[jira] [Commented] (HADOOP-13430) Optimize getFileStatus in S3A

["Steve Loughran (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-13430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16236409#comment-16236409 ] 

Steve Loughran commented on HADOOP-13430:
-----------------------------------------

Interesting thought that we could actually override isDirectory(path) and isFile(path) and so actually fail fast if the conditions not met. 

isDirectory: do a LIST path first, then HEAD path +"/". 
isFile(), HEAD path & return true/map FNFE to false without bothering with the two other checks

> Optimize 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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