TrueNAS Glossary — Key Terms and Definitions

TrueNAS: The leading open-source storage operating system, available in CORE (FreeBSD-based) and SCALE (Linux-based) variants.

TrueNAS CORE: The FreeBSD-based variant, mature and widely deployed, using jails and plugins for additional services.

TrueNAS SCALE: The Linux-based variant, actively developed, using Docker containers and Kubernetes for apps.

NAS (Network Attached Storage): A device providing file-based storage to networked computers.

Web Interface: TrueNAS’s browser-based management interface for configuration and monitoring.

Jail: An isolated FreeBSD environment for running additional services (CORE only).

Plugin: A pre-packaged application that can be installed into a jail (CORE only).

Container: A Docker-based application running on TrueNAS SCALE.

Kubernetes (k3s): The lightweight Kubernetes distribution used by TrueNAS SCALE for app management.

App Catalog: The collection of applications available for installation on TrueNAS SCALE.

ZFS (Zettabyte File System): The most advanced filesystem, combining volume management, RAID, and data integrity features.

Pool (zpool): The top-level storage container, made up of one or more virtual devices (vdevs).

Dataset: A logical division within a pool, similar to a partition but more flexible.

Zvol: A block device exported from a pool, used for iSCSI and VM storage.

Snapshot: A point-in-time, read-only copy of a dataset, nearly instant to create.

Scrub: A data integrity check that reads all blocks and verifies checksums.

Resilver: The process of rebuilding a degraded vdev after replacing a failed disk.

Copy-on-Write (CoW): ZFS writes new data to free blocks rather than overwriting existing data.

Checksum: A mathematical value verifying data integrity. ZFS checksums every block.

ARC (Adaptive Replacement Cache): ZFS’s in-memory read cache, using RAM for caching.

L2ARC: A second-level cache using SSDs to extend the ARC.

ZIL (ZFS Intent Log): A write cache for synchronous operations, optionally on a separate device (SLOG).

Single Disk: No redundancy, maximum capacity. Use only for non-critical data.

Mirror (RAID 1): Two or more identical disks, all storing the same data. Best performance, 50% capacity.

RAIDZ1: Single parity, tolerates 1 disk failure. Good balance of capacity and redundancy.

RAIDZ2: Double parity, tolerates 2 disk failures. Recommended default for most use cases.

RAIDZ3: Triple parity, tolerates 3 disk failures. Maximum redundancy for irreplaceable data.

Stripe of Mirrors: Mirrors striped together, excellent performance and redundancy.

Vdev (Virtual Device): A group of disks in a pool. The basic building block of ZFS pools.

Metadata: Data describing data — file names, permissions, timestamps, directory structure.

Record Size: The maximum size of a single block written to disk. Default 128 KB; tune for workload.

Ashift: The physical sector size of disks, set when creating a vdev.

Quota: Limits disk usage per dataset. Prevents any single dataset from consuming all pool space.

Reservation: Guarantees minimum disk space for a dataset, even if other datasets need space.

Compression: Algorithm applied to data before writing (lz4, zstd, gzip). Reduces storage usage.

lz4: Fast compression with minimal CPU overhead. Default and recommended for most workloads.

zstd: Better compression ratios than lz4 with moderate CPU cost.

gzip: High compression ratios but significant CPU cost. Use for cold archive data.

Deduplication: Remove duplicate blocks across datasets. Memory-intensive; generally not recommended.

Snapshot Policy: Automatic snapshots at defined intervals with retention rules.

Replication Target: Where snapshots are sent for backup.

Access Time (atime): Records when files are last read. Disable to improve performance.

Recordsize: Per-dataset record size override.

SMB (Server Message Block): The standard protocol for Windows file sharing. Also works with macOS and Linux.

NFS (Network File System): The standard protocol for Linux/Unix file sharing.

WebDAV: File access over HTTP/HTTPS, useful for remote access without VPN.

AFP (Apple Filing Protocol): Legacy Apple file sharing. SMB is now preferred for macOS.

iSCSI (Internet Small Computer System Interface): Block-level storage over IP networks, used for VM storage.

SMB Share: A directory exposed to the network via the SMB protocol.

NFS Export: A directory shared to Linux/Unix clients via NFS.

ACL (Access Control List): Fine-grained permissions for specific users and groups.

POSIX Permissions: Standard Unix permission model (owner, group, others).

