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https://issues.apache.org/jira/browse/HDFS-1224?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12881539#action_12881539
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Thanh Do commented on HDFS-1224:
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"Even so, does this cause any actual problems aside from a shorter pipeline?
I'm not sure, but based on the description, it sounds like dn2 thinks it has a
block (but it is incomplete), so a client might end up trying to get a block
from that node and get an incomplete block"
I think this does not create any problem aside from shorter pipeline.
dn2 has a block with old time stamp, because it misses updateBlock.
Hence the block at dn2 is finally deleted.
(but the append semantic is not guaranteed, right? because there are 3
alive datanodes, and write to all 3 is successful, but append only happen
successfully at 2 datanodes).
> Stale connection makes node miss append
> ---------------------------------------
>
> Key: HDFS-1224
> URL: https://issues.apache.org/jira/browse/HDFS-1224
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: data-node
> Affects Versions: 0.20-append
> Reporter: Thanh Do
>
> - Summary: if a datanode crashes and restarts, it may miss an append.
>
> - Setup:
> + # available datanodes = 3
> + # replica = 3
> + # disks / datanode = 1
> + # failures = 1
> + failure type = crash
> + When/where failure happens = after the first append succeed
>
> - Details:
> Since each datanode maintains a pool of IPC connections, whenever it wants
> to make an IPC call, it first looks into the pool. If the connection is not
> there,
> it is created and put in to the pool. Otherwise the existing connection is
> used.
> Suppose that the append pipeline contains dn1, dn2, and dn3. Dn1 is the
> primary.
> After the client appends to block X successfully, dn2 crashes and restarts.
> Now client writes a new block Y to dn1, dn2 and dn3. The write is successful.
> Client starts appending to block Y. It first calls dn1.recoverBlock().
> Dn1 will first create a proxy corresponding with each of the datanode in the
> pipeline
> (in order to make RPC call like getMetadataInfo( ) or updateBlock( )).
> However, because
> dn2 has just crashed and restarts, its connection in dn1's pool become stale.
> Dn1 uses
> this connection to make a call to dn2, hence an exception. Therefore, append
> will be
> made only to dn1 and dn3, although dn2 is alive and the write of block Y to
> dn2 has
> been successful.
> This bug was found by our Failure Testing Service framework:
> http://www.eecs.berkeley.edu/Pubs/TechRpts/2010/EECS-2010-98.html
> For questions, please email us: Thanh Do (than...@cs.wisc.edu) and
> Haryadi Gunawi (hary...@eecs.berkeley.edu)
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