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From "Andrew Purtell (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-3077) Quorum-based protocol for reading and writing edit logs
Date Thu, 15 Mar 2012 19:04:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-3077?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13230432#comment-13230432
] 

Andrew Purtell commented on HDFS-3077:
--------------------------------------

>From a user perspective.

bq. [Todd] I think a quorum commit is vastly superior for HA, especially given we'd like to
collocate the log replicas on machines doing other work. When those machines have latency
hiccups, or crash, we don't want the active NN to have to wait for long timeout periods before
continuing.

I think this is a promising direction. See next:

bq. [Eli] BK has two of the same main issues that we have depending on a an HA filer: (1)
many users don't want to admin a separate storage system (even if you "embed" BK it will be
discrete, fail independently etc) 

Perhaps we can go so far as to suggest the loggers be an additional thread added to the DataNodes.
Perhaps some subset of the DN pool is elected for the purpose. (Need we waste a whole disk
just for the transaction log? Maybe the log can be shared with DN storage. Or using a SSD
device for this purpose seems reasonable but the average user should not be expected to have
nodes with such on hand.) On the one hand, this would increase the internal complexity of
the DataNode implementation, even if the functionality can be pretty well partitioned -- separate
package, separate thread, etc. On the other hand, there would be not yet another moving part
to consider when deploying components around the cluster: ZooKeeper quorum peers, NameNodes,
DataNodes, the YARN AM, the Yarn NMs, HBase Masters, HBase RegionServers etc. etc. etc. 

This idea may go too far, but IMHO embedding BookKeeper goes enough in the other direction
to give me heartburn thinking about HA cluster ops.
                
> Quorum-based protocol for reading and writing edit logs
> -------------------------------------------------------
>
>                 Key: HDFS-3077
>                 URL: https://issues.apache.org/jira/browse/HDFS-3077
>             Project: Hadoop HDFS
>          Issue Type: New Feature
>          Components: ha, name-node
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>
> Currently, one of the weak points of the HA design is that it relies on shared storage
such as an NFS filer for the shared edit log. One alternative that has been proposed is to
depend on BookKeeper, a ZooKeeper subproject which provides a highly available replicated
edit log on commodity hardware. This JIRA is to implement another alternative, based on a
quorum commit protocol, integrated more tightly in HDFS and with the requirements driven only
by HDFS's needs rather than more generic use cases. More details to follow.

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