Apache Kudu Troubleshooting

Startup Errors

Errors During Hole Punching Test

Kudu requires hole punching capabilities in order to be efficient. Hole punching support depends upon your operation system kernel version and local filesystem implementation.

  • RHEL or CentOS 6.4 or later, patched to kernel version of 2.6.32-358 or later. Unpatched RHEL or CentOS 6.4 does not include a kernel with support for hole punching.

  • Ubuntu 14.04 includes version 3.13 of the Linux kernel, which supports hole punching.

  • Newer versions of the ext4 and xfs filesystems support hole punching. Older versions that do not support hole punching will cause Kudu to emit an error message such as the following and to fail to start:

    Error during hole punch test. The log block manager requires a
    filesystem with hole punching support such as ext4 or xfs. On el6,
    kernel version 2.6.32-358 or newer is required. To run without hole
    punching (at the cost of some efficiency and scalability), reconfigure
    Kudu to use the file block manager. Refer to the Kudu documentation for
    more details. WARNING: the file block manager is not suitable for
    production use and should be used only for small-scale evaluation and
    development on systems where hole-punching is not available. It's
    impossible to switch between block managers after data is written to the
    server. Raw error message follows
ext4 mountpoints may actually be backed by ext2 or ext3 formatted devices, which do not support hole punching. The hole punching test will fail when run on such filesystems. There are several different ways to determine whether an ext4 mountpoint is backed by an ext2, ext3, or ext4 formatted device; see this Stack Exchange post for details.

Without hole punching support, the log block manager is unsafe to use. It won’t ever delete blocks, and will consume ever more space on disk.

If you can’t use hole punching in your environment, you can still try Kudu. Enable the file block manager instead of the log block manager by adding the --block_manager=file flag to the commands you use to start the master and tablet servers. The file block manager does not scale as well as the log block manager.

The file block manager is known to scale and perform poorly, and should only be used for small-scale evaluation and development, and only on systems where hole punching is unavailable.

The file block manager uses one file per block. As multiple blocks are written for each rowset, the number of blocks can be very high, especially for actively written tablets. This can cause performance issues compared to the log block manager even with a small amount of data and it’s impossible to switch between block managers without wiping and reinitializing the tablet servers.

Already present: FS layout already exists

When Kudu starts, it checks each configured data directory, expecting either for all to be initialized or for all to be empty. If a server fails to start with a log message like

Check failed: _s.ok() Bad status: Already present: FS layout already exists; not overwriting existing layout: FSManager roots already exist: /data0/kudu/data

then this precondition has failed. This could be because Kudu was configured with non-empty data directories on first startup, or because a previously-running, healthy Kudu process was restarted and at least one data directory was deleted or is somehow corrupted, perhaps because of a disk error. If in the latter situation, consult the Changing Directory Configurations documentation.

NTP Clock Synchronization

For the master and tablet server daemons, the server’s clock must be synchronized using NTP. In addition, the maximum clock error (not to be mistaken with the estimated error) be below a configurable threshold. The default value is 10 seconds, but it can be set with the flag --max_clock_sync_error_usec.

If NTP is not installed, or if the clock is reported as unsynchronized, Kudu will not start, and will emit a message such as:

F0924 20:24:36.336809 14550 hybrid_clock.cc:191 Couldn't get the current time: Clock unsynchronized. Status: Service unavailable: Error reading clock. Clock considered unsynchronized.

If NTP is installed and synchronized, but the maximum clock error is too high, the user will see a message such as:

Sep 17, 8:13:09.873 PM FATAL hybrid_clock.cc:196 Couldn't get the current time: Clock synchronized, but error: 11130000, is past the maximum allowable error: 10000000


Sep 17, 8:32:31.135 PM FATAL tablet_server_main.cc:38 Check failed: _s.ok() Bad status: Service unavailable: Cannot initialize clock: Cannot initialize HybridClock. Clock synchronized but error was too high (11711000 us).

