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https://issues.apache.org/jira/browse/HDFS-767?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12790477#action_12790477
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Ning Zhang commented on HDFS-767:
---------------------------------
Thanks for the comments Steve and Todd.
I checked the JDK source code (1.6.0_16) and Random() uses a very simple
default seed:
/**
* Creates a new random number generator. This constructor sets
* the seed of the random number generator to a value very likely
* to be distinct from any other invocation of this constructor.
*/
public Random() { this(++seedUniquifier + System.nanoTime()); }
private static volatile long seedUniquifier = 8682522807148012L;
Based on the discussion in Sun's forum:
http://forums.sun.com/thread.jspa?threadID=5398150 , nanoTime is a native
method and is implemented based on the CPU clock cycles. So I guess the chance
of getting the same value from nanoTime is not that high even though all
machines boot up at the same time. I agree that adding the machine's MAC
address would greatly reduce the conflict probability, I am also fine to make
that change in the next version. The change would be fairly simple since JDK1.6
has support get the MAC address.
> Job failure due to BlockMissingException
> ----------------------------------------
>
> Key: HDFS-767
> URL: https://issues.apache.org/jira/browse/HDFS-767
> Project: Hadoop HDFS
> Issue Type: Improvement
> Reporter: Ning Zhang
> Assignee: Ning Zhang
> Attachments: HDFS-767.patch
>
>
> If a block is request by too many mappers/reducers (say, 3000) at the same
> time, a BlockMissingException is thrown because it exceeds the upper limit (I
> think 256 by default) of number of threads accessing the same block at the
> same time. The DFSClient wil catch that exception and retry 3 times after
> waiting for 3 seconds. Since the wait time is a fixed value, a lot of clients
> will retry at about the same time and a large portion of them get another
> failure. After 3 retries, there are about 256*4 = 1024 clients got the block.
> If the number of clients are more than that, the job will fail.
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