Yiqun Lin created HDFS-10716:
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Summary: The target node should be removed in balancer when the
scheduled size of bytes that firstly reduced to less than 0
Key: HDFS-10716
URL: https://issues.apache.org/jira/browse/HDFS-10716
Project: Hadoop HDFS
Issue Type: Improvement
Components: balancer & mover
Reporter: Yiqun Lin
Assignee: Yiqun Lin
In HDFS-10602, we found a failing case that the balancer moves data always
between 2 DNs. And it made the balancer can't be finished. I debug the code for
this, I found there seems a bug in choosing pending blocks in
{{Dispatcher.Source.chooseNextMove}}.
The codes:
{code}
private PendingMove chooseNextMove() {
for (Iterator<Task> i = tasks.iterator(); i.hasNext();) {
final Task task = i.next();
final DDatanode target = task.target.getDDatanode();
final PendingMove pendingBlock = new PendingMove(this, task.target);
if (target.addPendingBlock(pendingBlock)) {
// target is not busy, so do a tentative block allocation
if (pendingBlock.chooseBlockAndProxy()) {
long blockSize = pendingBlock.reportedBlock.getNumBytes(this);
incScheduledSize(-blockSize);
task.size -= blockSize;
// If the size of bytes that need to be moved was first reduced to
less than 0
// it should also be removed.
if (task.size == 0) {
i.remove();
}
return pendingBlock;
//...
{code}
The value of task.size was assigned in {{Balancer#matchSourceWithTargetToMove}}
{code}
long size = Math.min(source.availableSizeToMove(),
target.availableSizeToMove());
final Task task = new Task(target, size);
{code}
This value was depended on the source and target node, and this value will not
always can be reduced to 0 in choosing pending blocks. And then, it will still
move the data to the target node even if the size of bytes that needed to move
has been already reduced less than 0. And finally it will make the data
imbalance again in cluster.
We can opitimize for this as this title mentioned, I think this can speed the
balancer.
Can see the failling logs for this.
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