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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16498739#comment-16498739
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Xiao Chen commented on HDFS-12943:
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Thanks all for the for the work! (and sorry for the late response here) Just
read through the design doc and the comments, looks great!
I have 2 questions:
About 'Optimization 1':
Currently atime is created to be the same as mtime, and only gets updated if
"dfs.namenode.accesstime.precision" has passed. Does this mean we require a
really small atime precision? (Anecdotally, snapshot will capture a diff on the
inode if atime is different. So if someone takes daily snapshots for a week,
atime precision of a week will only resulting in 1 object being created while
atime precision < 1 day will resulting in 7.).
About Observer nodes:
How is the failover handled? Currently ANN <-> SBN is done by failover
controller racing to write to zookeeper. For the observer node <-> SBN
transition, how is it done?
> Consistent Reads from Standby Node
> ----------------------------------
>
> Key: HDFS-12943
> URL: https://issues.apache.org/jira/browse/HDFS-12943
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: hdfs
> Reporter: Konstantin Shvachko
> Priority: Major
> Attachments: ConsistentReadsFromStandbyNode.pdf,
> ConsistentReadsFromStandbyNode.pdf
>
>
> StandbyNode in HDFS is a replica of the active NameNode. The states of the
> NameNodes are coordinated via the journal. It is natural to consider
> StandbyNode as a read-only replica. As with any replicated distributed system
> the problem of stale reads should be resolved. Our main goal is to provide
> reads from standby in a consistent way in order to enable a wide range of
> existing applications running on top of HDFS.
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