Apache Kudu Administration

Kudu master processes serve their web interface on port 8051. The interface exposes several pages with information about the cluster state:

Each tablet server serves a web interface on port 8050. The interface exposes information about each tablet hosted on the server, its current state, and debugging information about maintenance background operations.

Both Kudu masters and tablet servers expose a common set of information via their web interfaces:

These interfaces are linked from the landing page of each daemon’s web UI.

The full set of available metrics for a Kudu server can be dumped via a special command line flag:

This will output a large JSON document. Each metric indicates its name, label, description, units, and type. Because the output is JSON-formatted, this information can easily be parsed and fed into other tooling which collects metrics from Kudu servers.

Metrics can be collected from a server process via its HTTP interface by visiting /metrics. The output of this page is JSON for easy parsing by monitoring services. This endpoint accepts several GET parameters in its query string:

For example:

Kudu may be configured to dump various diagnostics information to a local log file. The diagnostics log will be written to the same directory as the other Kudu log files, with a similar naming format, substituting diagnostics instead of a log level like INFO. After any diagnostics log file reaches 64MB uncompressed, the log will be rolled and the previous file will be gzip-compressed.

Each line in the diagnostics log consists of the following components:

Currently, the only type of diagnostics record is a periodic dump of the server metrics. Each record is encoded in compact JSON format, and the server attempts to elide any metrics which have not changed since the previous record. In addition, counters which have never been incremented are elided. Otherwise, the format of the JSON record is identical to the format exposed by the HTTP endpoint above.

The frequency with which metrics are dumped to the diagnostics log is configured using the --metrics_log_interval_ms flag. By default, Kudu logs metrics every 60 seconds.

As of Kudu 1.10.0, Kudu supports both full and incremental table backups via a job implemented using Apache Spark. Additionally it supports restoring tables from full and incremental backups via a restore job implemented using Apache Spark.

Given the Kudu backup and restore jobs use Apache Spark, ensure Apache Spark is installed in your environment following the Spark documentation. Additionally review the Apache Spark documentation for Submitting Applications.

Kudu does not yet provide built-in physical backup and restore functionality. However, it is possible to create a physical backup of a Kudu node (either tablet server or master) and restore it later.

For high availability and to avoid a single point of failure, Kudu clusters should be created with multiple masters. Many Kudu clusters were created with just a single master, either for simplicity or because Kudu multi-master support was still experimental at the time. This workflow demonstrates how to migrate to a multi-master configuration. It can also be used to migrate from two masters to three, with straightforward modifications. Note that the number of masters must be odd.

Kudu multi-master deployments function normally in the event of a master loss. However, it is important to replace the dead master; otherwise a second failure may lead to a loss of availability, depending on the number of available masters. This workflow describes how to replace the dead master.

In the event that a multi-master deployment has been overallocated nodes, the following steps should be taken to remove the unwanted masters.

To prevent long maintenance windows when replacing dead masters, DNS aliases should be used. If the cluster was set up without aliases, changing the host names can be done by following the below steps.

A common workflow when administering a Kudu cluster is adding additional tablet server instances, in an effort to increase storage capacity, decrease load or utilization on individual hosts, increase compute power, etc.

By default, any newly added tablet servers will not be utilized immediately after their addition to the cluster. Instead, newly added tablet servers will only be utilized when new tablets are created or when existing tablets need to be replicated, which can lead to imbalanced nodes. It’s recommended to run the rebalancer CLI tool just after adding a new tablet server into the cluster, as described in the enumerated steps below.

Avoid placing multiple tablet servers on a single node. Doing so nullifies the point of increasing the overall storage capacity of a Kudu cluster and increases the likelihood of tablet unavailability when a single node fails (the latter drawback is not applicable if the cluster is properly configured to use the location awareness feature).

To add additional tablet servers to an existing cluster, the following steps can be taken to ensure tablet replicas are uniformly distributed across the cluster:

The kudu CLI includes a tool named ksck that can be used for gathering information about the state of a Kudu cluster, including checking its health. ksck will identify issues such as under-replicated tablets, unreachable tablet servers, or tablets without a leader.

ksck should be run from the command line as the Kudu admin user, and requires the full list of master addresses to be specified:

To see a full list of the options available with ksck, use the --help flag. If the cluster is healthy, ksck will print information about the cluster, a success message, and return a zero (success) exit status.

If the cluster is unhealthy, for instance if a tablet server process has stopped, ksck will report the issue(s) and return a non-zero exit status, as shown in the abbreviated snippet of ksck output below:

To verify data integrity, the optional --checksum_scan flag can be set, which will ensure the cluster has consistent data by scanning each tablet replica and comparing results. The --tables or --tablets flags can be used to limit the scope of the checksum scan to specific tables or tablets, respectively. For example, checking data integrity on the my_table table can be done with the following command:

By default, ksck will attempt to use a snapshot scan of the table, so the checksum scan can be done while writes continue.

