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From aaron morton <>
Subject Re: Help on how to configure an off-site DR node.
Date Tue, 29 Mar 2011 22:00:10 GMT
Snapshots take use a hard link and do not take additional disk space

WRT losing a node, it's not the number of total nodes thats important is the number of replicas.
If you have 3 nodes with RF2 and you lose one of the replicas you will not be able to work
at Quorum level. 

You *may* be able to use the NetworkTopologySnitch to have 2 replicas in DC1 and 1 replica
in DC2. Then use the property file snitch to only put one node in the second DC. Finally work
against DC1 with LOCAL_QUORUM so you do not wait on DC 2 and you can tolerate the link to
DC2 failing. That also means there is no guarantee DC2 is up to date. If you were to ship
snapshots you would have a better idea of what you had in DC2. 

FWIW I'm not convinced that setting things up so that one node gets *all* the data in DC2
is a good idea. It would make an offsite replica that could only work at essentially CL ONE
and would require a lot of streaming to move to a cluster with more nodes. I don't have time
right now to think through all of the implications now (may be able to do some more thinking
tonight), but the data stax guide creates a warm fail over that is ready to work. I'm not
sure what this approach would give you in case of failure: a backup to be restored or a failover

Hope that helps. 

On 30 Mar 2011, at 00:38, Brian Lycett wrote:

> Hi.
> Cheers for your reply.
> Unfortunately there's too much data for snapshots to be practical.  The
> data set will be at least 400GB initially, and the offsite node will be
> on a 20Mbit leased line.
> However I don't need the consistency level to be quorum for read/writes
> in the production cluster, so am I right in still assuming that a
> replication factor of 2 in a three node cluster allows for one node to
> die without data loss?
> If that's the case, I still don't understand how to ensure that the
> offsite node will get a copy of the whole data set.
> I've read through the O'Reilly book, and that doesn't seem to address
> this scenario (unless I still don't get the Cassandra basics at a
> fundamental level).
> Does anyone know any tutorials/examples of such a set-up that would help
> me out?
> Cheers,
> Brian
> On Tue, 2011-03-29 at 21:56 +1100, aaron morton wrote:
>> Be aware that at RF 2 the Quorum is 2, so you cannot afford to lose a
>> replica when working at Quorum. 3 is really the starting point if you
>> want some redundancy. 
>> If you want to get your data offsite how about doing snapshots and
>> moving them off
>> site
>> The guide from Data Stax will give you a warm failover site, which
>> sounds a bit more than what you need.  
>> Hope that helps. 
>> Aaron
>> On 28 Mar 2011, at 22:47, Brian Lycett wrote:
>>> Hello.
>>> I'm setting up a cluster that has three nodes in our production
>>> rack.
>>> My intention is to have a replication factor of two for this.
>>> For disaster recovery purposes, I need to have another node (or
>>> two?)
>>> off-site.
>>> The off-site node is entirely for the purpose of having an offsite
>>> backup of the data - no clients will connect to it.
>>> My question is, is it possible to configure Cassandra so that the
>>> offsite node will have a full copy of the data set?
>>> That is, somehow guarantee that a replica of all data will be
>>> written to
>>> it, but without having to resort to an ALL consistency level for
>>> writes?
>>> Although the offsite node will on a 20Mbit leased line, I'd rather
>>> not
>>> have the risk that the link goes down and breaks the cluster.
>>> I've seen this suggestion here:
>>> but that configuration is vulnerable to the link breaking, and uses
>>> four
>>> nodes in the offsite location.
>>> Regards,
>>> Brian

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