On Wed, Jan 25, 2017, at 13:50, Dong Lin wrote: > Hey Colin, > > Good point! Yeah we have actually considered and tested this solution, > which we call one-broker-per-disk. It would work and should require no > major change in Kafka as compared to this JBOD KIP. So it would be a good > short term solution. > > But it has a few drawbacks which makes it less desirable in the long > term. > Assume we have 10 disks on a machine. Here are the problems:
Hi Dong, Thanks for the thoughtful reply. > > 1) Our stress test result shows that one-broker-per-disk has 15% lower > throughput > > 2) Controller would need to send 10X as many LeaderAndIsrRequest, > MetadataUpdateRequest and StopReplicaRequest. This increases the burden > on > controller which can be the performance bottleneck. Maybe I'm misunderstanding something, but there would not be 10x as many StopReplicaRequest RPCs, would there? The other requests would increase 10x, but from a pretty low base, right? We are not reassigning partitions all the time, I hope (or else we have bigger problems...) > > 3) Less efficient use of physical resource on the machine. The number of > socket on each machine will increase by 10X. The number of connection > between any two machine will increase by 100X. > > 4) Less efficient way to management memory and quota. > > 5) Rebalance between disks/brokers on the same machine will less > efficient > and less flexible. Broker has to read data from another broker on the > same > machine via socket. It is also harder to do automatic load balance > between > disks on the same machine in the future. > > I will put this and the explanation in the rejected alternative section. > I > have a few questions: > > - Can you explain why this solution can help avoid scalability > bottleneck? > I actually think it will exacerbate the scalability problem due the 2) > above. > - Why can we push more RPC with this solution? To really answer this question we'd have to take a deep dive into the locking of the broker and figure out how effectively it can parallelize truly independent requests. Almost every multithreaded process is going to have shared state, like shared queues or shared sockets, that is going to make scaling less than linear when you add disks or processors. (And clearly, another option is to improve that scalability, rather than going multi-process!) > - It is true that a garbage collection in one broker would not affect > others. But that is after every broker only uses 1/10 of the memory. Can > we be sure that this will actually help performance? The big question is, how much memory do Kafka brokers use now, and how much will they use in the future? Our experience in HDFS was that once you start getting more than 100-200GB Java heap sizes, full GCs start taking minutes to finish when using the standard JVMs. That alone is a good reason to go multi-process or consider storing more things off the Java heap. Disk failure is the "easy" case. The "hard" case, which is unfortunately also the much more common case, is disk misbehavior. Towards the end of their lives, disks tend to start slowing down unpredictably. Requests that would have completed immediately before start taking 20, 100 500 milliseconds. Some files may be readable and other files may not be. System calls hang, sometimes forever, and the Java process can't abort them, because the hang is in the kernel. It is not fun when threads are stuck in "D state" http://stackoverflow.com/questions/20423521/process-perminantly-stuck-on-d-state . Even kill -9 cannot abort the thread then. Fortunately, this is rare. Another approach we should consider is for Kafka to implement its own storage layer that would stripe across multiple disks. This wouldn't have to be done at the block level, but could be done at the file level. We could use consistent hashing to determine which disks a file should end up on, for example. best, Colin > > Thanks, > Dong > > On Wed, Jan 25, 2017 at 11:34 AM, Colin McCabe <cmcc...@apache.org> > wrote: > > > Hi Dong, > > > > Thanks for the writeup! It's very interesting. > > > > I apologize in advance if this has been discussed somewhere else. But I > > am curious if you have considered the solution of running multiple > > brokers per node. Clearly there is a memory overhead with this solution > > because of the fixed cost of starting multiple JVMs. However, running > > multiple JVMs would help avoid scalability bottlenecks. You could > > probably push more RPCs per second, for example. A garbage collection > > in one broker would not affect the others. It would be interesting to > > see this considered in the "alternate designs" design, even if you end > > up deciding it's not the way to go. > > > > best, > > Colin > > > > > > On Thu, Jan 12, 2017, at 10:46, Dong Lin wrote: > > > Hi all, > > > > > > We created KIP-112: Handle disk failure for JBOD. Please find the KIP > > > wiki > > > in the link https://cwiki.apache.org/confluence/display/KAFKA/KIP- > > > 112%3A+Handle+disk+failure+for+JBOD. > > > > > > This KIP is related to KIP-113 > > > <https://cwiki.apache.org/confluence/display/KAFKA/KIP- > > 113%3A+Support+replicas+movement+between+log+directories>: > > > Support replicas movement between log directories. They are needed in > > > order > > > to support JBOD in Kafka. Please help review the KIP. You feedback is > > > appreciated! > > > > > > Thanks, > > > Dong > >
