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 <xiaotao...@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.wagn...@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 >
