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From Stack <st...@duboce.net>
Subject Re: Does Hedged-read usually happen in real workloads?
Date Mon, 19 Aug 2019 20:13:06 GMT
On Mon, Aug 19, 2019 at 5:30 AM Daegyu Han <hdg9400@gmail.com> wrote:

> Hi all,
>
> To my understand, Hedged-Read is a technique for copying IO to a replica
> when pread () is delayed to mitigate IO latency such as speculative
> execution in MapReduce.
>
>
The release note on the feature has a pretty good explanation
https://issues.apache.org/jira/browse/HDFS-5776


> - Does Hedged-read really happen where it uses hbase databases?
> - Does hedged-read often occur because of a bottleneck in HDFS or a
> bottleneck in the storage itself?
> - What is the best number to set the timeout and thread for Hedged-Read?
>
>
On the above, yes, it happens; on #2, no good answer (varies); and on #3,
start with defaults and monitor the associated metrics (mentioned in the
above cited release notes).

Thanks,
S



> Thank you
> ᐧ
>

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