[HDFS-12130] Optimizing permission check for getContentSummary

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 2.9.0, 3.0.0-beta1
• Component/s: namenode
• Target Version/s: 3.0.0-alpha4
• Hadoop Flags: Reviewed
• Assignee: Chen Liang
• Reporter: Chen Liang
• Created: 12/Jul/17 22:12
• Updated: 08/Apr/20 19:30
• Resolved: 14/Jul/17 20:43

Description

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!

Issue Links

• causes: HDFS-15165 In Du missed calling getAttributesProvider