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https://issues.apache.org/jira/browse/HDFS-7575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14277895#comment-14277895
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Daryn Sharp commented on HDFS-7575:
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bq. I think it's frustrating for storage IDs to change without warning just
because HDFS was restarted. It will make diagnosing problems by reading log
files harder because storageIDs might morph at any time. It also sets a bad
precedent of not allowing downgrade and modifying VERSION files "on the fly"
during startup.
I'm confused. StorageIDs aren't going to repeatedly morph - unless there's a
UUID collision that you argue can't happen. The important part is you always
want unique storage ids. It's an internal default of hdfs that is not up to
the user to assign. Succinctly stated, what I'd like is for storage ids to be
generated if missing, re-generated if incorrectly formatted, or if there are
dups. I think the latest patch actually does the first two, just not the dup
check.
bq. I'm surprised to hear you say that rollback should not be an option. It
seems like the conservative thing to do here is to allow the user to restore to
the VERSION file. Obviously we believe there will be no problems. But we always
believe that, or else we wouldn't have made the change. Sometimes there are
problems.
I didn't say that. Rollback is for reverting an incompatible change. Changing
the storage id is not incompatible. Unique ids are the default for newly
formatted nodes. If you think unique storage ids may have subtle bugs
(different than shared storage ids), then new clusters or newly formatted nodes
are buggy.
> 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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