Installing NTP

To install NTP, use the appropriate command for your operating system:

OS Command


sudo apt-get install ntp


sudo yum install ntp

If NTP is installed but not running, start it using one of these commands:

OS Command


sudo service ntp restart


sudo /etc/init.d/ntpd restart

Monitoring NTP Status

When NTP is installed, you can monitor the synchronization status by running ntptime. For example, a healthy system may report:

ntp_gettime() returns code 0 (OK)
  time de24c0cf.8d5da274  Tue, Feb  6 2018 16:03:27.552, (.552210980),
  maximum error 224455 us, estimated error 383 us, TAI offset 0
ntp_adjtime() returns code 0 (OK)
  modes 0x0 (),
  offset 1279.543 us, frequency 2.500 ppm, interval 1 s,
  maximum error 224455 us, estimated error 383 us,
  status 0x2001 (PLL,NANO),
  time constant 10, precision 0.001 us, tolerance 500 ppm,

In particular, note the following most important pieces of output:

  • maximum error 22455 us: this value is well under the 10-second maximum error required by Kudu.

  • status 0x2001 (PLL,NANO): this indicates a healthy synchronization status.

In contrast, a system without NTP properly configured and running will output something like the following:

ntp_gettime() returns code 5 (ERROR)
  time de24c240.0c006000  Tue, Feb  6 2018 16:09:36.046, (.046881),
  maximum error 16000000 us, estimated error 16000000 us, TAI offset 0
ntp_adjtime() returns code 5 (ERROR)
  modes 0x0 (),
  offset 0.000 us, frequency 2.500 ppm, interval 1 s,
  maximum error 16000000 us, estimated error 16000000 us,
  status 0x40 (UNSYNC),
  time constant 10, precision 1.000 us, tolerance 500 ppm,

Note the UNSYNC status and the 16-second maximum error.

If more detailed information is needed, the ntpq or ntpdc tools can be used to dump further information about which network time servers are currently acting as sources:

$ ntpq -nc lpeers
     remote           refid      st t when poll reach   delay   offset  jitter
-     2 u   13   64    1   71.743    0.373   0.016
+      2 u   12   64    1   72.583   -0.426   0.028
-    3 u   11   64    1   15.741    2.641   0.021
-     3 u   10   64    1   43.502    0.199   0.029
-   2 u    9   64    1   53.885   -0.016   0.013
* .CDMA.           1 u    6   64    1    1.475   -0.400   0.012
+   2 u    7   64    1    1.342   -0.640   0.018     2 u    6   64    1   17.380   -0.754   0.051    2 u    5   64    1   57.796   -3.411   0.059   .GPSs.           1 u    4   64    1    1.024   -0.374   0.018  3 u    3   64    1   72.409    0.895   0.964    2 u    2   64    1  135.195   -0.329   0.171  2 u    1   64    1   28.570    0.693   0.306   .GPS.            1 u    2   64    1   55.652   -0.039   0.019    2 u    1   64    1  135.265   -0.413   0.037
$ ntpq -nc opeers
     remote           local      st t when poll reach   delay   offset    disp
-    2 u   17   64    1   71.743    0.373 187.573
+    2 u   16   64    1   72.583   -0.426 187.594
-    3 u   15   64    1   15.741    2.641 187.569
-    3 u   14   64    1   43.502    0.199 187.580
-    2 u   13   64    1   53.885   -0.016 187.561
*    1 u   10   64    1    1.475   -0.400 187.543
+    2 u   11   64    1    1.342   -0.640 187.588    2 u   10   64    1   17.380   -0.754 187.596    2 u    9   64    1   57.796   -3.411 187.541    1 u    8   64    1    1.024   -0.374 187.578    3 u    7   64    1   72.409    0.895 187.589    2 u    6   64    1  135.195   -0.329 187.584    2 u    5   64    1   28.570    0.693 187.606    1 u    4   64    1   55.652   -0.039 187.587    2 u    3   64    1  135.265   -0.413 187.621
Both opeers and lpeers may be helpful as lpeers lists refid and jitter, while lpeers lists dispersion.
Using chrony for time synchronization

Some operating systems offer chrony as an alternative to ntpd for network time synchronization. Kudu has been tested most thoroughly using ntpd and use of chrony is considered experimental.

In order to use chrony for synchronization, chrony.conf must be configured with the rtcsync option.

