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https://issues.apache.org/jira/browse/HDFS-7575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14286637#comment-14286637
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Colin Patrick McCabe commented on HDFS-7575:
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bq. I disagree. The implementation is a bug – it supposes to change the old ids
(in old id format) to use the new uuid format. The entire heterogeneous storage
design requires storage ID to be unique. Which implementation works correctly
with the duplicate storage IDs?
I don't think it's productive to argue about whether this represents a "true"
layout version change.... whether it is "layout version changey" enough.
Clearly we both agree that doing an LV change here would work and solve the
problem. At the end of the day, we have to make the decision based on which
way is more maintainable.
This does bring up a practical point, though. It will be easier to backport
the "silently modify the VERSION file" patch to 2.6.1 than the LV change. In
view of this, I think it's fine to backport the "silently change VERSION" fix
to 2.6.1. I just don't want to have to support it forever in 3.0 and onward.
bq. For the so called practical points you made (say, "Again, if I accidentally
duplicate a directory on a datanode, ...") , how could updating layout version
help?
If we check for directories with duplicate storage IDs and exclude them, then
the system administrator becomes aware that there is a problem. It "helps" by
not "harming"-- by not changing the VERSION file when we don't know for sure
the reasons why the VERSION file is wrong.
> 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
> Affects Versions: 2.4.0, 2.5.0, 2.6.0
> Reporter: Lars Francke
> Assignee: Arpit Agarwal
> Priority: Critical
> Attachments: HDFS-7575.01.patch, HDFS-7575.02.patch,
> HDFS-7575.03.binary.patch, HDFS-7575.03.patch, HDFS-7575.04.binary.patch,
> HDFS-7575.04.patch, HDFS-7575.05.binary.patch, HDFS-7575.05.patch,
> testUpgrade22via24GeneratesStorageIDs.tgz,
> testUpgradeFrom22GeneratesStorageIDs.tgz,
> testUpgradeFrom24PreservesStorageId.tgz
>
>
> 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());}}):
> {code:title=DatanodeStorageInfo}
> void updateState(StorageReport r) {
> capacity = r.getCapacity();
> dfsUsed = r.getDfsUsed();
> remaining = r.getRemaining();
> blockPoolUsed = r.getBlockPoolUsed();
> }
> {code}
> 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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