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https://issues.apache.org/jira/browse/HDFS-14102?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]
BELUGA BEHR updated HDFS-14102:
-------------------------------
Status: Patch Available (was: Open)
Attaching same patch again to see if unit tests pass on second attempt
> verifyBlockPlacement
> --------------------
>
> Key: HDFS-14102
> URL: https://issues.apache.org/jira/browse/HDFS-14102
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: BELUGA BEHR
> Assignee: BELUGA BEHR
> Priority: Minor
> Attachments: HDFS-14102.1.patch, HDFS-14102.2.patch,
> HDFS-14102.3.patch
>
>
>
> {code:java|title=BlockPlacementPolicyDefault.java}
> // 1. Check that all locations are different.
> // 2. Count locations on different racks.
> Set<String> racks = new TreeSet<>();
> for (DatanodeInfo dn : locs)
> racks.add(dn.getNetworkLocation());
> ...
> racks.size(){code}
>
> Here, the code is counting the number of distinct Network Locations.
> However, it is using a TreeSet which has overhead to maintain item order and
> uses a linked structure internally. This overhead is unneeded since all that
> is required here is a count.
> {quote}A NavigableSet implementation based on a TreeMap. The elements are
> ordered using their natural ordering, or by a Comparator provided at set
> creation time, depending on which constructor is used.
> This implementation provides guaranteed log(n) time cost for the basic
> operations (add, remove and contains).
> [https://docs.oracle.com/javase/7/docs/api/java/util/TreeSet.html]
> {quote}
>
> Use Java streams for readability and because it uses a {{HashSet}} under the
> covers to perform the distinct action. {{HashSet}} uses an array internally
> and has constant time performance for the {{add}} method.
> [https://github.com/apache/hadoop/blob/27978bcb66a9130cbf26d37ec454c0b7fcdc2530/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/blockmanagement/BlockPlacementPolicyDefault.java#L1042]
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