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From "Lars Francke (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HDFS-7575) NameNode not handling heartbeats properly after HDFS-2832
Date Tue, 30 Dec 2014 12:16:13 GMT
Lars Francke created HDFS-7575:

             Summary: NameNode not handling heartbeats properly after HDFS-2832
                 Key: HDFS-7575
                 URL: https://issues.apache.org/jira/browse/HDFS-7575
             Project: Hadoop HDFS
          Issue Type: Bug
            Reporter: Lars Francke

Before HDFS-2832 each DataNode would have a unique storageId which included its IP address.
Since HDFS-2832 the DataNodes have a unique storageId per storage directory which is just
a random UUID.

They send reports per storage directory in their heartbeats. This heartbeat is processed on
the NameNode in the {{DatanodeDescriptor#updateHeartbeatState}} method. Pre HDFS-2832 this
would just store the information per Datanode. After the patch though each DataNode can have
multiple different storages so it's stored in a map keyed by the storage Id.

This works fine for all clusters that have been installed post HDFS-2832 as they get a UUID
for their storage Id. So a DN with 8 drives has a map with 8 different keys. On each Heartbeat
the Map is searched and updated ({{DatanodeStorageInfo storage = storageMap.get(s.getStorageID());}}):

  void updateState(StorageReport r) {
    capacity = r.getCapacity();
    dfsUsed = r.getDfsUsed();
    remaining = r.getRemaining();
    blockPoolUsed = r.getBlockPoolUsed();

On clusters that were upgraded from a pre HDFS-2832 version though the storage Id has not
been rewritten (at least not on the four clusters I checked) so each directory will have the
exact same storageId. That means there'll be only a single entry in the {{storageMap}} and
it'll be overwritten by a random {{StorageReport}} from the DataNode. This can be seen in
the {{updateState}} method above. This just assigns the capacity from the received report,
instead it should probably sum it up per received heartbeat.

The Balancer seems to be one of the only things that actually uses this information so it
now considers the utilization of a random drive per DataNode for balancing purposes.

Things get even worse when a drive has been added or replaced as this will now get a new storage
Id so there'll be two entries in the storageMap. As new drives are usually empty it skewes
the balancers decision in a way that this node will never be considered over-utilized.

Another problem is that old StorageReports are never removed from the storageMap. So if I
replace a drive and it gets a new storage Id the old one will still be in place and used for
all calculations by the Balancer until a restart of the NameNode.

I can try providing a patch that does the following:

* Instead of using a Map I could just store the array we receive or instead of storing an
array sum up the values for reports with the same Id
* On each heartbeat clear the map (so we know we have up to date information)

Does that sound sensible?

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