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https://issues.apache.org/jira/browse/HDFS-16456?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17514492#comment-17514492
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caozhiqiang commented on HDFS-16456:
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[~tasanuma] I have modify this patch in [^HDFS-16456.010.patch], please review.
For question 4, in below scenario will get an error result:
# Decommission a datanode, which is the only one node in its rack. and the
numOfEmptyRacks will +1.
# Stop this datanode, and the numOfEmptyRacks will -1 because this rack will
also be removed from emptyRackMap.
# Start this datanode, this rack and this node will be both added to
emptyRackMap but decommissionNode() will not be called again. The
numOfEmptyRacks will not change. Error is occured, because this node is also
decommissioned and its rack should be considered empty and numOfEmptyRacks
should +1.
So I use decommissionNodes to check if a new added node is a decommissioned one.
> EC: Decommission a rack with only on dn will fail when the rack number is
> equal with replication
> ------------------------------------------------------------------------------------------------
>
> Key: HDFS-16456
> URL: https://issues.apache.org/jira/browse/HDFS-16456
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: ec, namenode
> Affects Versions: 3.4.0
> Reporter: caozhiqiang
> Priority: Critical
> Attachments: HDFS-16456.001.patch, HDFS-16456.002.patch,
> HDFS-16456.003.patch, HDFS-16456.004.patch, HDFS-16456.005.patch,
> HDFS-16456.006.patch, HDFS-16456.007.patch, HDFS-16456.008.patch,
> HDFS-16456.009.patch, HDFS-16456.010.patch
>
>
> In below scenario, decommission will fail by TOO_MANY_NODES_ON_RACK reason:
> # Enable EC policy, such as RS-6-3-1024k.
> # The rack number in this cluster is equal with or less than the replication
> number(9)
> # A rack only has one DN, and decommission this DN.
> The root cause is in
> BlockPlacementPolicyRackFaultTolerant::getMaxNodesPerRack() function, it will
> give a limit parameter maxNodesPerRack for choose targets. In this scenario,
> the maxNodesPerRack is 1, which means each rack can only be chosen one
> datanode.
> {code:java}
> protected int[] getMaxNodesPerRack(int numOfChosen, int numOfReplicas) {
> ...
> // If more replicas than racks, evenly spread the replicas.
> // This calculation rounds up.
> int maxNodesPerRack = (totalNumOfReplicas - 1) / numOfRacks + 1;
> return new int[] {numOfReplicas, maxNodesPerRack};
> } {code}
> int maxNodesPerRack = (totalNumOfReplicas - 1) / numOfRacks + 1;
> here will be called, where totalNumOfReplicas=9 and numOfRacks=9
> When we decommission one dn which is only one node in its rack, the
> chooseOnce() in BlockPlacementPolicyRackFaultTolerant::chooseTargetInOrder()
> will throw NotEnoughReplicasException, but the exception will not be caught
> and fail to fallback to chooseEvenlyFromRemainingRacks() function.
> When decommission, after choose targets, verifyBlockPlacement() function will
> return the total rack number contains the invalid rack, and
> BlockPlacementStatusDefault::isPlacementPolicySatisfied() will return false
> and it will also cause decommission fail.
> {code:java}
> public BlockPlacementStatus verifyBlockPlacement(DatanodeInfo[] locs,
> int numberOfReplicas) {
> if (locs == null)
> locs = DatanodeDescriptor.EMPTY_ARRAY;
> if (!clusterMap.hasClusterEverBeenMultiRack()) {
> // only one rack
> return new BlockPlacementStatusDefault(1, 1, 1);
> }
> // Count locations on different racks.
> Set<String> racks = new HashSet<>();
> for (DatanodeInfo dn : locs) {
> racks.add(dn.getNetworkLocation());
> }
> return new BlockPlacementStatusDefault(racks.size(), numberOfReplicas,
> clusterMap.getNumOfRacks());
> } {code}
> {code:java}
> public boolean isPlacementPolicySatisfied() {
> return requiredRacks <= currentRacks || currentRacks >= totalRacks;
> }{code}
> According to the above description, we should make the below modify to fix it:
> # In startDecommission() or stopDecommission(), we should also change the
> numOfRacks in class NetworkTopology. Or choose targets may fail for the
> maxNodesPerRack is too small. And even choose targets success,
> isPlacementPolicySatisfied will also return false cause decommission fail.
> # In BlockPlacementPolicyRackFaultTolerant::chooseTargetInOrder(), the first
> chooseOnce() function should also be put in try..catch..., or it will not
> fallback to call chooseEvenlyFromRemainingRacks() when throw exception.
> # In verifyBlockPlacement, we need to remove invalid racks from total
> numOfRacks, or isPlacementPolicySatisfied() will return false and cause fail
> to reconstruct data.
>
>
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