Cluster rebalancing

Tom Santero tsantero at gmail.com
Thu Jul 3 13:51:34 EDT 2014


responses inline


On Thu, Jul 3, 2014 at 2:45 AM, Chaim Solomon <chaim at itcentralstation.com>
wrote:

> Hi,
>
> I'm running a 2.0.0b cluster (small) and have been running out of space on
> one node.
> I had expected that adding a node would lead to freeing up of space on
> other nodes - but it's not working too fast.
>

Keep in mind that the speed of transfers is bound by the bandwidth
available on the network as well as the speed at which you can actually
read the data off disk. Once the transfers complete you should see the disk
freed.


>
> I would suggest to add to RIAK a way to have the distribution algorithm
> take free space into consideration and to move data to empty nodes fast.
> Another issue is that adding the node moved most nodes from 25% to 18.8% -
> but one stayed on 25% in the planner.
>

The algorithm Riak uses to determine vnode placement is non-deterministic;
if you don't like any given staged vnode distribution I might suggest you
run riak-admin cluster clear to undo any staged changed and attempt to add
the node again, until you're content with the new plan.


>
> And I would also suggest adding some way to force a rebalancing of the
> cluster to force nodes to take up more load if they don't have enough or
> hand off load to others.
>

The hashing algorithm used by Riak to determine object placement in the
ring is uniform--over time and with a greater number of total keys you'll
start to see a smoother distribution across all partitions.

On the fly rebalancing would be incredibly expensive, especially for users
who have lots of nodes and petabytes of data stored in Riak. Ad-hoc
partition handoff would most likely be brittle and error-prone, given the
unreliability of the network.

In my humble opinion the engineers at Basho work harder than most other
distributed systems developers, considering all the edge cases where
systems can fail unexpectedly; I say this not to boost their egos, but
rather to point out that their approach has the effect of making Riak more
robust and resilient than most other distributed datastores. But such
resiliency isn't free, and for these guarantees every user must pay the
price. Riak might not be the fastest database, and it may even underutilize
that really expensive hardware you might throw at it...but i'll be damned
if it doesn't lie to me, lose my data or pretend that failures like network
partitions don't happen.


>
> Chaim Solomon
>
>
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> riak-users at lists.basho.com
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>
>
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