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From "Yonger (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-13430) Optimize and fix getFileStatus in S3A
Date Mon, 07 Aug 2017 15:02:00 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-13430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16116700#comment-16116700

Yonger commented on HADOOP-13430:

[~stevel@apache.org] I really can't see any optimization for getFileStatus in ticket [https://issues.apache.org/jira/browse/HADOOP-13208]
, with my understand, there is still exist 2 head +1 list operations in this function. 

And I scan the code where call getFileStatus, and found that in some place, we can know the
path that input is a file or a directory, e.g. in create and open function, we don't need
to check the path is a directory with calling getfileStatus, just consider the path is a file(according
the implementation of Presto), thus when calling getFileStatus, we know the input path is
a file so that it don't need to call getmetadata method again with suffix "/".

Totally, can we reduce the s3 call over network as possible by tell getFileStatus the path
is a file or directory explicitly? 

> Optimize and fix getFileStatus in S3A
> -------------------------------------
>                 Key: HADOOP-13430
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13430
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.8.0
>            Reporter: Steven K. Wong
>            Assignee: Steven K. Wong
>            Priority: Minor
>         Attachments: HADOOP-13430.001.WIP.patch
> 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. The swapped HEAD request will be skipped in practically all cases.
> 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. (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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