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https://issues.apache.org/jira/browse/HDFS-13880?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16609387#comment-16609387
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Erik Krogen commented on HDFS-13880:
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[~vagarychen] -
{quote}
The original thought was that, getContentSummary already yields locks so could
already be returning results based on outdated info. Making it coordinated
seems to have little gain.
{quote}
Yielding the locks results in a different type of inconsistency than lacking
the use of the {{isCoordinated}} flag. Consider the following scenario:
* My home directory {{/user/xkrogen}} has a quota of 100GB and is currently
empty.
* I move some 100GB file, {{/tmp/largeFile.avro}}, into my home directory.
* I check my quota via {{getContentSummary}, it says I have 100GB remaining
quota (because it is not coordinated and reads stale data)
* I try to move another file into my home directory and it fails because I've
exceeded quota
* I check my quota again via {{getContentSummary}} and it still says I have
100GB remaining
While the lock yield means you could have misleading results for _concurrent_
modifications, the lack of {{isCoordinated}} means you can miss results for
actions that occurred strictly before the content summary, losing RYOW
semantics. I think this is much more confusing for a user.
Also, just for the sake of uniformity, it seems odd to leave as uncoordinated
this one operation which does actually read from the global state - other
uncoordinated ops deal with node-local state.
> Add mechanism to allow certain RPC calls to bypass sync
> -------------------------------------------------------
>
> Key: HDFS-13880
> URL: https://issues.apache.org/jira/browse/HDFS-13880
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: namenode
> Reporter: Chen Liang
> Assignee: Chen Liang
> Priority: Major
> Attachments: HDFS-13880-HDFS-12943.001.patch,
> HDFS-13880-HDFS-12943.002.patch, HDFS-13880-HDFS-12943.003.patch,
> HDFS-13880-HDFS-12943.004.patch
>
>
> Currently, every single call to NameNode will be synced, in the sense that
> NameNode will not process it until state id catches up. But in certain cases,
> we would like to bypass this check and allow the call to return immediately,
> even when the server id is not up to date. One case could be the to-be-added
> new API in HDFS-13749 that request for current state id. Others may include
> calls that do not promise real time responses such as {{getContentSummary}}.
> This Jira is to add the mechanism to allow certain calls to bypass sync.
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