Karthik Kambatla commented on YARN-2942:

(Canceled the patch to stop Jenkins from evaluating the design doc :) ) 

[~rkanter] - thanks for updating the design doc. A couple of comments:
# If there is an NM X actively concatenating its logs and NM Y can't acquire 
the lock, what happens? 
## Does it do a blocking-wait? If yes, this should likely be in a separate 
## I would like for it to be non-blocking. How about a LogConcatenationService 
in the NM? This service is brought up if you enable log concatenation. This 
service would periodically go through all of its past aggregated logs and 
concatenate those that it can acquire a lock for. Delayed concatenation should 
be okay because we are doing this primarily to handle the problem HDFS has with 
small files. Also, this way, we don't have do anything different for NM 
restart. Forward looking, this concat service could potentially take input on 
how busy HDFS is. 
# I didn't completely understand the point about a config to specify the 
format. Are you suggesting we have two different on/off configs - one to turn 
on concatenation and one to specify the format JHS should be reading. I think 
just one config that clearly states that the turning on this on an NM (writer) 
requires the JHS (reader) already has this enabled. In case of rolling 
upgrades, this translates to requiring a JHS upgrade prior to NM upgrade.  

> Aggregated Log Files should be combined
> ---------------------------------------
>                 Key: YARN-2942
>                 URL: https://issues.apache.org/jira/browse/YARN-2942
>             Project: Hadoop YARN
>          Issue Type: New Feature
>    Affects Versions: 2.6.0
>            Reporter: Robert Kanter
>            Assignee: Robert Kanter
>         Attachments: CombinedAggregatedLogsProposal_v3.pdf, 
> CompactedAggregatedLogsProposal_v1.pdf, 
> CompactedAggregatedLogsProposal_v2.pdf, 
> ConcatableAggregatedLogsProposal_v4.pdf, YARN-2942-preliminary.001.patch, 
> YARN-2942-preliminary.002.patch, YARN-2942.001.patch, YARN-2942.002.patch, 
> YARN-2942.003.patch
> Turning on log aggregation allows users to easily store container logs in 
> HDFS and subsequently view them in the YARN web UIs from a central place.  
> Currently, there is a separate log file for each Node Manager.  This can be a 
> problem for HDFS if you have a cluster with many nodes as you’ll slowly start 
> accumulating many (possibly small) files per YARN application.  The current 
> “solution” for this problem is to configure YARN (actually the JHS) to 
> automatically delete these files after some amount of time.  
> We should improve this by compacting the per-node aggregated log files into 
> one log file per application.

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