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https://issues.apache.org/jira/browse/ACCUMULO-4353?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15348409#comment-15348409
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Shawn Walker edited comment on ACCUMULO-4353 at 6/24/16 4:14 PM:
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bq. Are you attempting to design a mechanism that could be used to avoid
re-balancing and have the master keep assignments where they were previously,
knowing that servers will come back into operation?
That is the idea, yes. There is definitely a tradeoff here.
bq. If this is really about trying to make rolling-restarts better, I'd
encourage a look at ACCUMULO-1454.
As I mentioned before, I hadn't seen ACCUMULO-1454 before starting this. I've
now looked at the discussion of that ticket. What I implemented was
approximately what Christopher Tubbs and David Medinets were suggesting. I
also read through Keith Turner's design proposal summary. I have some
reservations with it:
* It requires that each planned restart involves tablet servers changing ports.
While the recent changes to Accumulo to support a narrow port range during
port search would make this more plausible, it might still prove difficult to
establish firewall rules for Accumulo. (Sean Busby raises this issue in the
discussion).
* What happens if a tablet is split after migration starts? It seems to me
there might be a race condition here which would lead to incomplete migration
between sibling tablet servers. Do we block assignment during the rolling
restart, too? That seems seems like a cure worse than the problem.
* Even barring those two concerns, I again raise the spectre of ops complexity.
To transition a single server, I need to know (a) which port the "old" tserver
was running on, and (b) which port the "new" tserver is running on. If I'm
using some sort of dynamic port assignment (which I would need to unless I
pointed the "new" tserver at an entirely different configuration), it could be
non-trivial to gather these pieces of information. While the burden on the
operator of a cluster of 5 tservers might not be significant, the burden on the
operator of a cluster of 200 tservers might make this approach infeasible. And
the non-triviality of determining the correct port migration mapping would also
make the process difficult to robustly automate.
bq. While seeing a pull request accompanying the issue reported, It seems a bit
premature to me to see code without some discussion on what the problems are
and how best to solve them.
Ahh, my mistake then. As a new contributor to Accumulo, I still don't have a
full grasp of the rules, either written or unwritten. My feeling from watching
the list was that primary modus operandi was to present a (fully implemented)
solution along with a proposed problem, and then to discuss the merits of the
solution.
was (Author: shawnwalker):
bq. Are you attempting to design a mechanism that could be used to avoid
re-balancing and have the master keep assignments where they were previously,
knowing that servers will come back into operation?
That is the idea, yes. There is definitely a tradeoff here.
bq. If this is really about trying to make rolling-restarts better, I'd
encourage a look at ACCUMULO-1454.
As I mentioned before, I hadn't seen ACCUMULO-1454 before starting this. I've
now looked at the discussion of that ticket. What I implemented was
approximately what Christopher Tubbs and David Medinets were suggesting. I
also read through Keith Turner's design proposal summary. I have some
reservations with it:
* It requires that each planned restart involves tablet servers changing ports.
While the recent changes to Accumulo to support a narrow port range during
port search would make this more plausible, it might still prove difficult to
establish firewall rules for Accumulo. (Sean Busby raises this issue in the
discussion).
* What happens if a tablet is split after migration starts? It seems to me
there might be a race condition here which would lead to incomplete migration
between sibling tablet servers. Do we block assignment during the rolling
restart, too? That seems seems like a cure worse than the problem.
* Even barring those two concerns, I again raise the spectre of ops complexity.
To transition a single server, I need to know (a) which port the "old" tserver
was running on, and (b) which port the "new" tserver is running on. If I'm
using some sort of dynamic port assignment (which I would need to unless I
pointed the "new" tserver at an entirely different configuration), it could be
non-trivial to gather these pieces of information. While the burden on the
operator of a cluster of 5 tservers might not be significant, the burden on the
operator of a cluster of 200 tservers might make this approach infeasible. And
the non-triviality of determining the correct port migration mapping would also
make the process difficult to robustly automate.
bq. While seeing a pull request accompanying the issue reported, It seems a bit
premature to me to see code without some discussion on what the problems are
and how best to solve them.
Ahh, my mistake then. As a new contributor to Accumulo, I'm still don't have a
full grasp of the rules, either written or unwritten. My feeling from watching
the list was that primary modus operandi was to present a (fully implemented)
solution along with a proposed problem, and then to discuss the merits of the
solution.
> 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: 0.5h
> 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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