NTP Configuration Best Practices

In order to provide stable time synchronization with low maximum error, follow these best NTP configuration best practices.

Always configure at least four time sources for NTP. In addition to providing redundancy in case one or more time sources becomes unavailable, The NTP protocol is designed to increase its accuracy with a diversity of sources. Even if your organization provides one or more local time servers, configuring additional remote servers is highly recommended for a robust setup.

Pick servers in your server’s local geography. For example, if your servers are located in Europe, pick servers from the European NTP pool. If your servers are running in a public cloud environment, consult the cloud provider’s documentation for a recommended NTP setup. Many cloud providers offer highly accurate clock synchronization as a service.

Use the iburst option for faster synchronization at startup. The iburst option instructs ntpd to send an initial "burst" of time queries at startup. This typically results in a faster time synchronization when a machine restarts.

An example NTP server list may appear as follows:

# Use my organization's internal NTP servers.
server ntp1.myorg.internal iburst
server ntp2.myorg.internal iburst
# Provide several public pool servers for
# redundancy and robustness.
server 0.pool.ntp.org iburst
server 1.pool.ntp.org iburst
server 2.pool.ntp.org iburst
server 3.pool.ntp.org iburst
After configuring NTP, use the ntpq tool described above to verify that ntpd was able to connect to a variety of peers. If no public peers appear, it is possiblbe that the NTP protocol is being blocked by a firewall or other network connectivity issue.

Troubleshooting NTP Stability Problems

As of Kudu 1.6.0, Kudu daemons are able to continue to operate during a brief loss of NTP synchronization. If NTP synchronization is lost for several hours, however, daemons may crash. If a daemon crashes due to NTP synchronization issues, consult the ERROR log for a dump of related information which may help to diagnose the issue.

Kudu 1.5.0 and earlier versions were less resilient to brief NTP outages. In addition, they contained a bug which could cause Kudu to incorrectly measure the maximum error, resulting in crashes. If you experience crashes related to clock synchronization on these earlier versions of Kudu and it appears that the system’s NTP configuration is correct, consider upgrading to Kudu 1.6.0 or later.
NTP requires a network connection and may take a few minutes to synchronize the clock at startup. In some cases a spotty network connection may make NTP report the clock as unsynchronized. A common, though temporary, workaround for this is to restart NTP with one of the commands above.

Disk Space Usage

When using the log block manager (the default on Linux), Kudu uses sparse files to store data. A sparse file has a different apparent size than the actual amount of disk space it uses. This means that some tools may inaccurately report the disk space used by Kudu. For example, the size listed by ls -l does not accurately reflect the disk space used by Kudu data files:

$ ls -lh /data/kudu/tserver/data
total 117M
-rw------- 1 kudu kudu 160M Mar 26 19:37 0b9807b8b17d48a6a7d5b16bf4ac4e6d.data
-rw------- 1 kudu kudu 4.4K Mar 26 19:37 0b9807b8b17d48a6a7d5b16bf4ac4e6d.metadata
-rw------- 1 kudu kudu  32M Mar 26 19:37 2f26eeacc7e04b65a009e2c9a2a8bd20.data
-rw------- 1 kudu kudu 4.3K Mar 26 19:37 2f26eeacc7e04b65a009e2c9a2a8bd20.metadata
-rw------- 1 kudu kudu 672M Mar 26 19:37 30a2dd2cd3554d8a9613f588a8d136ff.data
-rw------- 1 kudu kudu 4.4K Mar 26 19:37 30a2dd2cd3554d8a9613f588a8d136ff.metadata
-rw------- 1 kudu kudu  32M Mar 26 19:37 7434c83c5ec74ae6af5974e4909cbf82.data
-rw------- 1 kudu kudu 4.3K Mar 26 19:37 7434c83c5ec74ae6af5974e4909cbf82.metadata
-rw------- 1 kudu kudu 672M Mar 26 19:37 772d070347a04f9f8ad2ad3241440090.data
-rw------- 1 kudu kudu 4.4K Mar 26 19:37 772d070347a04f9f8ad2ad3241440090.metadata
-rw------- 1 kudu kudu 160M Mar 26 19:37 86e50a95531f46b6a79e671e6f5f4151.data
-rw------- 1 kudu kudu 4.4K Mar 26 19:37 86e50a95531f46b6a79e671e6f5f4151.metadata
-rw------- 1 kudu kudu  687 Mar 26 19:26 block_manager_instance

Notice that the total size reported is 117MiB, while the first file’s size is listed as 160MiB. Adding the -s option to ls will cause ls to output the file’s disk space usage.

