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From "Ashish Singhi (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-16499) slow replication for small HBase clusters
Date Thu, 01 Dec 2016 07:38:58 GMT

    [ https://issues.apache.org/jira/browse/HBASE-16499?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15711200#comment-15711200

Ashish Singhi commented on HBASE-16499:

Any update here, [~vik.karma] ?

> slow replication for small HBase clusters
> -----------------------------------------
>                 Key: HBASE-16499
>                 URL: https://issues.apache.org/jira/browse/HBASE-16499
>             Project: HBase
>          Issue Type: Bug
>          Components: Replication
>            Reporter: Vikas Vishwakarma
>            Assignee: Vikas Vishwakarma
> For small clusters 10-20 nodes we recently observed that replication is progressing very
slowly when we do bulk writes and there is lot of lag accumulation on AgeOfLastShipped / SizeOfLogQueue.
From the logs we observed that the number of threads used for shipping wal edits in parallel
comes from the following equation in HBaseInterClusterReplicationEndpoint
> int n = Math.min(Math.min(this.maxThreads, entries.size()/100+1),
>       replicationSinkMgr.getSinks().size());
> ... 
>       for (int i=0; i<n; i++) {
>         entryLists.add(new ArrayList<HLog.Entry>(entries.size()/n+1)); <-- batch
>       }
> ...
>         for (int i=0; i<entryLists.size(); i++) {
>          .....
>             // RuntimeExceptions encountered here bubble up and are handled in ReplicationSource
>             pool.submit(createReplicator(entryLists.get(i), i));  <-- concurrency

>             futures++;
>           }
>         }
> maxThreads is fixed & configurable and since we are taking min of the three values
n gets decided based replicationSinkMgr.getSinks().size() when we have enough edits to replicate
> replicationSinkMgr.getSinks().size() is decided based on 
> int numSinks = (int) Math.ceil(slaveAddresses.size() * ratio);
> where ratio is this.ratio = conf.getFloat("replication.source.ratio", DEFAULT_REPLICATION_SOURCE_RATIO);
> Currently DEFAULT_REPLICATION_SOURCE_RATIO is set to 10% so for small clusters of size
10-20 RegionServers  the value we get for numSinks and hence n is very small like 1 or 2.
This substantially reduces the pool concurrency used for shipping wal edits in parallel effectively
slowing down replication for small clusters and causing lot of lag accumulation in AgeOfLastShipped.
Sometimes it takes tens of hours to clear off the entire replication queue even after the
client has finished writing on the source side. 
> We are running tests by varying replication.source.ratio and have seen multi-fold improvement
in total replication time (will update the results here). I wanted to propose here that we
should increase the default value for replication.source.ratio also so that we have sufficient
concurrency even for small clusters. We figured it out after lot of iterations and debugging
so probably slightly higher default will save the trouble. 

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