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https://issues.apache.org/jira/browse/HADOOP-11708?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14603374#comment-14603374
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Sean Busbey commented on HADOOP-11708:
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This was the ticket that was left open for the thoughtful reworking of
synchronization handling and documentation of thread safety promises for the
input and output streams.
If Hadoop is no longer interested in making those updates or if you'd prefer to
have them handled in a different jira, then there's nothing left to do on this
ticket. But that's why HADOOP-11710 was originally made a subtask rather than
just done here.
> CryptoOutputStream synchronization differences from DFSOutputStream break
> HBase
> -------------------------------------------------------------------------------
>
> Key: HADOOP-11708
> URL: https://issues.apache.org/jira/browse/HADOOP-11708
> Project: Hadoop Common
> Issue Type: Bug
> Components: fs
> Affects Versions: 2.6.0
> Reporter: Sean Busbey
> Assignee: Sean Busbey
> Priority: Critical
>
> For the write-ahead-log, HBase writes to DFS from a single thread and sends
> sync/flush/hflush from a configurable number of other threads (default 5).
> FSDataOutputStream does not document anything about being thread safe, and it
> is not thread safe for concurrent writes.
> However, DFSOutputStream is thread safe for concurrent writes + syncs. When
> it is the stream FSDataOutputStream wraps, the combination is threadsafe for
> 1 writer and multiple syncs (the exact behavior HBase relies on).
> When HDFS Transparent Encryption is turned on, CryptoOutputStream is inserted
> between FSDataOutputStream and DFSOutputStream. It is proactively labeled as
> not thread safe, and this composition is not thread safe for any operations.
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