ZanderXu created HDFS-17480:
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Summary: [FGL] GetListing RPC supports fine-grained locking
Key: HDFS-17480
URL: https://issues.apache.org/jira/browse/HDFS-17480
Project: Hadoop HDFS
Issue Type: Sub-task
Reporter: ZanderXu
Assignee: ZanderXu
GetListing is a very common used RPC by end-users. But we should consider how
does GetListing support FGL.
For example, there is directory /a/b/c contains some children, such as d1, d2,
d3, f1, f2, f3.
Normally, we should hold the write lock iNode c for listing /a/b/c to make sure
that there is no other threads are updating children of iNode c. But if the
listing path is /, the entire directory tree will be locked, which will have a
great impact.
There are two solutions to fix this problem:
Solution 1:
* Hold the read lock of iNode c
* Loop through all children
** Hold the read lock of each child and return it's file status
The result may contains some stale file status, because the looped children may
be updated by other thread before the result of getListing is returned to
client.
Solution 2:
* Hold the write lock of parent and current Node when updating the current node
** Holding the write lock of iNode c and d1 when updating d1
* Hold the read lock of iNode c
* Loop through all children
This solution will increases the scope of lock, since the parent's write lock
is usually not required.
I prefer the first solution, since namenode always returns results in batches.
Changes may have occurred between batch and batch.
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