The du and df utilities report the actual disk space usage by default.

$ du -h /data/kudu/tserver/data
118M   /data/kudu/tserver/data

The apparent size can be shown with the --apparent-size flag to du.

$ du -h --apparent-size /data/kudu/tserver/data
1.7G  /data/kudu/tserver/data

Reporting Kudu Crashes

Kudu uses the Google Breakpad library to generate a minidump whenever Kudu experiences a crash. These minidumps are typically only a few MB in size and are generated even if core dump generation is disabled. At this time, generating minidumps is only possible in Kudu on Linux builds.

A minidump file contains important debugging information about the process that crashed, including shared libraries loaded and their versions, a list of threads running at the time of the crash, the state of the processor registers and a copy of the stack memory for each thread, and CPU and operating system version information.

It is also possible to force Kudu to create a minidump without killing the process by sending a USR1 signal to the kudu-tserver or kudu-master process. For example:

sudo pkill -USR1 kudu-tserver

By default, Kudu stores its minidumps in a subdirectory of its configured glog directory called minidumps. This location can be customized by setting the --minidump_path flag. Kudu will retain only a certain number of minidumps before deleting the oldest ones, in an effort to avoid filling up the disk with minidump files. The maximum number of minidumps that will be retained can be controlled by setting the --max_minidumps gflag.

Minidumps contain information specific to the binary that created them and so are not usable without access to the exact binary that crashed, or a very similar binary. For more information on processing and using minidump files, see scripts/dump_breakpad_symbols.py.

A minidump can be emailed to a Kudu developer or attached to a JIRA in order to help a Kudu developer debug a crash. In order for it to be useful, the developer will need to know the exact version of Kudu and the operating system where the crash was observed. Note that while a minidump does not contain a heap memory dump, it does contain stack memory and therefore it is possible for application data to appear in a minidump. If confidential or personal information is stored on the cluster, do not share minidump files.

Performance Troubleshooting

Kudu Tracing

The kudu-master and kudu-tserver daemons include built-in tracing support based on the open source Chromium Tracing framework. You can use tracing to help diagnose latency issues or other problems on Kudu servers.

Accessing the tracing interface

The tracing interface is accessed via a web browser as part of the embedded web server in each of the Kudu daemons.

Table 1. Tracing Interface URLs
Daemon URL

Tablet Server




The tracing interface is known to work in recent versions of Google Chrome. Other browsers may not work as expected.

Collecting a trace

After navigating to the tracing interface, click the Record button on the top left corner of the screen. When beginning to diagnose a problem, start by selecting all categories. Click Record to begin recording a trace.

During the trace collection, events are collected into an in-memory ring buffer. This ring buffer is fixed in size, so it will eventually fill up to 100%. However, new events are still being collected while older events are being removed. While recording the trace, trigger the behavior or workload you are interested in exploring.

After collecting for several seconds, click Stop. The collected trace will be downloaded and displayed. Use the ? key to display help text about using the tracing interface to explore the trace.

Saving a trace

You can save collected traces as JSON files for later analysis by clicking Save after collecting the trace. To load and analyze a saved JSON file, click Load and choose the file.

RPC Timeout Traces

If client applications are experiencing RPC timeouts, the Kudu tablet server WARNING level logs should contain a log entry which includes an RPC-level trace. For example:

