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https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13397700#comment-13397700
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Konstantin Shvachko commented on HDFS-3370:
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Sanjay, you are taking a quote out of context. It has been explained what
"hard" means above. Please scan through. One more example:
Well understood why traditional hard links are not allowed across volumes. A
distributed namespace is like dynamically changing volumes. You can restrict a
link to a single volume, but the names can flow to different volumes later on.
I am not proposing to remove the existing complexity from the system, I propose
not to introduce more of it. In distributed case consistent hard links need
PAXOS-like algorithms. They are not "elementary operations", which only should
compose the API.
Hard links can be implemented as a library using ZK, which will stand in
distributed case.
A couple of quotes from Snajay's (mine too) favorite author:
- When in doubt, leave it out. If there is a fundamental theorem of API design,
this is it. You can always add things later, but you can't take them away.
- APIs must coexist peacefully with the platform, so do what is customary. It
is almost always wrong to transliterate an API from one platform to another.
- Consider the performance consequences of API design decisions ...
> HDFS hardlink
> -------------
>
> Key: HDFS-3370
> URL: https://issues.apache.org/jira/browse/HDFS-3370
> Project: Hadoop HDFS
> Issue Type: New Feature
> Reporter: Hairong Kuang
> Assignee: Liyin Tang
> Attachments: HDFS-HardLink.pdf
>
>
> We'd like to add a new feature hardlink to HDFS that allows harlinked files
> to share data without copying. Currently we will support hardlinking only
> closed files, but it could be extended to unclosed files as well.
> Among many potential use cases of the feature, the following two are
> primarily used in facebook:
> 1. This provides a lightweight way for applications like hbase to create a
> snapshot;
> 2. This also allows an application like Hive to move a table to a different
> directory without breaking current running hive queries.
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