[jira] [Created] (HDFS-14476) lock too long when fix inconsistent blocks between disk and in-memory

["Sean Chow (JIRA)" <ji...@apache.org>]

Sean Chow created HDFS-14476:
--------------------------------

             Summary: lock too long when fix inconsistent blocks between disk and in-memory
                 Key: HDFS-14476
                 URL: https://issues.apache.org/jira/browse/HDFS-14476
             Project: Hadoop HDFS
          Issue Type: Improvement
          Components: datanode
    Affects Versions: 2.7.0, 2.6.0
            Reporter: Sean Chow


When directoryScanner have the results of differences between disk and in-memory blocks. it will try to run `checkAndUpdate` to fix it. However `FsDatasetImpl.checkAndUpdate` is a synchronized call

As I have about 6millions blocks for every datanodes and every 6hours' scan will have about 25000 abnormal blocks to fix. That leads to a long lock holding FsDatasetImpl object.

let's assume every block need 10ms to fix(because of latency of SAS disk), that will cost 250 seconds to finish. That means all reads and writes will be blocked for 3mins for that datanode.

 

```

2019-05-06 08:06:51,704 INFO org.apache.hadoop.hdfs.server.datanode.DirectoryScanner: BlockPool BP-1644920766-10.223.143.220-1450099987967 Total blocks: 6850197, missing metadata files:23574, missing block files:23574, missing blocks in memory:47625, mismatched blocks:0

...

2019-05-06 08:16:41,625 INFO org.apache.hadoop.hdfs.server.datanode.DataNode: Took 588402ms to process 1 commands from NN

```

Take long time to process command from nn because threads are blocked. And namenode will see long lastContact time for this datanode.

Maybe this affect all hdfs versions.

to fix:

just like process invalidate command from namenode with 1000 batch size, fix these abnormal block should be handled with batch too and sleep 2 seconds between the batch to allow normal reading/writing blocks.



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