Guest Access: Allowing unauthenticated access to a share (not recommended).

3-2-1 Rule: 3 copies of data, on 2 different media, with 1 offsite copy.

Replication: Sending snapshots to another TrueNAS system or remote location.

Local Replication: Copying snapshots to another pool on the same system.

Remote Replication: Sending snapshots to a TrueNAS system at a different location.

Cloud Backup: Backing up data to cloud storage services (S3, Backblaze B2, Azure).

Cloud Sync: Synchronizing data with cloud storage on a schedule.

Snapshot Retention Policy: Rules defining how many snapshots to keep and for how long.

Hourly Snapshots: Snapshots taken every hour, typically kept for 24 hours.

Daily Snapshots: Snapshots taken once daily, typically kept for 7 days.

Weekly Snapshots: Snapshots taken once weekly, typically kept for 4 weeks.

zfs send/receive: ZFS commands for replicating snapshots between pools or systems.

Backup Verification: Periodically testing that backups can actually be restored.

SMART (Self-Monitoring, Analysis and Reporting Technology): Disk health monitoring providing early warning of failures.

Short Self-Test: A quick SMART test checking basic disk health.

Long Self-Test: A thorough SMART test reading all disk surfaces.

Email Alerts: Notifications sent via email for system events and failures.

Webhook: Custom HTTP callbacks for integration with external services (Slack, Discord, PagerDuty).

Grafana + Prometheus: Open-source monitoring stack for visual dashboards and metrics.

Netdata: Real-time performance monitoring for system metrics.

S.M.A.R.T. Errors: Indicators of potential disk failure requiring attention.

Pool Status: The health state of a ZFS pool (ONLINE, DEGRADED, FAULTED).

Resilver Status: Progress of rebuilding a degraded vdev after disk replacement.

ARC Size: The amount of RAM used by ZFS for caching. More RAM = better read performance.

Record Size: Tune per-dataset for workload: 4 KB for databases, 1 MB for large files.

Compression: lz4 is fast with minimal CPU overhead; zstd offers better ratios.

Sync Writes: Synchronous writes ensure data is on disk before acknowledging. Use sync=standard or sync=always.

Jumbo Frames: Ethernet frames larger than 1500 bytes (typically 9000 MTU), improving throughput.

Link Aggregation (LACP): Combining multiple network interfaces for increased bandwidth.

SMB Multichannel: Parallel SMB transfers across multiple network connections.

Disk Temperature: Monitor to prevent overheating and premature failure.

IOPS (Input/Output Operations Per Second): Measure of disk performance.

Throughput: The rate of data transfer, measured in MB/s or GB/s.

SSH (Secure Shell): Encrypted remote access protocol. Use key authentication, disable password-based SSH.

HTTPS: Encrypted web interface access using SSL/TLS certificates.

SSL/TLS Certificate: Encrypts web traffic between your browser and the TrueNAS interface.

Two-Factor Authentication (2FA): Additional security layer requiring a code beyond password.

Firewall: Network rules restricting which ports and addresses can access the system.

Encryption: Encoding data so only authorized parties can read it.

GELI: FreeBSD disk encryption (TrueNAS CORE).

ZFS Native Encryption: Built-in ZFS encryption (TrueNAS SCALE).

Encryption Key: The secret used to encrypt/decrypt data. Store securely, not on the NAS.

User Accounts: Individual accounts for accessing the NAS, avoiding shared accounts.

Root Account: The administrative account. Change the default password immediately.

Block Storage: Data stored in fixed-size blocks. Used by ZFS and iSCSI.

File Storage: Data stored as files in a directory hierarchy. Used by SMB and NFS.

Object Storage: Data stored as objects with metadata. Used by S3 and similar services.

RAID (Redundant Array of Independent Disks): Combining multiple disks for performance, redundancy, or both.

Hot Spare: A standby disk that automatically replaces a failed disk.

Hot Swap: Replacing a disk while the system is running.

Un SHRINK: A feature allowing you to reduce pool size by removing vdevs.

Free Space: Unused capacity in a pool, available for new data.

Fragmentation: The scattering of data blocks across the disk, reducing sequential performance.

TRIM: A command informing SSDs which blocks are no longer in use.

• See Linux Glossary for Linux fundamentals
• See Networking Glossary for network configuration
• See Security Glossary for security hardening
• See Databases Glossary for database storage
• See Tools Glossary for Docker and Kubernetes