Finally, ksck also supports output in JSON format using the --ksck_format flag. JSON output contains the same information as the plain text output, but in a format that can be used by other tools. See kudu cluster ksck --help for more information.

For higher read parallelism and larger volumes of storage per server, users may want to configure servers to store data in multiple directories on different devices. Users can add or remove data directories to an existing master or tablet server by updating the --fs_data_dirs gflag configuration and restarting the server. Data is striped across data directories, and when a new data directory is added, new data will be striped across the union of the old and new directories.

If on a Kudu version below 1.12, once a server is started, users must go through the below steps to change the directory configuration:

Kudu nodes can only survive failures of disks on which certain Kudu directories are mounted. For more information about the different Kudu directory types, see the section on Kudu Directory Configurations. Below describes this behavior across different Apache Kudu releases.

When a disk failure occurs that does not lead to a crash, Kudu will stop using the affected directory, shut down tablets with blocks on the affected directories, and automatically re-replicate the affected tablets to other tablet servers. The affected server will remain alive and print messages to the log indicating the disk failure, for example:

While in this state, the affected node will avoid using the failed disk, leading to lower storage volume and reduced read parallelism. The administrator can remove the failed directory from the --fs_data_dirs gflag to avoid seeing these errors.

When the disk is repaired, remounted, and ready to be reused by Kudu, take the following steps:

Note that existing tablets will not stripe to the restored disk, but any new tablets will stripe to the restored disk.

By default, Kudu reserves a small amount of space (1% by capacity) in its directories; Kudu considers a disk full if there is less free space available than the reservation. Kudu nodes can only tolerate running out of space on disks on which certain Kudu directories are mounted. For more information about the different Kudu directory types, see Kudu Directory Configurations. The table below describes this behavior for each type of directory. The behavior is uniform across masters and tablet servers.

Prior to Kudu 1.7.0, Kudu stripes tablet data across all directories, and will avoid writing data to full directories. Kudu will crash if all data directories are full.

In 1.7.0 and later, new tablets are assigned a disk group consisting of --fs_target_data_dirs_per_tablet data dirs (default 3). If Kudu is not configured with enough data directories for a full disk group, all data directories are used. When a data directory is full, Kudu will stop writing new data to it and each tablet that uses that data directory will write new data to other data directories within its group. If all data directories for a tablet are full, Kudu will crash. Periodically, Kudu will check if full data directories are still full, and will resume writing to those data directories if space has become available.

If Kudu does crash because its data directories are full, freeing space on the full directories will allow the affected daemon to restart and resume writing. Note that it may be possible for Kudu to free some space by running

but this command may also fail if there is too little space left.

It’s also possible to allocate additional data directories to Kudu in order to increase the overall amount of storage available. See the documentation on updating a node’s directory configuration for more information. Note that existing tablets will not use new data directories, so adding a new data directory does not resolve issues with full disks.

If a tablet has permanently lost a majority of its replicas, it cannot recover automatically and operator intervention is required. If the tablet servers hosting a majority of the replicas are down (i.e. ones reported as "TS unavailable" by ksck), they should be recovered instead if possible.

Suppose a tablet has lost a majority of its replicas. The first step in diagnosing and fixing the problem is to examine the tablet’s state using ksck:

This output shows that, for tablet e822cab6c0584bc0858219d1539a17e6, the two tablet replicas on tserver-01 and tserver-02 failed. The remaining replica is not the leader, so the leader replica failed as well. This means the chance of data loss is higher since the remaining replica on tserver-00 may have been lagging. In general, to accept the potential data loss and restore the tablet from the remaining replicas, divide the tablet replicas into two groups:

For example, in the above ksck output, the replica on tablet server tserver-00 is healthy, while the replicas on tserver-01 and tserver-02 are unhealthy. On each tablet server with a healthy replica, alter the consensus configuration to remove unhealthy replicas. In the typical case of 1 out of 3 surviving replicas, there will be only one healthy replica, so the consensus configuration will be rewritten to include only the healthy replica.

where <tablet-id> is e822cab6c0584bc0858219d1539a17e6 and <tserver-00-uuid> is the uuid of tserver-00, 638a20403e3e4ae3b55d4d07d920e6de.

Once the healthy replicas' consensus configurations have been forced to exclude the unhealthy replicas, the healthy replicas will be able to elect a leader. The tablet will become available for writes, though it will still be under-replicated. Shortly after the tablet becomes available, the leader master will notice that it is under-replicated, and will cause the tablet to re-replicate until the proper replication factor is restored. The unhealthy replicas will be tombstoned by the master, causing their remaining data to be deleted.

