Hi all -
I have some questions about how I want my Riak stuff to work - I've already
asked these questions of some Basho people and gotten some answers, but
thought I would toss it out into the wider world to see what you all have
to say, too:
First off - I know 5 instances is the "magic number" of instances to have.
If I understand the thinking here, it's that at the default redundancy
level ('n'?) of 3, it is most likely to start getting me some scaling
(e.g., performance > just that of a single node), and yet also have
redundancy; whereby I can lose one box and not start to take a performance
hit.
My question is - I think I can only do 4 in a way that makes sense. I only
have 4 AZ's that I can use right now; AWS won't let me boot instances in
1a. My concern is if I try to do 5, I will be "doubling up" in one AZ - and
in AWS you're almost as likely to lose an entire AZ as you are a single
instance. And so, if I have instances doubled-up in one AZ (let's say
us-east-1e), and then I lose 1e, I've now lost two instances. What are the
chances that all three of my replicas of some chunk of my data are on those
two instances? I know that it's not guaranteed that all replicas are on
separate nodes.
So is it better for me to ignore the recommendation of 5 nodes, and just do
4? Or to ignore the fact that I might be doubling-up in one AZ? Also,
another note. These are designed to be 'durable' nodes, so if one should go
down I would expect to bring it back up *with* its data - or, if I
couldn't, I would do a force-replace or replace and rebuild it from the
other replicas. I'm definitely not doing instance-store. So I don't know if
that mitigates my need for a full 5 nodes. I would also consider losing one
node to be "degraded" and would probably seek to fix that problem as soon
as possible, so I wouldn't expect to be in that situation for long. I would
probably tolerate a drop in performance during that time, too. (Not a
super-severe one, but 20-30 percent? Sure.)
What do you folks think?
-B.
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