W0922 00:56:52.313848 10858 inbound_call.cc:193] Call kudu.consensus.ConsensusService.UpdateConsensus
from (request call id 3555909) took 1464ms (client timeout 1000).
W0922 00:56:52.314888 10858 inbound_call.cc:197] Trace:
0922 00:56:50.849505 (+     0us) service_pool.cc:97] Inserting onto call queue
0922 00:56:50.849527 (+    22us) service_pool.cc:158] Handling call
0922 00:56:50.849574 (+    47us) raft_consensus.cc:1008] Updating replica for 2 ops
0922 00:56:50.849628 (+    54us) raft_consensus.cc:1050] Early marking committed up to term: 8 index: 880241
0922 00:56:50.849968 (+   340us) raft_consensus.cc:1056] Triggering prepare for 2 ops
0922 00:56:50.850119 (+   151us) log.cc:420] Serialized 1555 byte log entry
0922 00:56:50.850213 (+    94us) raft_consensus.cc:1131] Marking committed up to term: 8 index: 880241
0922 00:56:50.850218 (+     5us) raft_consensus.cc:1148] Updating last received op as term: 8 index: 880243
0922 00:56:50.850219 (+     1us) raft_consensus.cc:1195] Filling consensus response to leader.
0922 00:56:50.850221 (+     2us) raft_consensus.cc:1169] Waiting on the replicates to finish logging
0922 00:56:52.313763 (+1463542us) raft_consensus.cc:1182] finished
0922 00:56:52.313764 (+     1us) raft_consensus.cc:1190] UpdateReplicas() finished
0922 00:56:52.313788 (+    24us) inbound_call.cc:114] Queueing success response

These traces can give an indication of which part of the request was slow. Please include them in bug reports related to RPC latency outliers.

Kernel Stack Watchdog Traces

Each Kudu server process has a background thread called the Stack Watchdog, which monitors the other threads in the server in case they have blocked for longer-than-expected periods of time. These traces can indicate operating system issues or bottlenecked storage.

When the watchdog thread identifies a case of thread blockage, it logs an entry in the WARNING log like the following:

W0921 23:51:54.306350 10912 kernel_stack_watchdog.cc:111] Thread 10937 stuck at /data/kudu/consensus/log.cc:505 for 537ms:
Kernel stack:
[<ffffffffa00b209d>] do_get_write_access+0x29d/0x520 [jbd2]
[<ffffffffa00b2471>] jbd2_journal_get_write_access+0x31/0x50 [jbd2]
[<ffffffffa00fe6d8>] __ext4_journal_get_write_access+0x38/0x80 [ext4]
[<ffffffffa00d9b23>] ext4_reserve_inode_write+0x73/0xa0 [ext4]
[<ffffffffa00d9b9c>] ext4_mark_inode_dirty+0x4c/0x1d0 [ext4]
[<ffffffffa00d9e90>] ext4_dirty_inode+0x40/0x60 [ext4]
[<ffffffff811ac48b>] __mark_inode_dirty+0x3b/0x160
[<ffffffff8119c742>] file_update_time+0xf2/0x170
[<ffffffff8111c1e0>] __generic_file_aio_write+0x230/0x490
[<ffffffff8111c4c8>] generic_file_aio_write+0x88/0x100
[<ffffffffa00d3fb1>] ext4_file_write+0x61/0x1e0 [ext4]
[<ffffffff81180f5b>] do_sync_readv_writev+0xfb/0x140
[<ffffffff81181ee6>] do_readv_writev+0xd6/0x1f0
[<ffffffff81182046>] vfs_writev+0x46/0x60
[<ffffffff81182102>] sys_pwritev+0xa2/0xc0
[<ffffffff8100b072>] system_call_fastpath+0x16/0x1b
[<ffffffffffffffff>] 0xffffffffffffffff

User stack:
    @       0x3a1ace10c4  (unknown)
    @          0x1262103  (unknown)
    @          0x12622d4  (unknown)
    @          0x12603df  (unknown)
    @           0x8e7bfb  (unknown)
    @           0x8f478b  (unknown)
    @           0x8f55db  (unknown)
    @          0x12a7b6f  (unknown)
    @       0x3a1b007851  (unknown)
    @       0x3a1ace894d  (unknown)
    @              (nil)  (unknown)

These traces can be useful for diagnosing root-cause latency issues when they are caused by systems below Kudu, such as disk controllers or filesystems.

Memory Limits

Kudu has a hard and soft memory limit. The hard memory limit is the maximum amount a Kudu process is allowed to use, and is controlled by the --memory_limit_hard_bytes flag. The soft memory limit is a percentage of the hard memory limit, controlled by the flag memory_limit_soft_percentage and with a default value of 80%, that determines the amount of memory a process may use before it will start rejecting some write operations.

