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From "Steven K. Wong (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HADOOP-13430) Optimize and fix getFileStatus in S3A
Date Tue, 26 Jul 2016 21:35:20 GMT
Steven K. Wong created HADOOP-13430:

             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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