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From aaron morton <aa...@thelastpickle.com>
Subject Re: advice for EC2 deployment
Date Wed, 27 Apr 2011 11:13:49 GMT
Using the EC2Snitch you could have one AZ in us-east-1 and one Az in us-west-1, treat each
AZ as a single rack and each region as a DC. The network topology is rack aware so will prefer
request that go to the same rack (not much of an issue when you have only one rack). 

If possible I would use the same RF in each DC, if you want the fail over to be as clean as
possible (see earlier comments about when number failed nodes in a dc). i.e. 3 replicas in
each dc / region. 

Until you find a reason otherwise use LOCAL_QUORUM, if that proves to be too slow or you get
more experience and feel comfortable with the trade offs then change to a lower CL.  

Dropping the CL level for write bursts does not make the cluster run any faster, it lets the
client think the cluster is running faster and can result in the client overloading (in a
good "this is what it should do" way) the cluster. This can result in more "eventual consistency"
work to be done later during maintenance or read requests. If that is a reasonable trade off,
you can write at CL ONE and read at CL ALL to ensure you get consistent reads (quorum is not
good enough in that case).

Jump in and test it at Quorum, you may find the write performance is good enough. There are
lots of dials to play with http://wiki.apache.org/cassandra/MemtableThresholds

Hope that helps. 
Aaron


On 27 Apr 2011, at 09:31, William Oberman wrote:

