Chen Liang created HDFS-12130:
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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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