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From Jörg Wagner <joerg.wagn...@1und1.de>
Subject Re: Amount of partitions
Date Mon, 07 Sep 2015 08:08:20 GMT
Thank you very much for both replies.

@Tao
Thanks, I am aware of and have read that article. I am asking because my 
experience is completely different :/. Everytime we go beyond 400 
partitions the cluster really starts breaking apart.

@Todd
Thank you, very informative.

Or Details:
3 Brokers: 192GB Ram, 27 Disks for log.dirs, 9 topics and estimated 50k 
requests / second on 3 of the topics, the others are negligible.
Ordering is not required, messages are not keyed

The 3 main topics are one per DC (3 DCs) and being mirrored to the others.


The issue arises when we use over 400 partitions, which I think we 
require due to performance and mirroring. Partitions get out of sync and 
the log is being spammed by replicator messages. At the core, we start 
having massive stability issues.

Additionally, the mirrormaker only gets 2k Messages per *minute* through 
with a stable setup of 81 partitions (for the 3 main topics).

Has anyone experienced this and can give more insight? We have been 
doing testing for weeks, compared configuration and setups, without 
finding the main cause.
Can this be a Kernel (version/configuration) or Java(7) issue?

Cheers
Jörg


On 04.09.2015 20:24, Todd Palino wrote:
> Jun's post is a good start, but I find it's easier to talk in terms of more
> concrete reasons and guidance for having fewer or more partitions per topic.
>
> Start with the number of brokers in the cluster. This is a good baseline
> for the minimum number of partitions in a topic, as it will assure balance
> over the cluster. Of course, if you have lots of topics, you can
> potentially skip past this as you'll end up with balanced load in the
> aggregate, but I think it's a good practice regardless. As with all other
> advice here, there are always exceptions. If you really, really, really
> need to assure ordering of messages, you might be stuck with a single
> partition for some use cases.
>
> In general, you should pick more partitions if a) the topic is very busy,
> or b) you have more consumers. Looking at the second case first, you always
> want to have at least as many partitions in a topic as you have individual
> consumers in a consumer group. So if you have 16 consumers in a single
> group, you will want the topic they consume to have at least 16 partitions.
> In fact, you may also want to always have a multiple of the number of
> consumers so that you have even distribution. How many consumers you have
> in a group is going to be driven more by what you do with the messages once
> they are consumed, so here you'll be looking from the bottom of your stack
> up, until you get to Kafka.
>
> How busy the topic is is looking from the top down, through the producer,
> to Kafka. It's also a little more difficult to provide guidance on. We have
> a policy of expanding partitions for a topic whenever the size of the
> partition on disk (full retention over 4 days) is larger than 50 GB. We
> find that this gives us a few benefits. One is that it takes a reasonable
> amount of time when we need to move a partition from one broker to another.
> Another is that when we have partitions that are larger than this, the rate
> tends to cause problems with consumers. For example, we see mirror maker
> perform much better, and have less spiky lag problems, when we stay under
> this limit. We're even considering revising the limit down a little, as
> we've had some reports from other wildcard consumers that they've had
> problems keeping up with topics that have partitions larger than about 30
> GB.
>
> The last thing to look at is whether or not you are producing keyed
> messages to the topic. If you're working with unkeyed messages, there is no
> problem. You can usually add partitions whenever you want to down the road
> with little coordination with producers and consumers. If you are producing
> keyed messages, there is a good chance you do not want to change the
> distribution of keys to partitions at various points in the future when you
> need to size up. This means that when you first create the topic, you
> probably want to create it with enough partitions to deal with growth over
> time, both on the produce and consume side, even if that is too many
> partitions right now by other measures. For example, we have one client who
> requested 720 partitions for a particular set of topics. The reasoning was
> that they are producing keyed messages, they wanted to account for growth,
> and they wanted even distribution of the partitions to consumers as they
> grow. 720 happens to have a lot of factors, so it was a good number for
> them to pick.
>
> As a note, we have up to 5000 partitions per broker right now on current
> hardware, and we're moving to new hardware (more disk, 256 GB of memory,
> 10gig interfaces) where we're going to have up to 12,000. Our default
> partition count for most clusters is 8, and we've got topics up to 512
> partitions in some places just taking into account the produce rate alone
> (not counting those 720-partition topics that aren't that busy). Many of
> our brokers run with over 10k open file handles for regular files alone,
> and over 50k open when you include network.
>
> -Todd
>
>
>
> On Fri, Sep 4, 2015 at 8:11 AM, tao xiao <xiaotao183@gmail.com> wrote:
>
>> Here is a good doc to describe how to choose the right number of partitions
>>
>>
>> http://www.confluent.io/blog/how-to-choose-the-number-of-topicspartitions-in-a-kafka-cluster/
>>
>> On Fri, Sep 4, 2015 at 10:08 PM, Jörg Wagner <joerg.wagner1@1und1.de>
>> wrote:
>>
>>> Hello!
>>>
>>> Regarding the recommended amount of partitions I am a bit confused.
>>> Basically I got the impression that it's better to have lots of
>> partitions
>>> (see information from linkedin etc). On the other hand, a lot of
>>> performance benchmarks floating around show only a few partitions are
>> being
>>> used.
>>>
>>> Especially when considering the difference between hdd and ssds and also
>>> the amount thereof, what is the way to go?
>>>
>>> In my case, I seem to have the best stability and performance issues with
>>> few partitions *per hdd*, and only one io thread per disk.
>>>
>>> What are your experiences and recommendations?
>>>
>>> Cheers
>>> Jörg
>>>
>>
>>
>> --
>> Regards,
>> Tao
>>

-- 
Mit freundlichem Gruß

Jörg Wagner

  
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