> I see what you're saying.  I was able to control write latency on mysql using insert
vs insert delayed (what I feel is MySQLs poor man's eventual consistency option) + the fact
that replication was a background asynchronous process.  In terms of read latency, I was able
to do up to a few hundred well indexed mysql queries (across AZs) on a view while keeping
the overall latency of the page around or less than a second. 
> 
> I basically am replacing two use cases, the cases with difficult to scale anticipated
write volumes.  The first case was previously using insert delayed (which I'm doing in cassandra
as ONE) as I wasn't getting consistent write/read operations before anyways.  The second case
was using traditional insert (which I was going to replace with some QUORUM-like level, I
was assuming LOCAL_QUORUM).  But, the latter case uses a write through memory cache (memcache),
so I don't know how often it really reads data from the persistent store.  But I definitely
need to make sure it is consistent.
> 
> In any case, it sounds like I'd be best served treating AZs as DCs, but then I don't
know what to make racks?  Or do racks not matter in a single AZ?  That way I can get an ack
from a LOCAL_QUORUM read/write before the (slightly) slower read/write to/from the other AZ
(for redundancy).  Then I'm only screwed if Amazon has a multi-AZ failure (so far, they've
kept it to "only" one!) :-)
> 
> will
> 
> On Tue, Apr 26, 2011 at 5:01 PM, aaron morton <aaron@thelastpickle.com> wrote:
> One difference between Cassandra and MySQL replication may be when the network IO happens.
Was the MySQL replication synchronous on transaction commit ?  I was only aware that it had
async replication, which means the client is not exposed to the network latency. In cassandra
the network latency is exposed to the client as it needs to wait for the CL number of nodes
to respond. 
> 
> If you use the PropertyFilePartitioner with the NetworkTopology you can manually assign
machines to racks / dc's based on IP. 
> See conf/cassandra-topology.property file there is also an Ec2Snitch which (from the
code) 
> /**
>  * A snitch that assumes an EC2 region is a DC and an EC2 availability_zone
>  *  is a rack. This information is available in the config for the node.
> 
> Recent discussion on DC aware CL levels 
> http://www.mail-archive.com/user@cassandra.apache.org/msg11414.html
> 
> Hope that helps.
> Aaron
>  
> 
> On 27 Apr 2011, at 01:18, William Oberman wrote:
> 
>> Thanks Aaron!
>> 
>> Unless no one on this list uses EC2, there were a few minor troubles end of last
week through the weekend which taught me a lot about obscure failure modes in various applications
I use :-)  My original post was trying to be more redundant than fast, which has been by overall
goal from even before moving to Cassandra (my downtime from the EC2 madness was minimal, and
due to only having one single point of failure == the amazon load balancer).  My secondary
goal was  trying to make moving to a second region easier, but is that is causing problems
I can drop the idea.
>> 
>> I might be downplaying the cost of inter-AZ communication, but I've lived with that
for quite some time, for example my current setup of MySQL in Master-Master replication is
split over zones, and my webservers live in yet different zones.  Maybe Cassandra is "chattier"
than I'm used to?  (again, I'm fairly new to cassandra)
>> 
>> Based on that article, the discussion, and the recent EC2 issues, it sounds like
it would be better to start with:
>> -6 nodes split in two AZs 3/3
>> -Configure replication to do 2 in one AZ and one in the other (NetworkTopology treats
AZs as racks, so does RF=3,us-east=3 make this happen naturally?)
>> -What does LOCAL_QUORUM do in this case?  Is there a "rack quorum"?  Or does the
natural latencies of AZs make LOCAL_QUORUM behave like a rack quorum?
>> 
>> will
>> 
>> On Tue, Apr 26, 2011 at 1:14 AM, aaron morton <aaron@thelastpickle.com> wrote:
>> For background see this article:
>> http://www.datastax.com/dev/blog/deploying-cassandra-across-multiple-data-centers
>> 
>> And this recent discussion 
>> http://www.mail-archive.com/user@cassandra.apache.org/msg12502.html
>> 
>> Issues that may be a concern:
>> - lots of cross AZ latency in us-east, e.g. LOCAL_QUORUM ops must wait cross AZ .
Also consider it during maintenance tasks, how much of a pain is it going to be to have latency
between every node.   
>> - IMHO not having sufficient (by that I mean 3) replicas in a cassandra DC to handle
a single node failure when working at Quorum reduces the utility of the DC. e.g. with a local
RF of 2 in the west, the quorum is 2, and if you lose one node from the replica set you will
not be able to use local QUORUM for keys in that range. Or consider a failure mode where the
west is disconnected from the east.
>> 
>> Could you start simple with 3 replicas in one AZ in us-east and 3 replicas in an
AZ+Region ?  Then work through some failure scenarios.  
>> 
>> Hope that helps. 
>> Aaron
>>   
>> 
>> On 22 Apr 2011, at 03:28, William Oberman wrote:
>> 
>>> Hi,
>>> 
>>> My service is not yet ready to be fully multi-DC, due to how some of my legacy
MySQL stuff works.  But, I wanted to get cassandra going ASAP and work towards multi-DC. 
I have two main cassandra use cases: one where I can handle eventual consistency (and all
of the writes/reads are currently ONE), and one where I can't (writes/reads are currently
QUORUM).  My test cluster is currently 4 smalls all in us-east with RF=3 (more to prove I
can clustering, than to have an exact production replica).  All of my unit tests, and "load
tests" (again, not to prove true max load, but to more to tease out concurrency issues) are
passing now.
>>> 
>>> For production, I was thinking of doing:
>>> -4 cassandra larges in us-east (where I am now), once in each AZ
>>> -1 cassandra large in us-west (where I have nothing)
>>> For now, my data can fit into a single large's 2 disk ephemeral using RAID0,
and I was then thinking of doing a RF=3 with us-east=2 and us-west=1.  If I do eventual consistency
at ONE, and consistency at LOCAL_QUORUM, I was hoping:
>>> -eventual consistency ops would be really fast
>>> -consistent ops would be pretty fast (what does LOCAL_QUORUM do in this case?
 return after 1 or 2 us-east nodes ack?)
>>> -us-west would contain a complete copy of my data, so it's a good eventually
consistent "close to real time" backup  (assuming it can keep up over long periods of time,
but I think it should)
>>> -eventually, when I'm ready to roll out in us-west I'll be able to change the
replication settings and that server in us-west could help seed new cassandra instances faster
than the ones in us-east
>>> 
>>> Or am I missing something really fundamental about how cassandra works making
this a terrible plan?  I should have plenty of time to get my multi-DC working before the
instance in us-west fills up (but even then, I should be able to add instances over there
to stall fairly trivially, right?).
>>> 
>>> Thanks!
>>> 
>>> will
>> 
>> 
>> 
>> 
>> -- 
>> Will Oberman
>> Civic Science, Inc.
>> 3030 Penn Avenue., First Floor
>> Pittsburgh, PA 15201
>> (M) 412-480-7835
>> (E) oberman@civicscience.com
> 
> 
> 
> 
> -- 
> Will Oberman
> Civic Science, Inc.
> 3030 Penn Avenue., First Floor
> Pittsburgh, PA 15201
> (M) 412-480-7835
> (E) oberman@civicscience.com


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