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From "Dave Marion (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-4353) Stabilize tablet assignment during transient failure
Date Thu, 23 Jun 2016 22:32:16 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-4353?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15347329#comment-15347329
] 

Dave Marion commented on ACCUMULO-4353:
---------------------------------------

bq. Can you expand on this some more? Given that assignment is arguably the most important
thing for the Master to do, why are we concerned about letting the master do that as fast
as it can (for the aforementioned reason)? Do we need to come up with a more efficient way
for the master to handle the reassignment of many tablets?

Reading through this, and bringing some first-hand experience, I don't think the issue is
the the Master assigning tablets. It's the issue of tablet servers that are down for a short
period of time. When a tserver goes down, the Master re-assigns the tablets. When the tserver
comes back up, it goes through several rounds of balancing which could take a long time and
cause a lot of churn.

bq. I'm a little worried about this as a configuration knob – I feel like it kind of goes
against the highly-available distributed database which we expect Accumulo to be. When we
don't reassign tablets fast, that is a direct lack of availability for clients to read data.

I don't see any harm done here as long as the default behavior is what happens today. Allowing
an administrator to choose to delay tablet reassignment may not fit most use cases, but it
could fit some.

My 2 cents.

> Stabilize tablet assignment during transient failure
> ----------------------------------------------------
>
>                 Key: ACCUMULO-4353
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-4353
>             Project: Accumulo
>          Issue Type: Improvement
>            Reporter: Shawn Walker
>            Assignee: Shawn Walker
>            Priority: Minor
>          Time Spent: 10m
>  Remaining Estimate: 0h
>
> When a tablet server dies, Accumulo attempts to reassign the tablets it was hosting as
quickly as possible to maintain availability.  If multiple tablet servers die in quick succession,
such as from a rolling restart of the Accumulo cluster or a network partition, this behavior
can cause a storm of reassignment and rebalancing, placing significant load on the master.
> To avert such load, Accumulo should be capable of maintaining a steady tablet assignment
state in the face of transient tablet server loss.  Instead of reassigning tablets as quickly
as possible, Accumulo should be await the return of a temporarily downed tablet server (for
some configurable duration) before assigning its tablets to other tablet servers.



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