What causes "filesystem type cannot be empty" in containerd/containerd?

Explicitly set filesystem_type to "ext4" (the common default) in the plugin config.

filesystem type cannot be empty

Error message

filesystem type cannot be empty

What it means

Validate() treats an empty FilesystemType as invalid: unlike optional fields, the snapshotter requires an explicit filesystem because it formats thin devices at snapshot creation. It throws this in the else branch of the filesystem-type check.

Source

Thrown at plugins/snapshots/devmapper/config.go:122

		result = append(result, fmt.Errorf("pool_name is required"))
	}

	if c.RootPath == "" {
		result = append(result, fmt.Errorf("root_path is required"))
	}

	if c.BaseImageSize == "" {
		result = append(result, fmt.Errorf("base_image_size is required"))
	}

	if c.FileSystemType != "" {
		switch c.FileSystemType {
		case fsTypeExt4, fsTypeXFS, fsTypeExt2:
		default:
			result = append(result, fmt.Errorf("unsupported Filesystem Type: %q", c.FileSystemType))
		}
	} else {
		result = append(result, fmt.Errorf("filesystem type cannot be empty"))
	}

	return errors.Join(result...)
}

View on GitHub (pinned to 4246446a2b)

Solutions

Explicitly set filesystem_type to "ext4" (the common default) in the plugin config.
Check the plugin version's docs for whether a default was previously applied and add the key after upgrade.
Run Validate locally (or LoadConfig) to confirm the full joined error list is resolved.
Add filesystem_type alongside pool_name, root_path, and base_image_size in config templates.

Example fix

// before
{"config":{"pool_name":"pool","root_path":"/var/lib/devmapper","base_image_size":"10GB"}}
// after
{"config":{"pool_name":"pool","root_path":"/var/lib/devmapper","base_image_size":"10GB","filesystem_type":"ext4"}}

Defensive patterns

Strategy: validation

Validate before calling

func validateFSTypePresent(cfg Config) error {
    if cfg.FileSystemType == "" {
        return errors.New("config: filesystem_type is required (set to \"ext4\" if unsure)")
    }
    return nil
}

Try / catch

if err := cfg.Validate(); err != nil {
    if strings.Contains(err.Error(), "filesystem type cannot be empty") {
        cfg.FileSystemType = "ext4" // apply default and retry once
    } else { return err }
}

Prevention

Never rely on implicit defaults for filesystem_type
Set filesystem_type explicitly in all environments (dev, staging, prod)
Re-diff config files after plugin upgrades — previously optional keys may now be required
Keep the four required keys (pool_name, root_path, base_image_size, filesystem_type) in one snippet

When it happens

Trigger: LoadConfig/NewSnapshotter called with c.FileSystemType == "" — i.e. filesystem_type key absent or empty in the plugin config.

Common situations: Minimal config files omitting filesystem_type assuming a default; older configs predating the required-field change; programmatic config construction with zero values.

Related errors

AI-assisted analysis of containerd/containerd@4246446a2b (2026-09-02). Data as JSON: /api/errors/f593bd8aa09dcb2e. Report an issue: GitHub.