Status

Current state: [One of "Under Discussion", "Accepted", "Rejected"]

Discussion thread: 

JIRA: KAFKA-20595 - Getting issue details... STATUS

Motivation

During broker startup, LogManager parallelizes log loading across partitions using a per-data-directory thread pool controlled by num.recovery.threads.per.data.dir. However, within each partition, LogLoader.loadSegmentFiles() processes segment files strictly sequentially: each LogSegment is opened and sanity-checked one at a time before proceeding to the next segment. This design becomes a startup bottleneck for brokers with a large number of segments per partition.

The sequential per-partition loading phase becomes the dominant startup cost. Increasing num.recovery.threads.per.data.dir only increases the number of partitions loaded concurrently, while segment loading within each partition remains serialized. As a result, broker startup time scales approximately linearly with the maximum segment count of any partition, regardless of the number of available CPU cores or recovery threads.

This KIP proposes parallelizing the per-partition segment loading phase. A new broker configuration, num.segment.loading.threads.per.data.dir, controls the thread pool size used to concurrently open and sanity-check segment files within a partition.

Public Interfaces

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Test Plan

Describe in few sentences how the KIP will be tested. We are mostly interested in system tests (since unit-tests are specific to implementation details). How will we know that the implementation works as expected? How will we know nothing broke?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.