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https://issues.apache.org/jira/browse/HDFS-4808?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13654929#comment-13654929
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Suresh Srinivas commented on HDFS-4808:
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bq. 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.)
FileSystem as I said earlier is a both User API and file system implementors
API. Owing it to being user API, a lot of methods that it currently has will
run into the same argument. Are methods such as getDelegationToken,
addDelegationToken, getFileBlockLocation, getBlockSize, getFileChecksum,
checksum verification related methods, getServerDefaults, concat etc. generic?
Perhaps most file systems do snapshots differently. Some do it with a strong
notion of volume level snapshots. They could chose to accordingly
implement/tailor the methods in FileSystem.
bq. If literally all other implementers of FileSystem will throw
UnsupportedOperationException, what's the point of putting it in FileSystem at
all?
The reason is exactly why the other such methods exist. FileSystem is a user
API using which applications access the file system. I intend to write
enhancements (for example another version of distcp) using snapshot
capabilities, especially around snapshot diffs. For this exposing it using
FileSystem APIs is useful.
bq. 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.
I am not sure I agree with this. See my arguments above. Our FileSystem
interface, how much ever we pretend it to be generic is heavily influenced by
HDFS. One thing that could help here is, some of the specification work and
ideas [[email protected]] has about ability to query capabilities etc.
> 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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