>>  - waits for ack
>>
>> BlockReceiver.run
>>  - if (lastPacketInBlock && !receiver.finalized) calls 
>> FSDataset.finalizeBlock calls FSDataset.finalizeBlockInternal calls 
>> FSVolume.addBlock calls FSDir.addBlock calls FSDir.addBlock
>>  - renames block from "blocksBeingWritten" tmp dir to "current" dest 
>> dir
>>
>> This looks to me as I would expect a synchronous chain from a DFS 
>> client to moving the file from blocksBeingWritten to the current dir 
>> so that once the file is closed that it the block files would be in 
>> the proper directory
>> - even if the contents of the file are still in the OS buffer rather 
>> than synced to disk.  It's only after this moving of blocks that 
>> NameNode.complete file is called.  There are several conditions and 
>> loops in there that I'm not certain this chain is fully reliable in 
>> all cases without a greater understanding of the code.
>>
>> Could it be the case that the rename operation itself is not synced 
>> and that ext3 lost the fact that the block files were moved?
>> Or is there a bug in the close file logic that for some reason the 
>> block files are not always moved into place when a file is closed?
>>
>> Thanks for your patience,
>> Dave
>>
>>
>> On Mon, Jul 1, 2013 at 3:35 PM, Dave Latham <latham@davelink.net> wrote:
>>
>>> Thanks for the response, Suresh.
>>>
>>> I'm not sure that I understand the details properly.  From my 
>>> reading of
>>> HDFS-744 the hsync API would allow a client to make sure that at any 
>>> point in time it's writes so far hit the disk.  For example, for 
>>> HBase it could apply a fsync after adding some edits to its WAL to 
>>> ensure those edits are fully durable for a file which is still open.
>>>
>>> However, in this case the dfs file was closed and even renamed.  Is 
>>> it the case that even after a dfs file is closed and renamed that 
>>> the data blocks would still not be synced and would still be stored 
>>> by the datanode in "blocksBeingWritten" rather than in "current"?  
>>> If that is case, would it be better for the NameNode not to reject 
>>> replicas that are in blocksBeingWritten, especially if it doesn't 
>>> have any other replicas available?
>>>
>>> Dave
>>>
>>>
>>> On Mon, Jul 1, 2013 at 3:16 PM, Suresh Srinivas <suresh@hortonworks.com>wrote:
>>>
>>>> Yes this is a known issue.
>>>>
>>>> The HDFS part of this was addressed in
>>>> https://issues.apache.org/jira/browse/HDFS-744 for 2.0.2-alpha and 
>>>> is not available in 1.x  release. I think HBase does not use this 
>>>> API yet.
>>>>
>>>>
>>>> On Mon, Jul 1, 2013 at 3:00 PM, Dave Latham <latham@davelink.net>
>>>> wrote:
>>>>
>>>> > We're running HBase over HDFS 1.0.2 on about 1000 nodes.  On 
>>>> > Saturday
>>>> the
>>>> > data center we were in had a total power failure and the cluster 
>>>> > went
>>>> down
>>>> > hard.  When we brought it back up, HDFS reported 4 files as CORRUPT.
>>>>  We
>>>> > recovered the data in question from our secondary datacenter, but 
>>>> > I'm trying to understand what happened and whether this is a bug 
>>>> > in HDFS
>>>> that
>>>> > should be fixed.
>>>> >
>>>> > From what I can tell the file was created and closed by the dfs 
>>>> > client (hbase).  Then HBase renamed it into a new directory and 
>>>> > deleted some
>>>> other
>>>> > files containing the same data.  Then the cluster lost power.  
>>>> > After
>>>> the
>>>> > cluster was restarted, the datanodes reported into the namenode 
>>>> > but
>>>> the
>>>> > blocks for this file appeared as "blocks being written" - the 
>>>> > namenode rejected them and the datanodes deleted the blocks.  At 
>>>> > this point
>>>> there
>>>> > were no replicas for the blocks and the files were marked CORRUPT.