If the logs or RPC traces contain messages like

Service unavailable: Soft memory limit exceeded (at 96.35% of capacity)

then Kudu is rejecting writes due to memory backpressure. This may result in write timeouts. There are several ways to relieve the memory pressure on Kudu:

  • If the host has more memory available for Kudu, increase --memory_limit_hard_bytes.

  • Increase the rate at which Kudu can flush writes from memory to disk by increasing the number of disks or increasing the number of maintenance manager threads --maintenance_manager_num_threads. Generally, the recommended ratio of maintenance manager threads to data directories is 1:3.

  • Reduce the volume of writes flowing to Kudu on the application side.

Finally, on versions of Kudu prior to 1.8, check the value of --block_cache_capacity_mb. This setting determines the maximum size of Kudu’s block cache. While a higher value can help with read and write performance, do not raise --block_cache_capacity_mb above the memory pressure threshold, which is --memory_pressure_percentage (default 60%) of --memory_limit_hard_bytes, as this will cause Kudu to flush aggressively even if write throughput is low. Keeping --block_cache_capacity_mb below 50% of the memory pressure threshold is recommended. With the defaults, this means --block_cache_capacity_mb should not exceed 30% of --memory_limit_hard_bytes. On Kudu 1.8 and higher, servers will refuse to start if the block cache capacity exceeds the memory pressure threshold.

Block Cache Size

Kudu uses an LRU cache for recently read data. On workloads that scan a subset of the data repeatedly, raising the size of this cache can offer significant performance benefits. To increase the amount of memory dedicated to the block cache, increase the value of the flag --block_cache_capacity_mb. The default is 512MiB.

Kudu provides a set of useful metrics for evaluating the performance of the block cache, which can be found on the /metrics endpoint of the web UI. An example set:

  "name": "block_cache_inserts",
  "value": 64
  "name": "block_cache_lookups",
  "value": 512
  "name": "block_cache_evictions",
  "value": 0
  "name": "block_cache_misses",
  "value": 96
  "name": "block_cache_misses_caching",
  "value": 64
  "name": "block_cache_hits",
  "value": 0
  "name": "block_cache_hits_caching",
  "value": 352
  "name": "block_cache_usage",
  "value": 6976

To judge the efficiency of the block cache on a tablet server, first wait until the server has been running and serving normal requests for some time, so the cache is not cold. Unless the server stores very little data or is idle, block_cache_usage should be equal or nearly equal to block_cache_capacity_mb. Once the cache has reached steady state, compare block_cache_lookups to block_cache_misses_caching. The latter metric counts the number of blocks that Kudu expected to read from cache but which weren’t found in the cache. If a significant amount of lookups result in misses on expected cache hits, and the block_cache_evictions metric is significant compared to block_cache_inserts, then raising the size of the block cache may provide a performance boost. However, the utility of the block cache is highly dependent on workload, so it’s necessary to test the benefits of a larger block cache.

Do not raise the block cache size --block_cache_capacity_mb higher than the memory pressure threshold (defaults to 60% of --memory_limit_hard_bytes). As this would cause poor flushing behavior, Kudu servers version 1.8 and higher will refuse to start when misconfigured in this way.

Heap Sampling

For advanced debugging of memory usage, released builds of Kudu enable Heap Sampling by default. This allows Kudu developers to associate memory usage with the specific lines of code and data structures responsible. When reporting a bug related to memory usage or an apparent memory leak, heap profiling can give quantitative data to pinpoint the issue.

If heap sampling is enabled, the current sampled heap occupancy can be retrieved over HTTP by visiting http://tablet-server.example.com:8050/pprof/heap or http://master.example.com:8051/pprof/heap. The output is a machine-readable dump of the stack traces with their associated heap usage.

Rather than visiting the heap profile page directly in a web browser, it is typically more useful to use the pprof tool that is distributed as part of the gperftools open source project. For example, a developer with a local build tree can use the following command to collect the sampled heap usage and output an SVG diagram:

thirdparty/installed/uninstrumented/bin/pprof -svg  'http://localhost:8051/pprof/heap' > /tmp/heap.svg

The resulting SVG may be visualized in a web browser or sent to the Kudu community to help troubleshoot memory occupancy issues.

