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From "Chen Liang (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-12130) Optimizing permission check for getContentSummary
Date Wed, 12 Jul 2017 22:13:00 GMT
Chen Liang created HDFS-12130:

             Summary: Optimizing permission check for getContentSummary
                 Key: HDFS-12130
                 URL: https://issues.apache.org/jira/browse/HDFS-12130
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: namenode
            Reporter: Chen Liang
            Assignee: Chen Liang

Currently, {{getContentSummary}} takes two phases to complete:
- phase1. check the permission of the entire subtree. If any subdirectory does not have {{READ_EXECUTE}},
an access control exception is thrown and {{getContentSummary}} terminates here (unless it's
super user).
- phase2. If phase1 passed, it will then traverse the entire tree recursively to get the actual
content summary.

An issue is, both phases currently hold the fs lock.

Phase 2 has already been written that, it will yield the fs lock over time, such that it does
not block other operations for too long. However phase 1 does not yield. Meaning it's possible
that the permission check phase still blocks things for long time.

One fix is to add lock yield to phase 1. But a simpler fix is to merge phase 1 into phase
2. Namely, instead of doing a full traversal for permission check first, we start with phase
2 directly, but for each directory, before obtaining its summary, check its permission first.
This way we take advantage of existing lock yield in phase 2 code and still able to check
permission and terminate on access exception.

Thanks [~szetszwo] for the offline discussions!

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