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https://issues.apache.org/jira/browse/HADOOP-13430?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16153415#comment-16153415
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Steve Loughran commented on HADOOP-13430:
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yes, those existence checks [are really
slow|https://steveloughran.blogspot.co.uk/2016/12/how-long-does-filesystemexists-take.html].
# open can just do a HEAD of the path, only if that's not found can it fall
back to to a full getFileStatus (HADOOP-13712).
# in create(overwrite=true) we don't need to look for the path existing, just
verify that it's not a directory/got children, and that there isn't a parent
file (no need to look for any dir status). See HADOOP-13884 for some discussion
there. Essentially: one HEAD dest + "/", one LIST dest " /", one HEAD
parent(dest) to make sure it's not a file. There's no need to verify the parent
is a dir (HEAD + LIST), because create() does a mkdirs() & will delete any
empty parent dirs after the close() in a single POST. createNonRecursive will
need that check, but as it avoids doing a full recursive "is there a parent
file" check, it's actually more robust than what we have today.
The reason we've avoided the optimisation until now is that we've been looking
@ S3guard for delivering significantly higher performance. We've also gone
through a lot of downstream code and eliminated a lot of the pairs of existence
& action checks (if file.exists() file.delete()), etc, which are doiing a full
needless getFileStatus call. Now we've got a lot of those out the way, time to
look at the core operations.
See also
> 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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