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From Jonathan Hsieh <...@cloudera.com>
Subject Re: Multiple RS for serving one region
Date Tue, 22 Jan 2013 00:55:38 GMT
The main motivation is to maintain good performance on RS failovers.
This is also tied with hdfs and its block placement policy.  Let me
explain as I understand it.   If we control the hdfs block placement
strategy we can write all blocks for a hfile (or for all hfiles
related to a region) to the same set of data nodes.  If the RS fails,
they favor failover to a node that has a local copy of all the blocks.

Today, when you write an hfile to hdfs, for each block the first
replica goes to the local data node but the others get disbursed
around the cluster randomly at a per block granularity. The problem
here is that if the rs fails, the new rs that gets the responsibility
for the region has to read files that are spread all over the cluster
and with roughly 1/nth of the data local.  This means that the
recovered region is slower until a compaction localizes the data gain.

They've gone in and modified hdfs and their hbase to take advantage of
this idea.  I believe the randomization policy is enforced per region
-- if an rs serves 25 region, all the files within a each region are
sent to the same set of secondary/tertiary nodes, but each region
sends to a different set of secondary/tertiary nodes.


On Mon, Jan 21, 2013 at 3:48 PM, Devaraj Das <ddas@hortonworks.com> wrote:
> In 0.89-fb branch I stumbled upon stuff that indicated that there is a
> concept of secondary and tertiary regionserver. Could someone with
> more insights please shed some light on this?
> Might be useful to do the analysis on whether it makes sense for trunk..
> Thanks
> Devaraj

// Jonathan Hsieh (shay)
// Software Engineer, Cloudera
// jon@cloudera.com

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