apache/cassandra · warning

IO exception while reading file

Error message

IO exception while reading file {}.

What it means

In DiskCheck/CheckReadAhead machinery, getBlockDevices walks each configured data directory to resolve its backing block device by reading sysfs paths. If an IOException occurs while reading the directory/files, it logs 'IO exception while reading file {dataDirectory}.' with the exception, skips that directory, and continues. The startup check degrades gracefully rather than failing startup.

Solutions

  1. Inspect the logged exception for the actual failing path and fix permissions (chown cassandra:cassandra on the data directory).
  2. Mount /sys read-write or at least readable into containers running Cassandra.
  3. Verify data_file_directories in cassandra.yaml point to existing, readable paths.
  4. If the warning is expected for your storage topology, treat it as informational; read-ahead checks are skipped for that device.

Example fix

// before
data_file_directories: [/mnt/missing-disk/data]
// after
data_file_directories: [/var/lib/cassandra/data]  # existing, readable path
Defensive patterns

Strategy: try-catch

Validate before calling

for (String dir : dataDirs) {
    File f = new File(dir);
    if (!f.canRead()) System.out.println("Unreadable data directory: " + dir);
}
if (!new File("/sys/block").canRead()) System.out.println("/sys/block not accessible; device checks will be skipped");

Try / catch

try {
    // device/directory probing
} catch (IOException e) {
    logger.warn("IO exception while reading file {}.", dataDirectory, e);
    // fall back: skip read-ahead check for this directory
}

Prevention

When it happens

Trigger: IOException while listing/reading the sysfs block-device path or data directory during getBlockDevices (e.g. /sys entries missing, unreadable /proc/self/mountinfo, I/O error on the data directory).

Common situations: Containers without /sys/block mounted, unusual storage layouts (LVM/loop devices), permission-restricted data directories, NFS or ephemeral mounts where sysfs resolution fails.

Understand the failure class

Background: "failed to read file", EACCES, ENOENT and "could not read <path>" errors: when a program can't read a file from disk — this error's family across 49 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/ec47dc5ad649def3. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/service/StartupChecks.java:726

        private Map<String, String> getBlockDevices(String[] dataDirectories) {
            Map<String, String> blockDevices = new HashMap<String, String>();

            for (String dataDirectory : dataDirectories)
            {
                try
                {
                    Path p = File.getPath(dataDirectory);
                    FileStore fs = Files.getFileStore(p);

                    String blockDirectory = fs.name();
                    if(StringUtils.isNotEmpty(blockDirectory))
                    {
                        blockDevices.put(blockDirectory, dataDirectory);
                    }
                }
                catch (IOException e)
                {
                    logger.warn("IO exception while reading file {}.", dataDirectory, e);
                }
            }
            return blockDevices;
        }

        @Override
        public void execute(StartupChecksConfiguration configuration)
        {
            if (configuration.isDisabled(name()) || !FBUtilities.isLinux)
                return;

            String[] dataDirectories = DatabaseDescriptor.getRawConfig().data_file_directories;
            Map<String, String> blockDevices = getBlockDevices(dataDirectories);

            for (Map.Entry<String, String> entry: blockDevices.entrySet())
            {
                String blockDeviceDirectory = entry.getKey();
                String dataDirectory = entry.getValue();

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