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From "Aaron T. Myers (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-4808) Limit snapshot related methods to DistributedFileSystem
Date Thu, 09 May 2013 01:40:13 GMT

    [ https://issues.apache.org/jira/browse/HDFS-4808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13652647#comment-13652647
] 

Aaron T. Myers commented on HDFS-4808:
--------------------------------------

bq. Why cannot this be a general FileSystem functionality? I think this is general enough.
Currently looking at the snapshot related methods added, I do not see any HDFS specific semantics.

Do you know of any existing or future FileSystem implementations (LocalFileSystem, S3FileSystem,
etc.) which will support snapshots? If they do ever support snapshots, will they support sub-tree
snapshots? (Sub-tree snapshots is not something I'm aware of any other file system supporting.)

>From what I know of other file systems which support some form of snapshots (WAFL, ZFS,
etc.), their semantics, conceptual nouns, etc. all differ so substantially from one another
that I don't think it would be easy (possible?) to come up with a single general-purpose snapshot
API to cover all of them. All other file systems I'm aware of which support snapshots provide
their own file system-specific APIs for creating, deleting, managing their snapshots. This
suggests to me that HDFS should expose a file system-specific API as well.

bq. FileSystem currently is both the user API and file system implementors API. I think file
systems that can support snapshots can implement this feature and file systems that do not
can throw unsupported exception.

If literally all other implementers of FileSystem will throw UnsupportedOperationException,
what's the point of putting it in FileSystem at all?

How about this for a way forward: For now we move the snapshot create/delete APIs to an HDFS-specific
class. If/when another FileSystem implementation shows up which supports a similar API to
the HDFS snapshots API, we can move it to FileSystem at that point.
                
> Limit snapshot related methods to DistributedFileSystem
> -------------------------------------------------------
>
>                 Key: HDFS-4808
>                 URL: https://issues.apache.org/jira/browse/HDFS-4808
>             Project: Hadoop HDFS
>          Issue Type: Bug
>            Reporter: Suresh Srinivas
>
> HDFS-2802 introduces snapshots related methods into FileSystem. This jira is for discussing
whether these snapshots related methods are needed in FileSystem or should be limited to only
DistributedFileSystem.

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