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Accumulo >> mail # user >> Straggler problem in Accumulo BatchScans

Slater, David M. 2013-08-21, 23:09
James Hughes 2013-08-21, 23:29
Slater, David M. 2013-08-21, 23:47
Eric Newton 2013-08-21, 23:53
Dave Marion 2013-08-21, 23:28
Slater, David M. 2013-08-21, 23:54
Eric Newton 2013-08-22, 00:03
Slater, David M. 2013-08-22, 00:12
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Re: Straggler problem in Accumulo BatchScans
You can set it in the shell on the table. Just override the default tablet balancer for the table. I think the master has to use the Table load balancer also if it is not set by default.

----- Original Message -----

From: "David M. Slater" <[EMAIL PROTECTED]>
Sent: Wednesday, August 21, 2013 8:12:46 PM
Subject: RE: Straggler problem in Accumulo BatchScans

Thanks Eric,

Just to make sure I’m going in the right direction, this would involve extending the TabletBalancer class, correct? How do I add it to the table after that (and remove the old one)? I don’t see it under the Connector’s TableOperations().

Is using a load-balancer what you would recommend if I wanted to make sure that two different tables stored related information (e.g. data and indexes) on the same tablets?


From: Eric Newton [mailto:[EMAIL PROTECTED]]
Sent: Wednesday, August 21, 2013 8:03 PM
Subject: Re: Straggler problem in Accumulo BatchScans
A new balancer is a plug-in class that instructs the Master process where to place tablets.

If you know you need your tablets spread out over servers based on time (row id), you can do that. It's pretty common, in fact.


On Wed, Aug 21, 2013 at 7:54 PM, Slater, David M. < [EMAIL PROTECTED] > wrote:
Hi Dave,

The table is currently organizing netflow data with its rowID of timestamp_netflowRecordID, some columns corresponding to various netflow quantites, and one column representing the entire netflow in binary form.

The table is about 1.2 TB, and I am scanning 5-40 GB per scan, which scans about 7-28 tablets.

What do you mean by a custom load balancer? Do you mean balancing the data on ingest, or balancing the query load? What would you recommend for balancing the query load if I can only retrieve the data from a particular tablet?

I’ve played with index/data caches, though I haven’t used readahead threads or max open files. Is that referring to rfiles?

I’m noticing that most of the queries are CPU bound, and that read i/o is not being hit very hard. Is that a typical behavior for scans?

From: Dave Marion [mailto: [EMAIL PROTECTED] ]
Sent: Wednesday, August 21, 2013 7:29 PM
Subject: RE: Straggler problem in Accumulo BatchScans
How is the table organized?

What percent of the table are you scanning in these large operations?

Have you considered writing a custom load balancer?

I don’t think that a tablet can be hosted on multiple servers. But you might be able to play around with the index/data caches, readahead threads (concurrent queries), and max open files to achieve better performance.
From: Slater, David M. [ mailto:[EMAIL PROTECTED] ]
Sent: Wednesday, August 21, 2013 7:09 PM
Subject: Straggler problem in Accumulo BatchScans
Hey, I have a 7 node network running accumulo 1.4.1 and hadoop 1.0.4.

When I run large BatchScanner operations, the number of tablets scanned per node is not uniform, leading to the overloaded nodes taking much longer to finish than the others. For queries that require all of the scans to finish before returning, this is a major latency issue. What are some practical means of load-balancing this to reduce delay?

Is it possible for tablets to be hosted on multiple tablet servers, up to the replication factor of the underlying hdfs? Are there reasons this might be an undesirable design?

Thanks in advance,
James Hughes 2013-08-22, 14:13