In the event that critical files are lost, i.e. WALs or tablet-specific metadata, all Kudu directories on the server must be deleted and rebuilt to ensure correctness. Doing so will destroy the copy of the data for each tablet replica hosted on the local server. Kudu will automatically re-replicate tablet replicas removed in this way, provided the replication factor is at least three and all other servers are online and healthy.

If a single tablet server is brought down temporarily in a healthy cluster, all tablets will remain available and clients will function as normal, after potential short delays due to leader elections. However, if the downtime lasts for more than --follower_unavailable_considered_failed_sec (default 300) seconds, the tablet replicas on the down tablet server will be replaced by new replicas on available tablet servers. This will cause stress on the cluster as tablets re-replicate and, if the downtime lasts long enough, significant reduction in the number of replicas on the down tablet server, which would require the rebalancer to fix.

To work around this, in Kudu versions from 1.11 onwards, the kudu CLI contains a tool to put tablet servers into maintenance mode. While in this state, the tablet server’s replicas are not re-replicated due to its downtime alone, though re-replication may still occur in the event that the server in maintenance suffers from a disk failure or if a follower replica on the tablet server falls too far behind its leader replica. Upon exiting maintenance, re-replication is triggered for any remaining under-replicated tablets.

The kudu tserver state enter_maintenance and kudu tserver state exit_maintenance tools are added to orchestrate tablet server maintenance. The following can be run from a tablet server to put it into maintenance:

The tablet server maintenance mode is shown in the "Tablet Servers" page of the Kudu leader master’s web UI, and in the output of kudu cluster ksck. To exit maintenance mode, run the following:

In versions prior to 1.11, a different approach must be used to prevent unwanted re-replication. Increase --follower_unavailable_considered_failed_sec on all tablet servers so the amount of time before re-replication starts is longer than the expected downtime of the tablet server, including the time it takes the tablet server to restart and bootstrap its tablet replicas. To do this, run the following command for each tablet server:

where <num_seconds> is the number of seconds that will encompass the downtime. Once the downtime is finished, reset the flag to its original value.

As of Kudu 1.12, tooling is available to restart a cluster with no downtime. To perform such a "rolling restart", perform the following sequence:

The kudu CLI contains a rebalancing tool that can be used to rebalance tablet replicas among tablet servers. For each table, the tool attempts to balance the number of replicas per tablet server. It will also, without unbalancing any table, attempt to even out the number of replicas per tablet server across the cluster as a whole. The rebalancing tool should be run as the Kudu admin user, specifying all master addresses:

When run, the rebalancer will report on the initial tablet replica distribution in the cluster, log the replicas it moves, and print a final summary of the distribution when it terminates:

If more details are needed in addition to the replica distribution summary, use the --output_replica_distribution_details flag. If added, the flag makes the tool print per-table and per-tablet server replica distribution statistics as well.

Use the --report_only flag to get a report on table- and cluster-wide replica distribution statistics without starting any rebalancing activity.

The rebalancer can also be restricted to run on a subset of the tables by supplying the --tables flag. Note that, when running on a subset of tables, the tool will not attempt to balance the cluster as a whole.

The length of time rebalancing is run for can be controlled with the flag --max_run_time_sec. By default, the rebalancer will run until the cluster is balanced. To control the amount of resources devoted to rebalancing, modify the flag --max_moves_per_server. See kudu cluster rebalance --help for more.

It’s safe to stop the rebalancer tool at any time. When restarted, the rebalancer will continue rebalancing the cluster.

The rebalancer requires all registered tablet servers to be up and running to proceed with the rebalancing process. That’s to avoid possible conflicts and races with the automatic re-replication and keep replica placement optimal for current configuration of the cluster. If a tablet server becomes unavailable during the rebalancing session, the rebalancer will exit. As noted above, it’s safe to restart the rebalancer after resolving the issue with unavailable tablet servers.

The rebalancing tool can rebalance Kudu clusters running older versions as well, with some restrictions. Consult the following table for more information. In the table, "RF" stands for "replication factor".

If the rebalancer is running against a cluster where rebalancing replication factor one tables is not supported, it will rebalance all the other tables and the cluster as if those singly-replicated tables did not exist.

As detailed in the rack awareness section, it’s possible to use the kudu cluster rebalance tool to establish the placement policy on a cluster. This might be necessary when the rack awareness feature is first configured or when re-replication violated the placement policy. The rebalancing tool breaks its work into three phases:

By using the --report_only flag, it’s also possible to check if all tablets in the cluster conform to the placement policy without attempting any replica movement.

Starting with Kudu 1.12, the Kudu rebalancer tool can be used to decommission a tablet server by supplying the --ignored_tservers and --move_replicas_from_ignored_tservers arguments.

When using the kudu command line tool, it can be difficult to remember the precise list of Kudu master RPC addresses needed to communicate with a cluster, especially when managing multiple clusters. As an alternative, the command line tool can identify clusters by name. To use this functionality: