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https://issues.apache.org/jira/browse/HDFS-9600?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15136526#comment-15136526
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Yongjun Zhang commented on HDFS-9600:
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Thanks for looking into and commenting Andrew!
I agree that how the status is set is confusing here.
{quote}
For open files, we only let it decom if the UC block stays above minReplication
(default 1). Note the curReplicas > minReplication check.
{quote}
The {{isReplicationInProgress}} method initialize {{status}} to false, for
replicationFactor==1 case, the check {{curReplicas > minReplication}} will be
false, thus the method will return false, and the node that contains the single
replica will be marked as decommissioned, thus the block will be lost , unless
we over replicate all replicas on the node before decommissioning for
replicationFactor==1 case. Am I right? Did we do over-replicate before
decommissioning?
One point in my earlier comment is, If a replica is currently being written to
(under construction), in order for us to decommission this node, either 1, the
client need to reconstruct the write pipeline without this node, or 2, we need
to wait for the block to be complete, and satisfy the replication count before
this node can be decommissioned. Since it may take long to finish writing a
block, I think likely it's case 1. If we go with case 1, then for
replicationFactor==1 case, the client need to rewrite all data of the same
block to other node. I wonder if this is the current behavior?
Hope other folks who are more familiar with the code can answer your question.
It'd be nice to have some good document about the handling.
Thanks.
> do not check replication if the block is under construction
> -----------------------------------------------------------
>
> Key: HDFS-9600
> URL: https://issues.apache.org/jira/browse/HDFS-9600
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Phil Yang
> Assignee: Phil Yang
> Priority: Critical
> Fix For: 2.8.0, 2.7.3, 2.6.4
>
> Attachments: HDFS-9600-branch-2.6.patch, HDFS-9600-branch-2.7.patch,
> HDFS-9600-branch-2.patch, HDFS-9600-v1.patch, HDFS-9600-v2.patch,
> HDFS-9600-v3.patch, HDFS-9600-v4.patch
>
>
> When appending a file, we will update pipeline to bump a new GS and the old
> GS will be considered as out of date. When changing GS, in
> BlockInfo.setGenerationStampAndVerifyReplicas we will remove replicas having
> old GS which means we will remove all replicas because no DN has new GS until
> the block with new GS is added to blockMaps again by
> DatanodeProtocol.blockReceivedAndDeleted.
> If we check replication of this block before it is added back, it will be
> regarded as missing. The probability is low but if there are decommissioning
> nodes the DecommissionManager.Monitor will scan all blocks belongs to
> decommissioning nodes with a very fast speed so the probability of finding
> missing block is very high but actually they are not missing.
> Furthermore, after closing the appended file, in
> FSNamesystem.finalizeINodeFileUnderConstruction, it will checkReplication. If
> some of nodes are decommissioning, this block with new GS will be added to
> UnderReplicatedBlocks map so there are two blocks with same ID in this map,
> one is in QUEUE_WITH_CORRUPT_BLOCKS and the other is in
> QUEUE_HIGHEST_PRIORITY or QUEUE_UNDER_REPLICATED. And there will be many
> missing blocks warning in NameNode website but there is no corrupt files...
> Therefore, I think the solution is we should not check replication if the
> block is under construction. We only check complete blocks.
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