sodonnel commented on pull request #3928:
URL: https://github.com/apache/hadoop/pull/3928#issuecomment-1023076178
This is the scan logic:
```
for (DatanodeStorageInfo s : storage) {
namesystem.readLock();
try {
// As the lock is dropped and re-taken between each storage, we need
// to check the storage is still present before processing it, as it
// may have been removed.
if (dn.getStorageInfo(s.getStorageID()) == null) {
continue;
}
Iterator<BlockInfo> it = s.getBlockIterator();
while (it.hasNext()) {
BlockInfo b = it.next();
if (!initialScan || dn.isEnteringMaintenance()) {
// this is a rescan, so most blocks should be replicated now,
// or this node is going into maintenance. On a healthy
// cluster using racks or upgrade domain, a node should be
// able to go into maintenance without replicating many blocks
// so we will check them immediately.
if (!isBlockReplicatedOk(dn, b, false, null)) {
blockList.put(b, null);
}
} else {
blockList.put(b, null);
}
numBlocksChecked++;
}
} finally {
namesystem.readUnlock();
}
}
```
The rescan would be more expensive as it needs to check if each block is
correctly replicated, and also if you are putting a node to maintenance. The
rescan happens at the end. The initial scan basically puts all the entries into
a list, so it should already be lightweight.
Do you know if the pause is happening at the end of decommission during the
re-scan? If so, we should probably refactor it to not do the
`isBlockReplicatedOk()` check under the lock.
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