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https://issues.apache.org/jira/browse/HDFS-3370?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13294934#comment-13294934
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Lars Hofhansl commented on HDFS-3370:
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This is a good discussion.
Couple of points:
bq. Or provide use cases which cannot be solved without it.
This seems to be the key question: What services should a file system provide?
The same argument could be made for symbolic links. The application could
implement those (in fact it's quite simple).
bq. but they are very hard to support when the namespace is distributed
But isn't that an implementation detail, which should not inform the feature
set?
Hardlinks could be only supported per distinct namespace (namespace in
federated HDFS or a volume in MapR - I think). This is not unlike Unix where
hardlinks are per distinct filesystem (i.e. not across mount points).
@M.C. Srivas:
If you create 15 backups without hardlinks you get 15 times the metadata *and*
15 times the data... Unless you assume some other feature such as snapshots
with copy-on-write or backup-on-write semantics. (Maybe I did not get the
argument)
Immutable files are very a common and useful design pattern (not just for
HBase) and while not strictly needed, hardlinks are very useful together with
immutable files.
Just my $0.02.
> 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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