Hi Reid,
Thank you very much for clearing these concepts for me.
https://community.datastax.com/comments/1133/view.html I posted this
question on the datastax forum regarding our cluster that it is unbalanced
and the reply was related that the *number of racks should be a multiplier
of the replication factor *in order to be balanced or 1. I thought then if
I have 3 availability zones I should have 3 racks for each datacenter and
not 2 (us-east-1b, us-east-1a) as I have right now or in the easiest way, I
should have a rack for each datacenter.
1. Datacenter: live
================
Status=Up/Down
|/ State=Normal/Leaving/Joining/Moving
-- Address Load Tokens Owns Host ID
Rack
UN 10.1.20.49 289.75 GiB 256 ?
be5a0193-56e7-4d42-8cc8-5d2141ab4872 us-east-1a
UN 10.1.30.112 103.03 GiB 256 ?
e5108a8e-cc2f-4914-a86e-fccf770e3f0f us-east-1b
UN 10.1.19.163 129.61 GiB 256 ?
3c2efdda-8dd4-4f08-b991-9aff062a5388 us-east-1a
UN 10.1.26.181 145.28 GiB 256 ?
0a8f07ba-a129-42b0-b73a-df649bd076ef us-east-1b
UN 10.1.17.213 149.04 GiB 256 ?
71563e86-b2ae-4d2c-91c5-49aa08386f67 us-east-1a
DN 10.1.19.198 52.41 GiB 256 ?
613b43c0-0688-4b86-994c-dc772b6fb8d2 us-east-1b
UN 10.1.31.60 195.17 GiB 256 ?
3647fcca-688a-4851-ab15-df36819910f4 us-east-1b
UN 10.1.25.206 100.67 GiB 256 ?
f43532ad-7d2e-4480-a9ce-2529b47f823d us-east-1b
So each rack label right now matches the availability zone and we have 3
Datacenters and 2 Availability Zone with 2 racks per DC but the above is
clearly unbalanced
If I have a keyspace with a replication factor = 3 and I want to
minimize the number of nodes to scale up and down the cluster and keep it
balanced should I consider an approach like OPTION A)
2. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1a
3. 3 read ONE us-east-1a
4. 4 write ONE us-east-1b 5 write ONE us-east-1b
5. 6 write ONE us-east-1b
6. OPTION B)
7. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1a
8. 3 read ONE us-east-1a
9. 4 write TWO us-east-1b 5 write TWO us-east-1b
10. 6 write TWO us-east-1b
11. *7 read ONE us-east-1c 8 write TWO us-east-1c*
12. *9 read ONE us-east-1c* Option B looks to be unbalanced and I would
exclude it OPTION C)
13. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1b
14. 3 read ONE us-east-1c
15. 4 write TWO us-east-1a 5 write TWO us-east-1b
16. 6 write TWO us-east-1c
17.
so I am thinking of A if I have the restriction of 2 AZ but I guess that
option C would be the best. If I have to add another DC for reads because
we want to assign a new DC for each new microservice it would look like:
OPTION EXTRA DC For Reads
1. Node DC RACK AZ 1 read ONE us-east-1a 2 read ONE us-east-1b
2. 3 read ONE us-east-1c
3. 4 write TWO us-east-1a 5 write TWO us-east-1b
4. 6 write TWO us-east-1c 7 extra-read THREE us-east-1a
5. 8 extra-read THREE us-east-1b
6.
7.
1. 9 extra-read THREE us-east-1c
2.
The DC for *write* will replicate the data in the other datacenters. My
scope is to keep the *read* machines dedicated to serve reads and *write*
machines to serve writes. Cassandra will handle the replication for me. Is
there any other option that is I missing or wrong assumption? I am thinking
that I will write a blog post about all my learnings so far, thank you very
much for the replies Best, Sergio
Il giorno mer 23 ott 2019 alle ore 10:57 Reid Pinchback <
[email protected]> ha scritto:
> No, that’s not correct. The point of racks is to help you distribute the
> replicas, not further-replicate the replicas. Data centers are what do the
> latter. So for example, if you wanted to be able to ensure that you always
> had quorum if an AZ went down, then you could have two DCs where one was in
> each AZ, and use one rack in each DC. In your situation I think I’d be
> more tempted to consider that. Then if an AZ went away, you could fail
> over your traffic to the remaining DC and still be perfectly fine.
>
>
>
> For background on replicas vs racks, I believe the information you want is
> under the heading ‘NetworkTopologyStrategy’ at:
>
> http://cassandra.apache.org/doc/latest/architecture/dynamo.html
>
>
>
> That should help you better understand how replicas distribute.
>
>
>
> As mentioned before, while you can choose to do the reads in one DC,
> except for concerns about contention related to network traffic and
> connection handling, you can’t isolate reads from writes. You can _
> *mostly*_ insulate the write DC from the activity within the read DC, and
> even that isn’t an absolute because of repairs. However, your mileage may
> vary, so do what makes sense for your usage pattern.
