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https://issues.apache.org/jira/browse/HDFS-3979?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13469952#comment-13469952
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Tsz Wo (Nicholas), SZE commented on HDFS-3979:
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{quote}
For applications like HBase we'd like API4 as well as API5.
(API4 allows a hypothetical kill -9 of all DNs without loss of acknowledged
data, API5 allows HW failures of all data nodes - i.e. a DC outage - with loss
of acknowledged data)
{quote}
Why API4 is needed for HBase?
As everyone known, there are usually 3 replicas in HDFS. If only one of the
datanodes is killed, the data is still available in the other two datanodes.
That's why we have invented "hflush" (i.e. API 3) in HDFS-265.
> Fix hsync and hflush semantics.
> -------------------------------
>
> Key: HDFS-3979
> URL: https://issues.apache.org/jira/browse/HDFS-3979
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: data-node, hdfs client
> Affects Versions: 0.22.0, 0.23.0, 2.0.0-alpha
> Reporter: Lars Hofhansl
> Assignee: Lars Hofhansl
> Attachments: hdfs-3979-sketch.txt, hdfs-3979-v2.txt
>
>
> See discussion in HDFS-744. The actual sync/flush operation in BlockReceiver
> is not on a synchronous path from the DFSClient, hence it is possible that a
> DN loses data that it has already acknowledged as persisted to a client.
> Edit: Spelling.
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