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From Jeff Jirsa <jji...@gmail.com>
Subject Re: Re[6]: Modify keyspace replication strategy and rebalance the nodes
Date Tue, 19 Sep 2017 14:01:21 GMT
The replicas will exist, they just may not have the data we expect them to
have. But the read queries will still get the right data through read
repair, so it'll be fine.


On Tue, Sep 19, 2017 at 5:29 AM, Myron A. Semack <msemack@kcftech.com>
wrote:

> But if you use ALL, and RF=3, it will be expecting 3 replicas.  After you
> switch to NetworkTopologyStrategy, you potentially only have 1 replica
> until the repair is done.  Won’t the query fail until the repair is done
> because the other two replicas won’t be ready yet (the ALL condition can’t
> be satisfied)?
>
>
>
> Sincerely,
>
> Myron A. Semack
>
>
>
> *From:* Jeff Jirsa [mailto:jjirsa@gmail.com]
> *Sent:* Monday, September 18, 2017 6:02 PM
> *To:* cassandra <user@cassandra.apache.org>
>
> *Subject:* Re: Re[6]: Modify keyspace replication strategy and rebalance
> the nodes
>
>
>
> Using CL:ALL basically forces you to always include the first replica in
> the query.
>
>
>
> The first replica will be the same for both SimpleStrategy/SimpleSnitch
> and NetworkTopologyStrategy/EC2Snitch.
>
>
>
> It's basically the only way we can guarantee we're not going to lose a row
> because it's only written to the second and third replicas while the first
> replica is down, in case the second and third replicas change to different
> hosts (racks / availability zones) during the ALTER.
>
>
>
>
>
>
>
>
>
>
>
>
>
> On Mon, Sep 18, 2017 at 1:57 PM, Myron A. Semack <msemack@kcftech.com>
> wrote:
>
> How would setting the consistency to ALL help?  Wouldn’t that just cause
> EVERY read/write to fail after the ALTER until the repair is complete?
>
>
>
> Sincerely,
>
> Myron A. Semack
>
>
>
> *From:* Jeff Jirsa [mailto:jjirsa@gmail.com]
> *Sent:* Monday, September 18, 2017 2:42 PM
> *To:* user@cassandra.apache.org
> *Subject:* Re: Re[6]: Modify keyspace replication strategy and rebalance
> the nodes
>
>
>
> The hard part here is nobody's going to be able to tell you exactly what's
> involved in fixing this because nobody sees your ring
>
>
>
> And since you're using vnodes and have a nontrivial number of instances,
> sharing that ring (and doing anything actionable with it) is nontrivial.
>
>
>
> If you weren't using vnodes, you could just fix the distribution and decom
> extra nodes afterward.
>
>
>
> I thought - but don't have time or energy to check - that the ec2snitch
> would be rack aware even when using simple strategy - if that's not the
> case (as you seem to indicate), then you're in a weird spot - you can't go
> to NTS trivially because doing so will reassign your replicas to be rack/as
> aware, certainly violating your consistency guarantees.
>
>
>
> If you can change your app to temporarily write with ALL and read with
> ALL, and then run repair, then immediately ALTER the keyspace, then run
> repair again, then drop back to whatever consistency you're using, you can
> probably get through it. The challenge is that ALL gets painful if you lose
> any instance.
>
>
>
> But please test in a lab, and note that this is inherently dangerous, I'm
> not advising you to do it, though I do believe it can be made to work.
>
>
>
>
>
>
>
>
>
>
> --
>
> Jeff Jirsa
>
>
>
>
> On Sep 18, 2017, at 11:18 AM, Dominik Petrovic <dominik.petrovic@mail.ru.
> INVALID> wrote:
>
> @jeff what do you think is the best approach here to fix this problem?
> Thank you all for helping me.
>
> Thursday, September 14, 2017 3:28 PM -07:00 from kurt greaves <
> kurt@instaclustr.com>:
>
> Sorry that only applies our you're using NTS. You're right that simple
> strategy won't work very well in this case. To migrate you'll likely need
> to do a DC migration to ensuite no downtime, as replica placement will
> change even if RF stays the same.
>
>
>
> On 15 Sep. 2017 08:26, "kurt greaves" <kurt@instaclustr.com> wrote:
>
> If you have racks configured and lose nodes you should replace the node
> with one from the same rack. You then need to repair, and definitely don't
> decommission until you do.
>
>
>
> Also 40 nodes with 256 vnodes is not a fun time for repair.
>
>
>
> On 15 Sep. 2017 03:36, "Dominik Petrovic" <dominik.petrovic@mail.ru.invalid>
> wrote:
>
> @jeff,
> I'm using 3 availability zones, during the life of the cluster we lost
> nodes, retired others and we end up having some of the data
> written/replicated on a single availability zone. We saw it with nodetool
> getendpoints.
> Regards
>
> Thursday, September 14, 2017 9:23 AM -07:00 from Jeff Jirsa <
> jjirsa@gmail.com>:
>
> With one datacenter/region, what did you discover in an outage you think
> you'll solve with network topology strategy? It should be equivalent for a
> single D.C.
>
> --
>
> Jeff Jirsa
>
>
>
>
> On Sep 14, 2017, at 8:47 AM, Dominik Petrovic <dominik.petrovic@mail.ru.
> INVALID> wrote:
>
> Thank you for the replies!
>
> @jeff my current cluster details are:
> 1 datacenter
> 40 nodes, with vnodes=256
> RF=3
> What is your advice? is it a production cluster, so I need to be very
> careful about it.
> Regards
>
> Thu, 14 Sep 2017 -2:47:52 -0700 from Jeff Jirsa <jjirsa@gmail.com>:
>
> The token distribution isn't going to change - the way Cassandra maps
> replicas will change.
>
>
>
> How many data centers/regions will you have when you're done? What's your
> RF now? You definitely need to run repair before you ALTER, but you've got
> a bit of a race here between the repairs and the ALTER, which you MAY be
> able to work around if we know more about your cluster.
>
>
>
> How many nodes
>
> How many regions
>
> How many replicas per region when you're done?
>
>
>
>
>
>
>
> --
>
> Jeff Jirsa
>
>
>
>
> On Sep 13, 2017, at 2:04 PM, Dominik Petrovic <dominik.petrovic@mail.ru.
> INVALID> wrote:
>
> Dear community,
> I'd like to receive additional info on how to modify a keyspace
> replication strategy.
>
> My Cassandra cluster is on AWS, Cassandra 2.1.15 using vnodes, the
> cluster's snitch is configured to Ec2Snitch, but the keyspace the
> developers created has replication class SimpleStrategy = 3.
>
> During an outage last week we realized the discrepancy between the
> configuration and we would now fix the issue using NetworkTopologyStrategy.
>
> What are the suggested steps to perform?
> For Cassandra 2.1 I found only this doc: http://docs.datastax.com/
> en/cassandra/2.1/cassandra/operations/opsChangeKSStrategy.html
> that does not mention anything about repairing the cluster
>
> For Cassandra 3 I found this other doc: https://docs.datastax.
> com/en/cassandra/3.0/cassandra/operations/opsChangeKSStrategy.html
> That involves also the cluster repair operation.
>
> On a test cluster I tried the steps for Cassandra 2.1 but the token
> distribution in the ring didn't change so I'm assuming that wasn't the
> right think to do.
> I also perform a nodetool repair -pr but nothing changed as well.
> Some advice?
>
> --
> Dominik Petrovic
>
>
>
> --
> Dominik Petrovic
>
>
>
> --
> Dominik Petrovic
>
>
>
> --
> Dominik Petrovic
>
>
>

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