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https://issues.apache.org/jira/browse/KAFKA-16344?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17830685#comment-17830685
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Greg Harris commented on KAFKA-16344:
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Hi [~janardhanag] Yes, you should consider increasing offset.lag.max. 0 is best
for precise offset translation, but can significantly increase the amount of
traffic on the offset-syncs topic.
For topics without offset gaps (e.g. not compacted, not transactional) the
offset.lag.max currently behaves as a ratio between the amount of records/sec
in a mirrored topic, and that topic's additional load on the offset-syncs
topic. There's a semaphore
https://github.com/apache/kafka/blob/f8ce7feebcede6c6da93c649413a64695bc8d4c1/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L95
that prevents this load from slowing down the mirror source connector, but it
won't protect the brokers holding the mm2 offsets topic from being overloaded.
For example, if you were to mirror 1000000 records, offset.lag.max=0 could
write 1000000 records to the offset-syncs topic, generating huge load. For
1000000 records with offset.lag.max=100, the number of offset syncs could be
10000, 100x less. For offset.lag.max=10000, the number of offset syncs could be
100, 10000x less. To answer your questions:
> 1. What is the proportion of mm2-offsetsyncsinternal topic writes to overall
>MM2 traffic?
Configurable with offset.lag.max for topics with contiguous offsets. For
transactional topics it isn't configurable, as each transaction may cause 1 or
2 offset syncs and that isn't limited by the offset.lag.max, so reducing the
throughput may require increasing the transaction intervals on your source
applications.
> 2. Is there a way to tune the internal topic writes with increased traffic
> MM2?
As the throughput of your MM2 instance increases, you should decrease the
offset.lag.max to keep the offset-syncs topic fixed.
> 3. I want to understand the expected mm2-offsetsyncsinternal write TPS for a
> given amount of MM2 traffic. Are there any tunable parameters to reduce these
> writes, and what are the consequences of tuning if any?
There isn't a closed formula, because a sync may be initiated by multiple
conditions
[https://github.com/apache/kafka/blob/f8ce7feebcede6c6da93c649413a64695bc8d4c1/connect/mirror/src/main/java/org/apache/kafka/connect/mirror/MirrorSourceTask.java#L360-L363]
and prevented by the semaphore when the producer latency becomes significant.
The consequence of decreasing throughput on this topic is less precise offset
translation in the MirrorCheckpointConnector, and increased redelivery when
failing over from source to target consumer offsets. If you aren't using this
connector, then the offset syncs topic isn't read from at all. You currently
can't turn the topic off, but there is a KIP open for that:
[https://cwiki.apache.org/confluence/display/KAFKA/KIP-1031%3A+Control+offset+translation+in+MirrorSourceConnector]
. If for example you were satisfied with 30 seconds of redelivery for a topic
which had 1000 records/second, then you could set offset.lag.max to 30*1000.
> 4. In a larger Kafka cluster, if a single broker is overloaded with
> mm2-offsetsyncsinternal traffic, can it lead to a broker crash? Are there any
> guidelines available for such scenarios? Currently, we have 6 brokers with
> 24K MM2 traffic, and internal writes are at 10K, resulting in a 20% CPU
> increase on one broker.
I'm not familiar with the failure conditions for overloaded brokers. If you are
seeing failures then I would definitely recommend trying to tune MM2, or try to
use quotas [https://kafka.apache.org/documentation/#design_quotas] to get this
topic-partition under control.
> 5. Are there any limitations on Kafka brokers scaling , I mean how much kafka
>broker can be expanded in single Kakfa cluster due to the
>mm2-offsetsyncsinternal topic?
Because the offset syncs topic only supports a single partition, adding brokers
will not solve this problem, other than having a broker dedicated to only
serving this one topic. At some point, you would need two separate offset-syncs
topics and separate MirrorSourceConnectors to shard the load.
> 6. How can the system be dimensioned to handle MM2 internal topic writes
> effectively? Are there any recommended figures available? For instance, for a
> given amount of traffic (X), what percentage increase in CPU (Y) should each
> broker have to handle MM2 internal topic writes? Note that in other pods,
> this resource may not be utilized.
The Kafka project doesn't publish "expected" dimensions, other than the default
values of configurations. We don't publish performance analysis on any
particular hardware setups, because the diversity of hardware running Kafka is
just too vast to capture properly.
> Internal topic mm2-offset-syncs<clustername>internal created with single
> partition is putting more load on the broker
> ---------------------------------------------------------------------------------------------------------------------
>
> Key: KAFKA-16344
> URL: https://issues.apache.org/jira/browse/KAFKA-16344
> Project: Kafka
> Issue Type: Bug
> Components: connect
> Affects Versions: 3.5.1
> Reporter: Janardhana Gopalachar
> Priority: Major
>
> We are using Kafka 3.5.1 version, we see that the internal topic created by
> mirrormaker
> mm2-offset-syncs<clustername>internal is created with single partition due to
> which the CPU load on the broker which will be leader for this partition is
> increased compared to other brokers. Can multiple partitions be created for
> the topic so that the CPU load would get distributed
>
> Topic: mm2-offset-syncscluster-ainternal TopicId: XRvTDbogT8ytNhqX2YTyrA
> PartitionCount: 1ReplicationFactor: 3 Configs:
> min.insync.replicas=2,cleanup.policy=compact,message.format.version=3.0-IV1
> Topic: mm2-offset-syncscluster-ainternal Partition: 0 Leader: 2
> Replicas: 2,1,0 Isr: 2,1,0
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