Daisuke Kobayashi created HDFS-14315:
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             Summary: Add more detailed log message when decreasing replication 
factor < max in snapshots
                 Key: HDFS-14315
                 URL: https://issues.apache.org/jira/browse/HDFS-14315
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: hdfs
    Affects Versions: 3.1.2
            Reporter: Daisuke Kobayashi


When changing replication factor for a given file, the following 3 types of 
logging appear in the namenode log, but more detailed message would be useful 
especially when the file is in snapshot(s).
{noformat}
Decreasing replication from X to Y for FILE
Increasing replication from X to Y for FILE
Replication remains unchanged at X for FILE
{noformat}
I have added additional log messages as well as further test scenarios to 
org.apache.hadoop.hdfs.server.namenode.snapshot.TestSnapshotReplication#testReplicationWithSnapshot.

The test sequence is:

1) A file is created with replication factor 1 (there are 5 datanodes)

2) Take a snapshot and increase the factor by 1. Continue this loop up to 5.

3) Setrep back to 3, but the target replication is decided to 4, which is the 
maximum in snapshots.
{noformat}
2019-02-25 17:17:26,800 [IPC Server handler 9 on default port 55726] INFO  
namenode.FSDirectory (FSDirAttrOp.java:unprotectedSetReplication(408)) - 
Decreasing replication from 5 to 4 for /TestSnapshot/sub1/file1. Requested 
value is 3, but 4 is the maximum in snapshots
{noformat}
4) Setrep to 3 again, but it's unchanged as follows.
{noformat}
2019-02-25 17:17:26,804 [IPC Server handler 6 on default port 55726] INFO  
namenode.FSDirectory (FSDirAttrOp.java:unprotectedSetReplication(420)) - 
Replication remains unchanged at 4 for /TestSnapshot/sub1/file1 . Requested 
value is 3, but 4 is the maximum in snapshots.
{noformat}
5) Make sure the number of replicas in datanodes remains 4.



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