Large ring_creation_size

Greg Nelson grourk at dropcam.com
Thu Apr 14 13:48:40 EDT 2011


We have a exact idea of the amount of data we'll be storing, and the kinds of machines we'll be storing them on. The simple math of (total data we'll be storing 6 months from now) / (total capacity of a single node) * (number of duplicates of each datum we'd like to store for redundancy) gives us a number a concrete number of machines we'll need, whether we're using Riak or something else...

However, that's a little off track from the question I'm trying to answer. Which is: 

Why is it when I start two nodes with a large-ish ring_creation_size -- as soon as I join the second node to the first, CPU and memory usage of both nodes goes through the roof? No data stored yet. This even happens with a ring_creation_size I wouldn't consider huge, like 4096.

So that is the small configuration I'm starting with and that is the limit I'm hitting.

I realize that the feasibility of building out a single 1000 node cluster is a larger question. But I can tell you we'll get there; having to shard across 10 100-node clusters is an option. Regardless, I'd like to have an understanding of what the resource overhead of each vnode is...
On Thursday, April 14, 2011 at 6:38 AM, Sean Cribbs wrote:
Good points, Dave.
> 
> Also, it's worth mentioning that we've seen that many customers and open-source users think they will need many more nodes than they actually do. Many are able to start with 5 nodes and are happy for quite a while. The only way to tell what you actually need is to start with a baseline configuration and simulate some percentage above your current load. Once you've figured out what size that initial cluster is, start with (number of nodes) * 50 as the ring_creation_size (rounded to the nearest power of 2 of course). This gives you a growth factor of about 5 before you need to consider changing.
> 
> As well, there's some ops "common sense" that says the lifetime of any single architecture is 18 months or less. That doesn't necessarily mean that you'll be building a new cluster with a larger ring size in 18 months, but just that your needs will be different at that time and are hard to predict. Plan for now, worry about the 1000 node cluster when you actually need it.
> 
> Sean Cribbs <sean at basho.com>
> Developer Advocate
> Basho Technologies, Inc.
> http://basho.com/
> 
> 
> On Apr 14, 2011, at 9:09 AM, Dave Barnes wrote:
> > Sorry I feel compelled to chime in.
> > 
> > Maybe you could assess your physical node limits and start with a small configuration, then increase  it and increase it until you hit a limit.
> > 
> > Work small to large.
> > 
> >  Once you find the pain point, lets us know what resource ran out.
> > 
> > You will learn a lot along the way on how your servers behave and we'll discover a lot when you share the results.
> > 
> > Thanks for digging in,
> > 
> > Dave
> > 
> > On Wed, Apr 13, 2011 at 5:11 PM, Greg Nelson <grourk at dropcam.com> wrote:
> > > Ok, how about in this case I described? It runs out of memory with a single pair of nodes...
> > > 
> > > (Or did you mean there's a connection between each pair of vnodes?) 
> > > On Wednesday, April 13, 2011 at 1:56 PM, Jon Meredith wrote:
> > > > Hi Greg et al,
> > > > 
> > > > As you say largest known is not largest possible. Internally within Basho, the largest cluster we've experimented with so far had 50 nodes. 
> > > > 
> > > > Going beyond that it's speculation from me about pain points. 
> > > > 
> > > > 1) It is true that you need enough file descriptors to start up all partitions when a node restarts - Riak checks if there is any handoff data pending for each partition. We have work scheduled to address that in the medium term. The plan is to only spin up partitions the node owns and any that have been started as fallbacks that handoff has not completed for. Until that work is done you will need a high ulimit with large ring sizes. 
> > > > 
> > > > 2) It is also true that Erlang runs a fully connected network, so there will be connections between each node pair in the cluster. We haven't determined the point at which it becomes a problem. 
> > > > 
> > > > So it looks like you'll be pushing the known limits. Basho will do our very best to help overcome any obstacles as you encounter them.
> > > > 
> > > > Jon Meredith
> > > > Basho Technologies.
> > > > 
> > > > On Wed, Apr 13, 2011 at 1:41 PM, Greg Nelson <grourk at dropcam.com> wrote:
> > > > > The largest known riak cluster != the largest possible riak cluster. ;-)
> > > > > 
> > > > > The inter node communication of the cluster depends on the data set and usage pattern, doesn't it? Or is there some constant overhead that tops out at a few hundred nodes? I should point out that we'll have big data, but not a huge number of keys. 
> > > > > 
> > > > > The number of vnodes in the cluster should be equal to the ring_creation_size under normal circumstances, shouldn't it? So when I have a one node cluster, that node is running ring_creation_size vnodes... File descriptors probably isn't a problem -- these machines won't be doing anything else, and the limits are set to 65536. 
> > > > > 
> > > > > Thinking about the internode communication you mentioned, that's probably where the resource hog is.. socket buffers, etc.
> > > > > 
> > > > > Anyway, I'd also love to hear more from basho. :)
> > > > > On Wednesday, April 13, 2011 at 12:33 PM, siculars at gmail.com wrote:
> > > > > > Ill just chime in and say that this is not practical for a few reasons. The largest known riak cluster has like 50 or 60 nodes. Afaik, inter node communication of erlang clusters top out at a few hundred nodes. I'm also under the impression that each physical node has to have enough file descriptors to accommodate every virtual node in the cluster. 
> > > > > > 
> > > > > > I'd love to hear more from basho. 
> > > > > > 
> > > > > > -alexander 
> > > > > > 
> > > > > > 
> > > > > > Sent from my Verizon Wireless BlackBerry
> > > > > > 
> > > > > > -----Original Message-----
> > > > > > From: Greg Nelson <grourk at dropcam.com>
> > > > > >  Sender: riak-users-bounces at lists.basho.com
> > > > > > Date: Wed, 13 Apr 2011 12:13:34 
> > > > > > To: <riak-users at lists.basho.com>
> > > > > >  Subject: Large ring_creation_size
> > > > > > 
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