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https://issues.apache.org/jira/browse/HDFS-8999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15083176#comment-15083176
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Tsz Wo Nicholas Sze commented on HDFS-8999:
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> Is it strange? If we gonna do this, does it mean addBlock(..) can apply the
> same change?
[~walter.k.su], your example is interesting. As you mentioned, addBlock(..)
waits for the second-last block. close() still waits for the second-last
block. Two methods are the same in this sense.
Indeed, we may change addBlock(..) to wait for the third-last block. However,
we don't see a need for the moment.
> If block size is small or client writes lots of small files, we have lots of
> committed blocks. ...
Within a short period of time, it is correct that we have a lot of committed
blocks. This is the problem we try to solve here -- datanode send an
accumulated block receipt instead of a block receipt for each block within a
short period of time in order to reduce the number of RPCs to NN.
> ... And, what's the meaning of "minRepl"? Why we need "committed" and
> "completed"? ...
Historically, the notion of minRepl existed before we introduced the notions of
COMMITTED and COMPLETE blocks for append. These two states are still useful
for append after the change.
> Namenode need not wait for {{blockReceived}} for the last block before
> completing a file.
> -----------------------------------------------------------------------------------------
>
> Key: HDFS-8999
> URL: https://issues.apache.org/jira/browse/HDFS-8999
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: namenode
> Reporter: Jitendra Nath Pandey
> Assignee: Tsz Wo Nicholas Sze
> Attachments: h8999_20151228.patch
>
>
> This comes out of a discussion in HDFS-8763. Pasting [~jingzhao]'s comment
> from the jira:
> {quote}
> ...whether we need to let NameNode wait for all the block_received msgs to
> announce the replica is safe. Looking into the code, now we have
> # NameNode knows the DataNodes involved when initially setting up the
> writing pipeline
> # If any DataNode fails during the writing, client bumps the GS and
> finally reports all the DataNodes included in the new pipeline to NameNode
> through the updatePipeline RPC.
> # When the client received the ack for the last packet of the block (and
> before the client tries to close the file on NameNode), the replica has been
> finalized in all the DataNodes.
> Then in this case, when NameNode receives the close request from the client,
> the NameNode already knows the latest replicas for the block. Currently the
> checkReplication call only counts in all the replicas that NN has already
> received the block_received msg, but based on the above #2 and #3, it may be
> safe to also count in all the replicas in the
> BlockUnderConstructionFeature#replicas?
> {quote}
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