>
>
>
> R
>
>
>
> *From: *Sergio <[email protected]>
> *Reply-To: *"[email protected]" <[email protected]>
> *Date: *Wednesday, October 23, 2019 at 12:50 PM
> *To: *"[email protected]" <[email protected]>
> *Subject: *Re: Cassandra Rack - Datacenter Load Balancing relations
>
>
>
> *Message from External Sender*
>
> Hi Reid,
>
> Thanks for your reply. I really appreciate your explanation.
>
> We are in AWS and we are using right now 2 Availability Zone and not 3. We
> found our cluster really unbalanced because the keyspace has a replication
> factor = 3 and the number of racks is 2 with 2 datacenters.
> We want the writes spread across all the nodes but we wanted the reads
> isolated from the writes to keep the load on that node low and to be able
> to identify problems in the consumers (reads) or producers (writes)
> applications.
> It looks like that each rack contains an entire copy of the data so this
> would lead to replicate for each rack and then for each node the
> information. If I am correct if we have a keyspace with 100GB and
> Replication Factor = 3 and RACKS = 3 => 100 * 3 * 3 = 900GB
> If I had only one rack across 2 or even 3 availability zone I would save
> in space and I would have 300GB only. Please correct me if I am wrong.
>
> Best,
>
> Sergio
>
>
>
> Il giorno mer 23 ott 2019 alle ore 09:21 Reid Pinchback <
> [email protected]> ha scritto:
>
> Datacenters and racks are different concepts. While they don't have to be
> associated with their historical meanings, the historical meanings probably
> provide a helpful model for understanding what you want from them.
>
> When companies own their own physical servers and have them housed
> somewhere, the questions arise on where you want to locate any particular
> server. It's a balancing act on things like network speed of related
> servers being able to talk to each other, versus fault-tolerance of having
> many servers not all exposed to the same risks.
>
> "Same rack" in that physical world tended to mean something like "all
> behind the same network switch and all sharing the same power bus". The
> morning after an electrical glitch fries a power bus and thus everything in
> that rack, you realize you wished you didn't have so many of the same type
> of server together. Well, they were servers. Now they are door stops.
> Badness and sadness.
>
> That's kind of the mindset to have in mind with racks in Cassandra. It's
> an artifact for you to separate servers into pools so that the disparate
> pools have hopefully somewhat independent infrastructure risks. However,
> all those servers are still doing the same kind of work, are the same
> version, etc.
>
> Datacenters are amalgams of those racks, and how similar or different they
> are from each other depends on what you want to do with them. What is true
> is that if you have N datacenters, each one of them must have enough disk
> storage to house all the data. The actual physical footprint of that data
> in each DC depends on the replication factors in play.
>
> Note that you sorta can't have "one datacenter for writes" because the
> writes will replicate across the data centers. You could definitely choose
> to have only one that takes read queries, but best to think of writing as
> being universal. One scenario you can have is where the DC not taking live
> traffic read queries is the one you use for maintenance or performance
> testing or version upgrades.
>
> One rack makes your life easier if you don't have a reason for multiple
> racks. It depends on the environment you deploy into and your fault
> tolerance goals. If you were in AWS and wanting to spread risk across
> availability zones, then you would likely have as many racks as AZs you
> choose to be in, because that's really the point of using multiple AZs.
>
> R
>
>
> On 10/23/19, 4:06 AM, "Sergio Bilello" <[email protected]> wrote:
>
> Message from External Sender
>
> Hello guys!
>
> I was reading about
> https://urldefense.proofpoint.com/v2/url?u=https-3A__cassandra.apache.org_doc_latest_architecture_dynamo.html-23networktopologystrategy&d=DwIBaQ&c=9Hv6XPedRSA-5PSECC38X80c1h60_XWA4z1k_R1pROA&r=OIgB3poYhzp3_A7WgD7iBCnsJaYmspOa2okNpf6uqWc&m=xmgs1uQTlmvCtIoGJKHbByZZ6aDFzS5hDQzChDPCfFA&s=9ZDWAK6pstkCQfdbwLNsB-ZGsK64RwXSXfAkOWtmkq4&e=
>
> I would like to understand a concept related to the node load
> balancing.
>
> I know that Jon recommends Vnodes = 4 but right now I found a cluster
> with vnodes = 256 replication factor = 3 and 2 racks. This is unbalanced
> because the racks are not a multiplier of the replication factor.
>
> However, my plan is to move all the nodes in a single rack to
> eventually scale up and down the node in the cluster once at the time.
>
> If I had 3 racks and I would like to keep the things balanced I should
> scale up 3 nodes at the time one for each rack.
>
> If I would have 3 racks, should I have also 3 different datacenters so
> one datacenter for each rack?
>
> Can I have 2 datacenters and 3 racks? If this is possible one
> datacenter would have more nodes than the others? Could it be a problem?
>
> I am thinking to split my cluster in one datacenter for reads and one
> for writes and keep all the nodes in the same rack so I can scale up once
> node at the time.
>
>
>
> Please correct me if I am wrong
>
>
>
> Thanks,
>
>
>
> Sergio
>
>
>
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