Ashutosh Chauhan created HDFS-9763:
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Summary: Add merge api
Key: HDFS-9763
URL: https://issues.apache.org/jira/browse/HDFS-9763
Project: Hadoop HDFS
Issue Type: New Feature
Components: fs
Reporter: Ashutosh Chauhan
It will be good to add merge(Path dir1, Path dir2, ... ) api to HDFS. Semantics
will be to move all files under dir1 to dir2 and doing a rename of files in
case of collisions.
In absence of this api, Hive[1] has to check for collision for each file and
then come up unique name and try again and so on. This is inefficient in
multiple ways:
1) It generates huge number of calls on NN (atleast 2*number of source files in
dir1)
2) It suffers from TOCTOU[2] bug for client picked up name in case of collision.
3) Whole operation is not atomic.
A merge api outlined as above will be immensely useful for Hive and potentially
to other HDFS users.
[1]
https://github.com/apache/hive/blob/release-2.0.0-rc1/ql/src/java/org/apache/hadoop/hive/ql/metadata/Hive.java#L2576
[2]https://en.wikipedia.org/wiki/Time_of_check_to_time_of_use
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