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https://issues.apache.org/jira/browse/HDFS-6507?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14028996#comment-14028996
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Vinayakumar B commented on HDFS-6507:
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bq. From what I know so far, all the operations mentioned in the description
are idempotent, so when a command fails on one(some) of the NNs, users can just
retry the command. What we need to do is report the error clearly to users,
either by logs or exit codes. Rollback is too complex.
Then we can go ahead for implementation.
bq. The manual operations required before command execution can be done by
tools in batches, for example, if users want to do refreshNodes, they can
upload new excludes/includes file to all of the NNs without caring which NN is
Active or Standby. After uploading the files, they just run refreshNodes
command, it's enough.
Yes, in anyway correct updates required at all namenodes.
bq. if we decide to send set safemode for both NN, we should let the user know
whether there's failure clearly. If there's failure, users should do retry
until all succeed.
Yes.
HDFS-6318 patch implements this approach for refreshServiceAcls, may be similar
changes can be done for all other commands mentioned in description
> Improve DFSAdmin to support HA cluster better
> ---------------------------------------------
>
> Key: HDFS-6507
> URL: https://issues.apache.org/jira/browse/HDFS-6507
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: tools
> Affects Versions: 2.4.0
> Reporter: Zesheng Wu
> Assignee: Zesheng Wu
>
> Currently, the commands supported in DFSAdmin can be classified into three
> categories according to the protocol used:
> 1. ClientProtocol
> Commands in this category generally implement by calling the corresponding
> function of the DFSClient class, and will call the corresponding remote
> implementation function at the NN side finally. At the NN side, all these
> operations are classified into five categories: UNCHECKED, READ, WRITE,
> CHECKPOINT, JOURNAL. Active NN will allow all operations, and Standby NN only
> allows UNCHECKED operations. In the current implementation of DFSClient, it
> will connect one NN first, if the first NN is not Active and the operation is
> not allowed, it will failover to the second NN. So here comes the problem,
> some of the commands(setSafeMode, saveNameSpace, restoreFailedStorage,
> refreshNodes, setBalancerBandwidth, metaSave) in DFSAdmin are classified as
> UNCHECKED operations, and when executing these commands in the DFSAdmin
> command line, they will be sent to a definite NN, no matter it is Active or
> Standby. This may result in two problems:
> a. If the first tried NN is standby, and the operation takes effect only on
> Standby NN, which is not the expected result.
> b. If the operation needs to take effect on both NN, but it takes effect on
> only one NN. In the future, when there is a NN failover, there may have
> problems.
> Here I propose the following improvements:
> a. If the command can be classified as one of READ/WRITE/CHECKPOINT/JOURNAL
> operations, we should classify it clearly.
> b. If the command can not be classified as one of the above four operations,
> or if the command needs to take effect on both NN, we should send the request
> to both Active and Standby NNs.
> 2. Refresh protocols: RefreshAuthorizationPolicyProtocol,
> RefreshUserMappingsProtocol, RefreshUserMappingsProtocol,
> RefreshCallQueueProtocol
> Commands in this category, including refreshServiceAcl,
> refreshUserToGroupMapping, refreshSuperUserGroupsConfiguration and
> refreshCallQueue, are implemented by creating a corresponding RPC proxy and
> sending the request to remote NN. In the current implementation, these
> requests will be sent to a definite NN, no matter it is Active or Standby.
> Here I propose that we sent these requests to both NNs.
> 3. ClientDatanodeProtocol
> Commands in this category are handled correctly, no need to improve.
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