[jira] [Commented] (HADOOP-13208) S3A listFiles(recursive=true) to do a bulk listObjects instead of walking the pseudo-tree of directories

["Chris Nauroth (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HADOOP-13208?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15402954#comment-15402954 ] 

Chris Nauroth commented on HADOOP-13208:
----------------------------------------

{quote}
What is this for? I see it was introduced in HADOOP-13131, but don't see
any usage of the configuration flag it sets (fs.s3a.impl.disable.cache).
{quote}

[~fabbri], sorry, I missed this comment last time around.

This logic disables the {{FileSystem}} cache.  The logic for this is inside the {{FileSystem}} base class.  (See the static {{FileSystem#get(URI, Configuration)}} method.)  Whether or not the cache is in effect is determined by a dynamic configuration property: {{fs.<scheme>.impl.disable.cache}}.

In some cases, we might have a test suite with multiple test methods that all want to test slightly different configuration.  However, all tests in a suite execute in the same process, so there would be a risk of reusing a cached instance across multiple tests.  Unfortunately, the caching is not sensitive to changes in the {{Configuration}} instance.  (The cache key is the scheme, authority, and {{UserGroupInformation}}.)  To work around that, we disable the cache for these tests.

> S3A listFiles(recursive=true) to do a bulk listObjects instead of walking the pseudo-tree of directories
> --------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-13208
>                 URL: https://issues.apache.org/jira/browse/HADOOP-13208
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: fs/s3
>    Affects Versions: 2.8.0
>            Reporter: Steve Loughran
>            Assignee: Steve Loughran
>            Priority: Minor
>         Attachments: HADOOP-13208-branch-2-001.patch, HADOOP-13208-branch-2-007.patch, HADOOP-13208-branch-2-008.patch, HADOOP-13208-branch-2-009.patch, HADOOP-13208-branch-2-010.patch, HADOOP-13208-branch-2-011.patch, HADOOP-13208-branch-2-012.patch, HADOOP-13208-branch-2-017.patch, HADOOP-13208-branch-2-018.patch
>
>   Original Estimate: 24h
>  Remaining Estimate: 24h
>
> A major cost in split calculation against object stores turns out be listing the directory tree itself. That's because against S3, it takes S3A two HEADs and two lists to list the content of any directory path (2 HEADs + 1 list for getFileStatus(); the next list to query the contents).
> Listing a directory could be improved slightly by combining the final two listings. However, a listing of a directory tree will still be O(directories). In contrast, a recursive {{listFiles()}} operation should be implementable by a bulk listing of all descendant paths; one List operation per thousand descendants. 
> As the result of this call is an iterator, the ongoing listing can be implemented within the iterator itself



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