# Understand Debug & Error Logs

If you need to troubleshoot issues with your cluster, you can check a node's logs, which include details about certain node-level and range-level events, such as errors. For example, if CockroachDB crashes, it normally logs a stack trace to what caused the problem.

Tip:

For detailed information about queries being executed against your system, see SQL Audit Logging.

## Details

When a node processes a `cockroach` command, it produces a stream of messages about the command's activities. Each message's body describes the activity, and its envelope contains metadata such as the message's severity level.

As a command generates messages, CockroachDB uses the command's logging flags and the message's severity level to determine the appropriate location for it.

Each node's logs detail only the internal activity of that node without visibility into the behavior of other nodes in the cluster. When troubleshooting, this means that you must identify the node where the problem occurred or collect the logs from all active nodes in your cluster.

### Commands

All `cockroach` commands support logging. However, it's important to note:

- `cockroach start` generates most messages related to the operation of your cluster.
- Other commands do generate messages, but they're typically only interesting in troubleshooting scenarios.

### Severity levels

CockroachDB identifies each message with a severity level, letting operators know if they need to intercede:

1. `INFO` (lowest severity; no action necessary)
2. `WARNING`
3. `ERROR`
4. `FATAL` (highest severity; requires operator attention)

Default behavior by severity level

Command | `INFO` messages | `WARNING` and above messages
--- | --- | ---
`cockroach start` | Write to file | Write to file
All other commands | Discard | Print to `stderr`

### Output locations

Based on the command's flags and the message's severity level, CockroachDB does one of the following:

- Writes the message to a file
- Prints it to `stderr`
- Discards the message entirely

#### Write to file

CockroachDB can write messages to log files. The files are named using the following format:

    cockroach.[host].[user].[start timestamp in UTC].[process ID].log

For example:

    cockroach.richards-mbp.rloveland.2018-03-15T15_24_10Z.024338.log

Note:
All log file timestamps are in UTC because CockroachDB is designed to be deployed in a distributed cluster. Nodes may be located in different time zones, and using UTC makes it easy to correlate log messages from those nodes no matter where they are located.

Property | `cockroach start` | All other commands
--- | --- | ---
Enabled by | Default1 | Explicit `--log-dir` flag
Default File Destination | `[first` `store` `dir]/logs` | N/A
Change File Destination | `--log-dir=[destination]` | `--log-dir=[destination]`
Default Severity Level Threshold | `INFO` | N/A
Change Severity Threshold | `--log-file-verbosity=[severity level]` | `--log-file-verbosity=[severity level]`
Disabled by | `--log-dir=`1 | Default

Note:
1 If the `cockroach` process does not have access to on-disk storage, `cockroach start` does not write messages to log files; instead it prints all messages to `stderr`.

#### Print to `stderr`

CockroachDB can print messages to `stderr`, which normally prints them to the machine's terminal but does not store them.

Property | `cockroach start` | All other commands
--- | --- | ---
Enabled by | Explicit `--logtostderr` flag2 | Default
Default Severity Level Threshold | N/A | `WARNING`
Change Severity Threshold | `--logtostderr=[severity level]` | `--logtostderr=[severity level]`
Disabled by | Default2 | `--logtostderr=NONE`

Note:
2 `cockroach start` does not print any messages to `stderr` unless the `cockroach` process does not have access to on-disk storage, in which case it defaults to `--logtostderr=INFO` and prints all messages to `stderr`.

#### Discard message

Messages with severity levels below the `--logtostderr` and `--log-file-verbosity` flag's values are neither written to files nor printed to `stderr`, so they are discarded.

By default, commands besides `cockroach start` discard messages with the `INFO` severity level.

## Flags

Flag | Description
--- | ---
`--log-dir` | Enable logging to files and write logs to the specified directory. Setting `--log-dir` to a blank directory (`--log-dir=`) disables logging to files. Do not use `--log-dir=""`; this creates a new directory named `""` and stores log files in that directory.
`--log-dir-max-size` | After the log directory reaches the specified size, delete the oldest log file. The flag's argument takes standard file sizes, such as `--log-dir-max-size=1GiB`. Default: 100MiB
`--log-file-max-size` | After logs reach the specified size, begin writing logs to a new file. The flag's argument takes standard file sizes, such as `--log-file-max-size=2MiB`. Default: 10MiB
`--log-file-verbosity` | Only writes messages to log files if they are at or above the specified severity level, such as `--log-file-verbosity=WARNING`. Requires logging to files. Default: `INFO`
`--logtostderr` | Enable logging to `stderr` for messages at or above the specified severity level, such as `--logtostderr=ERROR` If you use this flag without specifying the severity level (e.g., `cockroach start --logtostderr`), it prints messages of all severities to `stderr`. Setting `--logtostderr=NONE` disables logging to `stderr`.
`--no-color` | Do not colorize `stderr`. Possible values: `true` or `false`. When set to `false`, messages logged to `stderr` are colorized based on severity level. Default: `false`
`--sql-audit-dir` | New in v2.0: If non-empty, create a SQL audit log in this directory. By default, SQL audit logs are written in the same directory as the other logs generated by CockroachDB. For more information, see SQL Audit Logging.

## See also

- Troubleshooting Overview
- Support Resources