Hi Jun, In addition to the Eno's reference of why rebuild time with RAID-5 is more expensive, another concern is that RAID-5 will fail if more than one disk fails. JBOD is still works with 1+ disk failure and has better performance with one disk failure. These seems like good argument for using JBOD instead of RAID-5.
If a leader replica goes offline, the broker should first take all actions (i.e. remove the partition from fetcher thread) as if it has received StopReplicaRequest for this partition because the replica can no longer work anyway. It will also respond with error to any ProduceRequest and FetchRequest for partition. The broker notifies controller by writing notification znode in ZK. The controller learns the disk failure event from ZK, sends LeaderAndIsrRequest and receives LeaderAndIsrResponse to learn that the replica is offline. The controller will then elect new leader for this partition and sends LeaderAndIsrRequest/MetadataUpdateRequest to relevant brokers. The broker should stop adjusting the ISR for this partition as if the broker is already offline. I am not sure there is any inconsistency in broker's behavior when it is leader or follower. Is there any concern with this approach? Thanks for catching this. I have removed that reference from the KIP. Hi Eno, Thank you for providing the reference of the RAID-5. In LinkedIn we have 10 disks per Kafka machine. It will not be a show-stopper operationally for LinkedIn if we have to deploy one-broker-per-disk. On the other hand we previously discussed the advantage of JBOD vs. one-broker-per-disk or one-broker-per-machine. One-broker-per-disk suffers from the problems described in the KIP and one-broker-per-machine increases the failure caused by disk failure by 10X. Since JBOD is strictly better than either of the two, it is also better then one-broker-per-multiple-disk which is somewhere between one-broker-per-disk and one-broker-per-machine. I personally think the benefits of JBOD design is worth the implementation complexity it introduces. I would also argue that it is reasonable for Kafka to manage this low level detail because Kafka is already exposing and managing replication factor of its data. But whether the complexity is worthwhile can be subjective and I can not prove my opinion. I am contributing significant amount of time to do this KIP because Kafka develops at LinkedIn believes it is useful and worth the effort. Yeah, it will be useful to see what everyone else think about it. Thanks, Dong On Mon, Feb 27, 2017 at 1:16 PM, Jun Rao <j...@confluent.io> wrote: > Hi, Dong, > > For RAID5, I am not sure the rebuild cost is a big concern. If a disk > fails, typically an admin has to bring down the broker, replace the failed > disk with a new one, trigger the RAID rebuild, and bring up the broker. > This way, there is no performance impact at runtime due to rebuild. The > benefit is that a broker doesn't fail in a hard way when there is a disk > failure and can be brought down in a controlled way for maintenance. While > the broker is running with a failed disk, reads may be more expensive since > they have to be computed from the parity. However, if most reads are from > page cache, this may not be a big issue either. So, it would be useful to > do some tests on RAID5 before we completely rule it out. > > Regarding whether to remove an offline replica from the fetcher thread > immediately. What do we do when a failed replica is a leader? Do we do > nothing or mark the replica as not the leader immediately? Intuitively, it > seems it's better if the broker acts consistently on a failed replica > whether it's a leader or a follower. For ISR churns, I was just pointing > out that if we don't send StopReplicaRequest to a broker to be shut down in > a controlled way, then the leader will shrink ISR, expand it and shrink it > again after the timeout. > > The KIP seems to still reference " > /broker/topics/[topic]/partitions/[partitionId]/controller_managed_state". > > Thanks, > > Jun > > On Sat, Feb 25, 2017 at 7:49 PM, Dong Lin <lindon...@gmail.com> wrote: > > > Hey Jun, > > > > Thanks for the suggestion. I think it is a good idea to know put created > > flag in ZK and simply specify isNewReplica=true in LeaderAndIsrRequest if > > repilcas was in NewReplica state. It will only fail the replica creation > in > > the scenario that the controller fails after > > topic-creation/partition-reassignment/partition-number-change but before > > actually sends out the LeaderAndIsrRequest while there is ongoing disk > > failure, which should be pretty rare and acceptable. This should simplify > > the design of this KIP. > > > > Regarding RAID-5, I think the concern with RAID-5/6 is not just about > > performance when there is no failure. For example, RAID-5 can support up > to > > one disk failure and it takes time to rebuild disk after one disk > > failure. RAID 5 implementations are susceptible to system failures > because > > of trends regarding array rebuild time and the chance of drive failure > > during rebuild. There is no such performance degradation for JBOD and > JBOD > > can support multiple log directory failure without reducing performance > of > > good log directories. Would this be a reasonable reason for using JBOD > > instead of RAID-5/6? > > > > Previously we discussed wether broker should remove offline replica from > > replica fetcher thread. I still think it should do it instead of > printing a > > lot of error in the log4j log. We can still let controller send > > StopReplicaRequest to the broker. I am not sure I undertand why allowing > > broker to remove offline replica from fetcher thread will increase churns > > in ISR. Do you think this is concern with this approach? > > > > I have updated the KIP to remove created flag from ZK and change the > filed > > name to isNewReplica. Can you check if there is any issue with the latest > > KIP? Thanks for your time! > > > > Regards, > > Dong > > > > > > On Sat, Feb 25, 2017 at 9:11 AM, Jun Rao <j...@confluent.io> wrote: > > > > > Hi, Dong, > > > > > > Thanks for the reply. > > > > > > Personally, I'd prefer not to write the created flag per replica in ZK. > > > Your suggestion of disabling replica creation if there is a bad log > > > directory on the broker could work. The only thing is that it may delay > > the > > > creation of new replicas. I was thinking that an alternative is to > extend > > > LeaderAndIsrRequest by adding a isNewReplica field per replica. That > > field > > > will be set when a replica is transitioning from the NewReplica state > to > > > Online state. Then, when a broker receives a LeaderAndIsrRequest, if a > > > replica is marked as the new replica, it will be created on a good log > > > directory, if not already present. Otherwise, it only creates the > replica > > > if all log directories are good and the replica is not already present. > > > This way, we don't delay the processing of new replicas in the common > > case. > > > > > > I am ok with not persisting the offline replicas in ZK and just > > discovering > > > them through the LeaderAndIsrRequest. It handles the cases when a > broker > > > starts up with bad log directories better. So, the additional overhead > of > > > rediscovering the offline replicas is justified. > > > > > > > > > Another high level question. The proposal rejected RAID5/6 since it > adds > > > additional I/Os. The main issue with RAID5 is that to write a block > that > > > doesn't match the RAID stripe size, we have to first read the old > parity > > to > > > compute the new one, which increases the number of I/Os ( > > > http://rickardnobel.se/raid-5-write-penalty/). I am wondering if you > > have > > > tested RAID5's performance by creating a file system whose block size > > > matches the RAID stripe size (https://www.percona.com/blog/ > > > 2011/12/16/setting-up-xfs-the-simple-edition/). This way, writing a > > block > > > doesn't require a read first. A large block size may increase the > amount > > of > > > data writes, when the same block has to be written to disk multiple > > times. > > > However, this is probably ok in Kafka's use case since we batch the I/O > > > flush already. As you can see, we will be adding some complexity to > > support > > > JBOD in Kafka one way or another. If we can tune the performance of > RAID5 > > > to match that of RAID10, perhaps using RAID5 is a simpler solution. > > > > > > Thanks, > > > > > > Jun > > > > > > > > > On Fri, Feb 24, 2017 at 10:17 AM, Dong Lin <lindon...@gmail.com> > wrote: > > > > > > > Hey Jun, > > > > > > > > I don't think we should allow failed replicas to be re-created on the > > > good > > > > disks. Say there are 2 disks and each of them is 51% loaded. If any > > disk > > > > fail, and we allow replicas to be re-created on the other disks, both > > > disks > > > > will fail. Alternatively we can disable replica creation if there is > > bad > > > > disk on a broker. I personally think it is worth the additional > > > complexity > > > > in the broker to store created replicas in ZK so that we allow new > > > replicas > > > > to be created on the broker even when there is bad log directory. > This > > > > approach won't add complexity in the controller. But I am fine with > > > > disabling replica creation when there is bad log directory that if it > > is > > > > the only blocking issue for this KIP. > > > > > > > > Whether we store created flags is independent of whether/how we store > > > > offline replicas. Per our previous discussion, do you think it is OK > > not > > > > store offline replicas in ZK and propagate the offline replicas from > > > broker > > > > to controller via LeaderAndIsrRequest? > > > > > > > > Thanks, > > > > Dong > > > > > > > > > >
