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From "Konstantin Shvachko (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-8999) Namenode need not wait for {{blockReceived}} for the last block before completing a file.
Date Sun, 06 Sep 2015 23:33:45 GMT

    [ https://issues.apache.org/jira/browse/HDFS-8999?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14733172#comment-14733172
] 

Konstantin Shvachko commented on HDFS-8999:
-------------------------------------------

Spent some time browsing jira. This issue was discussed earlier in HDFS-1172 (linking).
# NN cannot rely on locations reported by the client (or a primary DN) because it leads to
a race condition between the client report and block reports from the DN, that contains the
replica. The block report may not contain the replica that was reported by the client. As
noted in [Hairong's comment|https://issues.apache.org/jira/browse/HDFS-1172?focusedCommentId=12874030&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-12874030]
# [~hairong] proposed a solution, which makes NN place replicas that were not yet reported
by DNs into {{pendingReplication}} queue instead of {{neededRepication}}. This is absolutely
logical, because NN knows that missing replicas were in the succeeded pipeline and can assume
they will be reported soon.

I don't know why HDFS-1172 was never committed. May be it is time to revisit it now.

> 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
>
> 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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