Heap samples contain only summary information about allocations and do not contain any data from the heap. It is safe to share heap samples in public without fear of exposing confidential or sensitive data.

Slow DNS Lookups and nscd

For better scalability on nodes hosting many replicas, we recommend that you use nscd (name service cache daemon) to cache both DNS name resolution and static name resolution (via /etc/hosts).

When DNS lookups are slow, you will see a log message similar to the following:

W0926 11:19:01.339553 27231 net_util.cc:129] Time spent resolving address for kudu-tserver.example.com: real 4.647s    user 0.000s     sys 0.000s

nscd (name service cache daemon) can alleviate slow name resolution by providing a cache for the most common name service requests, such as for passwords, groups, and hosts.

Refer to your operating system documentation for how to install and enable nscd.

Issues using Kudu

ClassNotFoundException: com.cloudera.kudu.hive.KuduStorageHandler

Users will encounter this exception when trying to use a Kudu table via Hive. This is not a case of a missing jar, but simply that Impala stores Kudu metadata in Hive in a format that’s unreadable to other tools, including Hive itself and Spark. There is no workaround for Hive users. Spark users need to create temporary tables.

Runtime error: Could not create thread: Resource temporarily unavailable (error 11)

Users will encounter this error when Kudu is unable to create more threads, usually on versions of Kudu older than 1.7. It happens on tablet servers, and is a sign that the tablet server hosts too many tablet replicas. To fix the issue, users can raise the nproc ulimit as detailed in the documentation for their operating system or distribution. However, the better solution is to reduce the number of replicas on the tablet server. This may involve rethinking the table’s partitioning schema. For the recommended limits on number of replicas per tablet server, see the known issues and scaling limitations documentation for the appropriate Kudu release. The releases page has links to documentation for previous versions of Kudu; for the latest release, see the known issues page.

Tombstoned or STOPPED tablet replicas

Users may notice some replicas on a tablet server are in a STOPPED state, and remain on the server indefinitely. These replicas are tombstones. A tombstone indicates that the tablet server once held a bona fide replica of its tablet. For example, if a tablet server goes down and its replicas are re-replicated elsewhere, if the tablet server rejoins the cluster its replicas will become tombstones. A tombstone will remain until the table it belongs to is deleted, or a new replica of the same tablet is placed on the tablet server. A count of tombstoned replicas and details of each one are available on the /tablets page of the tablet server web UI.

The Raft consensus algorithm that Kudu uses for replication requires tombstones for correctness in certain rare situations. They consume minimal resources and hold no data. They must not be deleted.

Corruption: checksum error on CFile block

In versions prior to Kudu 1.8.0, if the data on disk becomes corrupt, users will encounter warnings containing "Corruption: checksum error on CFile block" in the tablet server logs and client side errors when trying to scan tablets with corrupt CFile blocks. Fixing this corruption is a manual process.

To fix the issue, users can first identify all the affected tablets by running a checksum scan on the affected tables or tablets using the ksck tool.

sudo -u kudu kudu cluster ksck <master_addresses> -checksum_scan -tables=<tables>
sudo -u kudu kudu cluster ksck <master_addresses> -checksum_scan -tablets=<tablets>

If there is at least one replica for each tablet that does not return a corruption error, you can repair the bad copies by deleting them and forcing them to be re-replicated from the leader using the remote_replica delete tool.

sudo -u kudu kudu remote_replica delete <tserver_address> <tablet_id> "Cfile Corruption"

If all of the replica are corrupt, then some data loss has occurred. Until KUDU-2526 is completed this can happen if the corrupt replica became the leader and the existing follower replicas are replaced.

If data has been lost, you can repair the table by replacing the corrupt tablet with an empty one using the unsafe_replace_tablet tool.

sudo -u kudu kudu tablet unsafe_replace_tablet <master_addresses> <tablet_id>

From versions 1.8.0 onwards, Kudu will mark the affected replicas as failed, leading to their automatic re-replication elsewhere.