>>>>  The
>>>> > underlying file systems are ext3.  Some questions that I would 
>>>> > love
>>>> get
>>>> > answers for if anyone with deeper understanding of HDFS can chime in:
>>>> >
>>>> >  - Is this a known scenario where data loss is expected?  (I 
>>>> > found
>>>> > HDFS-1539 but that seems different)
>>>> >  - When are blocks moved from blocksBeingWritten to current?  
>>>> > Does
>>>> that
>>>> > happen before a file close operation is acknowledged to a hdfs client?
>>>> >  - Could it be that the DataNodes actually moved the blocks to
>>>> current but
>>>> > after the restart ext3 rewound state somehow (forgive my 
>>>> > ignorance of underlying file system behavior)?
>>>> >  - Is there any other explanation for how this can happen?
>>>> >
>>>> > Here is a sequence of selected relevant log lines from the RS 
>>>> > (HBase Region Server) NN (NameNode) and DN (DataNode - 1 example 
>>>> > of 3 in question).  It includes everything that mentions the 
>>>> > block in
>>>> question in
>>>> > the NameNode and one DataNode log.  Please let me know if this 
>>>> > more information that would be helpful.
>>>> >
>>>> > RS 2013-06-29 11:16:06,812 DEBUG org.apache.hadoop.hbase.util.FSUtils:
>>>> > Creating
>>>> >
>>>> file=hdfs://hm3:9000/hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e
>>>> /.tmp/6e0cc30af6e64e56ba5a539fdf159c4c
>>>> > with permission=rwxrwxrwx
>>>> > NN 2013-06-29 11:16:06,830 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.allocateBlock:
>>>> >
>>>> /hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e/.tmp/6e0cc30af6e64e56ba5a539fdf159c4c.
>>>> > blk_1395839728632046111_357084589 DN 2013-06-29 11:16:06,832 INFO
>>>> > org.apache.hadoop.hdfs.server.datanode.DataNode: Receiving block
>>>> > blk_1395839728632046111_357084589 src: /10.0.5.237:14327 dest: /
>>>> > 10.0.5.237:50010
>>>> > NN 2013-06-29 11:16:11,370 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: blockMap updated: 10.0.6.1:50010 is 
>>>> > added
>>>> to
>>>> > blk_1395839728632046111_357084589 size 25418340 NN 2013-06-29 
>>>> > 11:16:11,370 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: blockMap updated: 10.0.6.24:50010 is
>>>> added to
>>>> > blk_1395839728632046111_357084589 size 25418340 NN 2013-06-29 
>>>> > 11:16:11,385 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: blockMap updated: 10.0.5.237:50010 is
>>>> added to
>>>> > blk_1395839728632046111_357084589 size 25418340 DN 2013-06-29 
>>>> > 11:16:11,385 INFO
>>>> > org.apache.hadoop.hdfs.server.datanode.DataNode: Received block
>>>> > blk_1395839728632046111_357084589 of size 25418340 from /
>>>> 10.0.5.237:14327
>>>>
>>>> > DN 2013-06-29 11:16:11,385 INFO
>>>> > org.apache.hadoop.hdfs.server.datanode.DataNode: PacketResponder 
>>>> > 2 for block blk_1395839728632046111_357084589 terminating NN 
>>>> > 2013-06-29 11:16:11,385 INFO org.apache.hadoop.hdfs.StateChange:
>>>> > Removing lease on  file
>>>> >
>>>> /hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e/.tmp/6e0cc30af6e64e
>>>> 56ba5a539fdf159c4c
>>>> > from client DFSClient_hb_rs_hs745,60020,1372470111932
>>>> > NN 2013-06-29 11:16:11,385 INFO org.apache.hadoop.hdfs.StateChange:
>>>> DIR*
>>>> > NameSystem.completeFile: file
>>>> >
>>>> /hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e/.tmp/6e0cc30af6e64e
>>>> 56ba5a539fdf159c4c
>>>> > is closed by DFSClient_hb_rs_hs745,60020,1372470111932
>>>> > RS 2013-06-29 11:16:11,393 INFO
>>>> > org.apache.hadoop.hbase.regionserver.Store: Renaming compacted 
>>>> > file at
>>>> >
>>>> hdfs://hm3:9000/hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e/.tmp
>>>> /6e0cc30af6e64e56ba5a539fdf159c4c
>>>> > to
>>>> >
>>>> hdfs://hm3:9000/hbase/users-6/b5b0820cde759ae68e333b2f4015bb7e/n/6e
>>>> 0cc30af6e64e56ba5a539fdf159c4c
>>>> > RS 2013-06-29 11:16:11,505 INFO
>>>> > org.apache.hadoop.hbase.regionserver.Store: Completed major
>>>> compaction of 7
>>>> > file(s) in n of
>>>> > users-6,\x12\xBDp\xA3,1359426311784.b5b0820cde759ae68e333b2f4015bb7e.
