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From "Jonathan Gray (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HBASE-2240) Balancer should not reassign (because of overloading) a region that was just opened
Date Wed, 01 Sep 2010 17:45:56 GMT

    [ https://issues.apache.org/jira/browse/HBASE-2240?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12905115#action_12905115

Jonathan Gray commented on HBASE-2240:

This seems invalid now, in my mind.

What is the reasoning behind wanting to have a history of rebalances?  Is there some reason
we would never want to rebalance a region that was just recently opened (for some definition
of recent).

I'm pretty sure this jira was dealing with the region assignment churn because we are determining
to balance on each heartbeat, so it was literally balancing a region to be on a server, then
unassigning for another server almost immediately.  Now that it's in the background, waits
for no regions to be in transition, and will only ever do one move of a given region per load-balance,
this does not seem valid anymore.

A load-balance will run periodically (minutes).  Is it the case that we would not want to
move a region from A to B in one balance.  And then 30 minutes later to move the same region
from B to C (or wherever)?  Not sure I follow why this is very important to not move a region
in successive balances (if our algorithm, now or later, decides this is best to achieve "balance").

We should put smarts into that algorithm.  A high cost could be associated with moving a region
if, for example, it had a large MemStore so it would require a big flush.  Or, even to implement
what's being discussed here, a high cost could be related to how long ago a region opened
at its current location (but again, I'm having a hard time knowing whether we'd bias old or
new ones... other stuff is probably better to make decisions based on)

> Balancer should not reassign (because of overloading) a region that was just opened
> -----------------------------------------------------------------------------------
>                 Key: HBASE-2240
>                 URL: https://issues.apache.org/jira/browse/HBASE-2240
>             Project: HBase
>          Issue Type: Bug
>            Reporter: stack
> I'm running a mapreduce job.  I see a region split and its daughters come on line and
then 8 seconds later, master judges the regionserver overloaded and so closes the just-opened
region.  This messes up clients.   They may have just picked up the new location for their
row query and now its moved again and client has to go hunting anew. 
> We need to assign the vintage regions first.
> I'm going to change balancer slop.  Its not sloppy enough and balancing cuts in too early.

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