sodonnel commented on pull request #2562:
URL: https://github.com/apache/hadoop/pull/2562#issuecomment-748053842
It looks like this same logic also exists in trunk - could you submit a
trunk PR / patch and then we can backport the change across all active branches?
I am also a little confused about this problem. The map
`outOfServiceNodeBlocks` is modified in a few places in the middle of the
Cyclic Iteration. If it threw an ConcurrentModificationException on
modification, then I would expect us to be seeing this a lot. Probably anytime
there is more than 1 node added to decommission / maintenance.
Eg, from trunk DatanodeAdminDefaultMonitor.java, here `it` is a
CyclicIterator over `outOfServiceNodeBlocks`
```
while (it.hasNext() && !exceededNumBlocksPerCheck() && namesystem
.isRunning()) {
numNodesChecked++;
final Map.Entry<DatanodeDescriptor, AbstractList<BlockInfo>>
entry = it.next();
final DatanodeDescriptor dn = entry.getKey();
try {
AbstractList<BlockInfo> blocks = entry.getValue();
boolean fullScan = false;
if (dn.isMaintenance() && dn.maintenanceExpired()) {
// If maintenance expires, stop tracking it.
dnAdmin.stopMaintenance(dn);
toRemove.add(dn);
continue;
}
if (dn.isInMaintenance()) {
// The dn is IN_MAINTENANCE and the maintenance hasn't expired yet.
continue;
}
if (blocks == null) {
// This is a newly added datanode, run through its list to schedule
// under-replicated blocks for replication and collect the blocks
// that are insufficiently replicated for further tracking
LOG.debug("Newly-added node {}, doing full scan to find " +
"insufficiently-replicated blocks.", dn);
blocks = handleInsufficientlyStored(dn);
outOfServiceNodeBlocks.put(dn, blocks); // **** Modifies
outOfServiceNodeBlocks
...
```
Note that outOfServiceNodeBlocks is modified on the first pass, and so
`it.next()` should throw an exception on the next iteration.
Have you seen the ConcurrentModificationException logged due to this problem?
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