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https://issues.apache.org/jira/browse/HDFS-12943?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16723631#comment-16723631
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Brahma Reddy Battula commented on HDFS-12943:
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Hi [~vagarychen]
{quote}I have never seen the second-call issue. Here is an output from our
cluster (log outpu part omitted), and I think you are right about lowering
dfs.ha.tail-edits.period, we had similar numbers here:
{quote}
you can see this issue if "dfs.ha.tail-edits.period" is default value.
{quote}Curious, how many NN you had in the testing? and was there any error
from NN logs?
{quote}
1 ANN,1 SNN,1 Obserserver. No error logs from NN's.
Hi [~csun]
{quote}I think we should document {{dfs.ha.tail-edits.period}} in the user
guide - the default value is just too large for observer reads. Filed
HDFS-14154.
{quote}
Yes, thanks for reporting the same.
Hi [~xkrogen]
{quote}Your concern about heavier load in the JournalNode would have previously
been valid, but with the completion of HDFS-13150 and
{{dfs.ha.tail-edits.in-progress}} enabled, the Standby/Observer no longer
creates a new stream to tail edits, instead polling for edits via RPC (and thus
making use of connection keepalive). This greatly reduces the overheads
involved with each iteration of edit tailing, enabling it to be done much more
frequently.
{quote}
Yes,this is one of my concern. Gone through *fast path*(HDFS-13150) thanks,it
can improve.
{quote}I'm not aware of any good benchmark numbers produced after finishing the
feature, maybe [~csun] can provide them?
{quote}
[~csun] can you provide..? I am sure this feature going to be great advantage
over rpc workload on ANN, just i want to know write benchmarks also ( as
getHAserviceState() and fast editing tailing edits are intrdouced).Sorry for
pitching very late..
> Consistent Reads from Standby Node
> ----------------------------------
>
> Key: HDFS-12943
> URL: https://issues.apache.org/jira/browse/HDFS-12943
> Project: Hadoop HDFS
> Issue Type: New Feature
> Components: hdfs
> Reporter: Konstantin Shvachko
> Priority: Major
> Attachments: ConsistentReadsFromStandbyNode.pdf,
> ConsistentReadsFromStandbyNode.pdf, HDFS-12943-001.patch,
> HDFS-12943-002.patch, TestPlan-ConsistentReadsFromStandbyNode.pdf
>
>
> StandbyNode in HDFS is a replica of the active NameNode. The states of the
> NameNodes are coordinated via the journal. It is natural to consider
> StandbyNode as a read-only replica. As with any replicated distributed system
> the problem of stale reads should be resolved. Our main goal is to provide
> reads from standby in a consistent way in order to enable a wide range of
> existing applications running on top of HDFS.
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