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From Daryn Sharp <da...@yahoo-inc.com>
Subject Re: relative symbolic links in HDFS
Date Mon, 31 Oct 2011 14:46:08 GMT
It's generally been a problem that filesystem operations mangle paths to be something other
than what the user provided.  FsShell has to go to some (unnecessary, imho) lengths to independently
track the user's given path so the output paths will match what the user provided.  Not displaying
the user-given path makes it difficult/impossible for scripts to accurately parse the output
for the results of an operation on the given paths.

I like getLinkTarget returning the exact target, but I'd also like a FileStatus to return
the given path both in the case of a normal path and a symlink.  If the user needs a fully
qualified path for an operation, my opinion is they should request it?

Daryn


On Oct 29, 2011, at 9:02 PM, Eli Collins wrote:

> Hey Chuck,
> 
> Why is it problematic for your use that the symlink is stored in
> FileStatus fully qualified - you'd like FileContext#getSymlink to
> return the same Path that you used as the target in createSymlink?
> 
> The current behavior is so getFileLinkStatus is consistent with
> getFileStatus(new Path("/some/file")) which returns a fully qualified
> path (eg hdfs://myhost:123/some/file).   Note that you can use
> FileContext#getLinkTarget to return the path used when creating the
> link. Some more background is in the design doc:
> https://issues.apache.org/jira/secure/attachment/12434745/design-doc-v4.txt
> 
> There's a jira for porting FsShell to FileContext (HADOOP-6424), if
> you have a patch (even partial) feel free to post it to the jira.
> Note that since symlinks are not implemented in FileSystem, clients
> that use FileSystem to access paths with symlinks will fail.
> 
> Btw when looking at the code you pointed out I noticed a bug in link
> resolution (HADOOP-7783), thanks!
> 
> Thanks,
> Eli
> 
> 
> On Fri, Oct 28, 2011 at 9:46 AM, Charles Baker <cbaker@sdl.com> wrote:
>> Hey guys. We are in the early stages of planning and evaluating a hadoop
>> 'cold-storage' cluster for medium to long term storage of mixed data (small
>> to large files, zips, tar, etc...) and tons of symlinks. We do realize that
>> small files aren't ideal in HDFS but it's for long-term storage and beats the
>> cost of more NetApps by potentially several hundred thousand dollars by
>> leveraging existing equipment. We are already successfully using Hadoop and
>> the MapReduce framework in a different project and have developed quite a bit
>> of in-house expertise when it comes to Hadoop.
>> 
>> 
>> 
>> Since this use-case is preserving and restoring an arbitrary directory
>> structure, I have been evaluating 0.21.0's support of symlinks and found that
>> although it happily creates relative symlinks, the code that is called to
>> retrieve the symlink 'FileContext.getFileLinkStatus()' always converts the
>> relative Path object to an absolute one through the use of the
>> qualifySymlinkTarget() method. Though I was easily able to work around this
>> limitation by changing the one line of code that calls this function from:
>> 
>> 
>> 
>> fi.setSymlink(qualifySymlinkTarget(fs, p, fi.getSymlink()));
>> 
>> 
>> 
>> to:
>> 
>> 
>> 
>> fi.setSymlink(fi.getSymlink());
>> 
>> 
>> 
>> It has made us curious as to why the decision was made to always return the
>> absolute path of a symlink in the first place. Is it that attempts to open
>> targets to relative symlinks throw exceptions and it saves having the user do
>> the work to construct the absolute path since that's the general use-case? Or
>> does this workaround violate some internal assumptions of the code or ideas
>> about how a URI should behave (even though relative paths are implicitly
>> supported by URI object)? Any insight you guys can shed on this would be
>> great. I've tested the above change by adding support for symlinks (into and
>> out of HDFS) into FsShell.copyToLocal() and copyFromLocal() using a mixed bag
>> of relative and absolute symlinks and symlinks->symlinks and have so far
>> found no ill effects.
>> 
>> 
>> 
>> Thanks!
>> 
>> 
>> 
>> -Chuck
>> 
>> 
>> 
>> </pre>
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