>>>> into
>>>> > 6e0cc30af6e64e56ba5a539fdf159c4c, size=24.2m; total size for 
>>>> > store is
>>>> 24.2m
>>>> >
>>>> > -------  CRASH, RESTART ---------
>>>> >
>>>> > NN 2013-06-29 12:01:19,743 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: addStoredBlock request received for
>>>> > blk_1395839728632046111_357084589 on 10.0.6.1:50010 size 21978112
>>>> but was
>>>> > rejected: Reported as block being written but is a block of 
>>>> > closed
>>>> file.
>>>> > NN 2013-06-29 12:01:19,743 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addToInvalidates: blk_1395839728632046111 is added to
>>>> invalidSet
>>>> > of 10.0.6.1:50010
>>>> > NN 2013-06-29 12:01:20,155 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: addStoredBlock request received for
>>>> > blk_1395839728632046111_357084589 on 10.0.5.237:50010 size 
>>>> > 16971264
>>>> but
>>>> > was rejected: Reported as block being written but is a block of
>>>> closed file.
>>>> > NN 2013-06-29 12:01:20,155 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addToInvalidates: blk_1395839728632046111 is added to
>>>> invalidSet
>>>> > of 10.0.5.237:50010
>>>> > NN 2013-06-29 12:01:20,175 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addStoredBlock: addStoredBlock request received for
>>>> > blk_1395839728632046111_357084589 on 10.0.6.24:50010 size 
>>>> > 21913088
>>>> but
>>>> > was rejected: Reported as block being written but is a block of
>>>> closed file.
>>>> > NN 2013-06-29 12:01:20,175 INFO org.apache.hadoop.hdfs.StateChange:
>>>> BLOCK*
>>>> > NameSystem.addToInvalidates: blk_1395839728632046111 is added to
>>>> invalidSet
>>>> > of 10.0.6.24:50010
>>>> > (note the clock on the server running DN is wrong after restart.  
>>>> > I believe timestamps are off by 6 hours:) DN 2013-06-29 
>>>> > 06:07:22,877 INFO
>>>> > org.apache.hadoop.hdfs.server.datanode.DataNode: Scheduling block
>>>> > blk_1395839728632046111_357084589 file
>>>> > /data/hadoop/dfs/data/blocksBeingWritten/blk_1395839728632046111 
>>>> > for deletion DN 2013-06-29 06:07:24,952 INFO
>>>> > org.apache.hadoop.hdfs.server.datanode.DataNode: Deleted block
>>>> > blk_1395839728632046111_357084589 at file
>>>> > /data/hadoop/dfs/data/blocksBeingWritten/blk_1395839728632046111
>>>> >
>>>> >
>>>> > Thanks,
>>>> > Dave
>>>> >
>>>>
>>>>
>>>>
>>>> --
>>>> http://hortonworks.com/download/
>>>>
>>>
>>>
>>
>
>
> --
> http://hortonworks.com/download/
>

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