'not found' after join

Alexander Sicular siculars at gmail.com
Thu May 5 15:22:14 EDT 2011


I'm really loving this thread. Generating great ideas for the way
things should be... in the future. It seems to me that "the ring
changes immediately" is actually the problem as Ryan astutely
mentions. One way the future could look is :

- a new node comes online
- introductions are made
- candidate vnodes are selected for migration (<- insert pixie dust magic here)
- the number of simultaneous migrations are configurable, fewer for
limited interruption or more for quicker completion
- vnodes are migrated
- once migration is completed, ownership is claimed

Selecting vnodes for migration is where the unicorn cavalry attack the
dragons den. If done right(er) the algorithm could be swappable to
optimize for different strategies. Don't ask me how to implement it,
I'm only a yellow belt in erlang-fu.

Cheers,
Alexander

On Thu, May 5, 2011 at 13:33, Ryan Zezeski <rzezeski at basho.com> wrote:
> John,
> All great points.  The problem is that the ring changes immediately when a
> node is added.  So now, all the sudden, the preflist is potentially pointing
> to nodes that don't have the data and they won't have that data until
> handoff occurs.  The faster that data gets transferred, the less time your
> clients have to hit 'notfound'.
> However, I agree completely with what you're saying.  This is just a side
> effect of how the system currently works.  In a perfect world we wouldn't
> care how long handoff takes and we would also do some sort of automatic
> congestion control akin to TCP Reno or something.  The preflist would still
> point to the "old" partitions until all data has been successfully handed
> off, and then and only then would we flip the switch for that vnode.  I'm
> pretty sure that's where we are heading (I say "pretty sure" b/c I just
> joined the team and haven't been heavily involved in these specific talks
> yet).
> It's all coming down the pipe...
> As for your specific I/O question re handoff_concurrecy, you might be right.
>  I would think it depends on hardware/platform/etc.  I was offering it as a
> possible stopgap to minimize Greg's pain.  It's certainly a cure to a
> symptom, not the problem itself.
> -Ryan
>
> On Thu, May 5, 2011 at 1:10 PM, John D. Rowell <me at jdrowell.com> wrote:
>>
>> Hi Ryan, Greg,
>>
>> 2011/5/5 Ryan Zezeski <rzezeski at basho.com>
>>>
>>> 1. For example, riak_core has a `handoff_concurrency` setting that
>>> determines how many vnodes can concurrently handoff on a given node.  By
>>> default this is set to 4.  That's going to take a while with your 2048
>>> vnodes and all :)
>>
>> Won't that make the handoff situation potentially worse? From the thread I
>> understood that the main problem was that the cluster was shuffling too much
>> data around and thus becoming unresponsive and/or returning unexpected
>> results (like "not founds"). I'm attributing the concerns more to an
>> excessive I/O situation than to how long the handoff takes. If the handoff
>> can be made transparent (no or little side effects) I don't think most
>> people will really care (e.g. the "fix the cluster tomorrow" anecdote).
>>
>> How about using a percentage of available I/O to throttle the vnode
>> handoff concurrency? Start with 1, and monitor the node's I/O (kinda like
>> 'atop' does, collection CPU, disk and network metrics), if it is below the
>> expected usage, then increase the vnode handoff concurrency, and vice-versa.
>>
>> I for one would be perfectly happy if the handoff took several hours (even
>> days) if we could maintain the core riak_kv characteristics intact during
>> those events. We've all seen looooong RAID rebuild times, and it's usually
>> better to just sit tight and keep the rebuild speed low (slower I/O) while
>> keeping all of the dependent systems running smoothly.
>>
>> cheers
>> -jd
>
>
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