[
https://issues.apache.org/jira/browse/HDFS-7575?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14286169#comment-14286169
]
Colin Patrick McCabe commented on HDFS-7575:
--------------------------------------------
This patch does change the layout format. It changes it from one where storage
ID may or may not be unique to one where it definitely is.
Can you response to the practical points I made above? I made a few points
that nobody has responded to yet.
* Changing the storage ID during startup basically changes storage ID from
being a permanent identifier to a temporary one... makes persisting this later
impossible. It commits us to an architecture where block locations can't be
persisted.
* With approach #1, we have to carry the burden of the dedupe code forever.
* Approach #1 degrades error handling. If you somehow end up with two volumes
that map to the same directory, the code silently does the wrong thing.
I would appreciate a response to these. thanks
> 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?
--
This message was sent by Atlassian JIRA
(v6.3.4#6332)
