Steven K. Wong created HADOOP-13430:
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Summary: Optimize and fix getFileStatus in S3A
Key: HADOOP-13430
URL: https://issues.apache.org/jira/browse/HADOOP-13430
Project: Hadoop Common
Issue Type: Improvement
Components: fs/s3
Affects Versions: 2.8.0
Reporter: Steven K. Wong
Priority: Minor
Currently, S3AFileSystem.getFileStatus(Path f) sends up to 3 requests to S3
when pathToKey(f) = key = "foo/bar" is a directory:
1. HEAD key=foo/bar \[continue if not found]
2. HEAD key=foo/bar/ \[continue if not found]
3. LIST prefix=foo/bar/ delimiter=/ max-keys=1
My experience (and generally true, I reckon) is that almost all directories are
nonempty directories without a "fake directory" file (e.g. "foo/bar/"). Under
this condition, request #2 is mostly unhelpful; it only slows down
getFileStatus. Therefore, I propose swapping the order of requests #2 and #3.
Furthermore, when key = "foo/bar" is a nonempty directory that contains a "fake
directory" file (in addition to actual files), getFileStatus currently returns
an S3AFileStatus with isEmptyDirectory=true, which is wrong. Swapping will fix
this. The swapped LIST request will use max-keys=2 to determine
isEmptyDirectory correctly. The swapped HEAD request will be skipped if the
directory is empty. (Removing the delimiter from the LIST request should make
the logic a little simpler than otherwise.)
Note that key = "foo/bar/" has the same problem with isEmptyDirectory. To fix
it, I propose skipping request #1 when key ends with "/". The price is this
will, for an empty directory, replace a HEAD request with a LIST request that's
generally more taxing on S3.
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