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https://issues.apache.org/jira/browse/HDFS-5776?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13877145#comment-13877145
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Liang Xie commented on HDFS-5776:
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[~saint....@gmail.com], [~cmccabe], hedged reads doesn't need to be aware of
weather local read or not.
pread -> fetchBlockByteRange/fetchBlockByteRangeSpeculative ->
actualGetFromOneDataNode -> getBlockReader
we don't need to handle weather local reader or not, the hedged reads only
focus on picking a secondary dn, then wait the winner.
In my test, the region server(rs) instance was in the same box with one dn(say
dn1). Take my first case(dfs.dfsclient.hedged.read.threshold.millis = 500ms)
for example, in that case, in deed, nearly all of the preads go to the local
dn1, i could observe io util% always is 100%(really io bound, aha, btw, i filed
HDFS-5727, hope with that we can alleviate the io somehow, i am planning to dig
that jira after this done, if you guys have any comments, just put there)
during the testing, and dn2/dn3's io util% keep 0%. and in following cases,
i could see the io util% of dn2/dn3 began to increment to around 5%~20%.
> Support 'hedged' reads in DFSClient
> -----------------------------------
>
> Key: HDFS-5776
> URL: https://issues.apache.org/jira/browse/HDFS-5776
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs-client
> Affects Versions: 3.0.0
> Reporter: Liang Xie
> Assignee: Liang Xie
> Attachments: HDFS-5776-v2.txt, HDFS-5776-v3.txt, HDFS-5776.txt
>
>
> This is a placeholder of hdfs related stuff backport from
> https://issues.apache.org/jira/browse/HBASE-7509
> The quorum read ability should be helpful especially to optimize read outliers
> we can utilize "dfs.dfsclient.quorum.read.threshold.millis" &
> "dfs.dfsclient.quorum.read.threadpool.size" to enable/disable the hedged read
> ability from client side(e.g. HBase), and by using DFSQuorumReadMetrics, we
> could export the interested metric valus into client system(e.g. HBase's
> regionserver metric).
> The core logic is in pread code path, we decide to goto the original
> fetchBlockByteRange or the new introduced fetchBlockByteRangeSpeculative per
> the above config items.
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