Status

Current state: Under Discussion

Discussion thread: here

JIRA: here

Please keep the discussion on the mailing list rather than commenting on the wiki (wiki discussions get unwieldy fast).

Motivation

Storage is one of the key resources in a Kafka cluster. Administrators typically monitor the disk usage of each log directory via metrics to enable them to properly manage the storage attached to brokers. In order to build advanced tooling and automation, it would be useful to also be able to retrieve disk capacity and usable space directly via the Kafka API. That would allow to easily validate whether disk operations (like a resize), or topic deletion (log deletion only happen after a short delay) have completed.

Public Interfaces

We already have the DescribeLogDirs API that returns logdirs and details about the replicas they contain. To expose logdirs total and usable space, this KIP proposes adding 2 new fields to the DescribeLogDirsResponse message and bumping its protocol version to 4. The LogDirDescription class will also be updated to expose these 2 new fields to the Admin API.

Proposed Changes

DescribeLogDirs v4

No changes in the Request. Two new fields are added to the Response: TotalSpace and UsableSpace

{
  "apiKey": 35,
  "type": "response",
  "name": "DescribeLogDirsResponse",
  // Starting in version 1, on quota violation, brokers send out responses before throttling.
  "validVersions": "0-4",
  // Version 2 is the first flexible version.
  // Version 3 adds the top-level ErrorCode field
  // Version 4 adds the TotalSpace and UsableSpace fields
  "flexibleVersions": "2+",
  "fields": [
    { "name": "ThrottleTimeMs", "type": "int32", "versions": "0+",
      "about": "The duration in milliseconds for which the request was throttled due to a quota violation, or zero if the request did not violate any quota." },
    { "name": "ErrorCode", "type": "int16", "versions": "3+", "about": "The error code, or 0 if there was no error." },
    { "name": "Results", "type": "[]DescribeLogDirsResult", "versions": "0+",
      "about": "The log directories.", "fields": [
      { "name": "ErrorCode", "type": "int16", "versions": "0+",
        "about": "The error code, or 0 if there was no error." },
      { "name": "LogDir", "type": "string", "versions": "0+",
        "about": "The absolute log directory path." },
      { "name": "Topics", "type": "[]DescribeLogDirsTopic", "versions": "0+",
        "about": "Each topic.", "fields": [
          ...
        ]}
      ]},
      { "name": "TotalSpace", "type": "int64", "versions": "4+", "ignorable": true, "default": "-1",
        "about": "The total size in bytes of the log directory."
      },
      { "name": "UsableSpace", "type": "int64", "versions": "4+", "ignorable": true, "default": "-1",
        "about": "The usable size in bytes of the log directory."
      }
    ]}
  ]
}

ReplicaManager

When handling a DescribeLogDirs request, ReplicaManager will retrieve the totalSpace and usableSpace in bytes from each logdir. In case these sizes are larger than Long.MAX_VALUE, (see https://bugs.openjdk.java.net/browse/JDK-8162520), brokers will return Long.MAX_VALUE.

LogDirDescription

LogDirDescription is used by the Admin API to represent the results from describeLogDirs(). Two new methods are added to this type as well as a new constant:

public static final long UNKNOWN_SPACE = -1L;


public long totalSpace() { return totalSpace; }

public long usableSpace() { return usableSpace; }

When calling describeLogDirs() on a broker that does not support this feature, totalSpace() and usableSpace() will return UNKNOWN_SPACE.

Compatibility, Deprecation, and Migration Plan

Only new clients will use the new version, this does not change the behavior of existing clients.

Rejected Alternatives

None