ZFS/Features

The following is a near-exhaustive list of features that ZFS offers:

• Simplified administration (two main administration tools, zpool and zfs)
• A hierarchical namespace for management of all mountpoints (datasets) and block devices (zvols).
• Online management (no downtime required for routine administrative tasks)
• Partitioning is replaced by ZFS storage pools that span multiple disks
• Dynamic allocation of storage across mountpoints (no need to repartition)
• Integrated Volume management (zvol block devices like LVM logical volumes)
  • Supports thin provisioning of storage
• Snapshots (maintains a copy of data as it was at a specific point in time)
• Clones (write-able copies of snapshots that store only changes from the original)
• Special .zfs directory for viewing contents of snapshots.
• ZFS Send/Recv of snapshots (online backup without the consistency issues of rsync)
• Incremental Send/Recv of snapshots (reads list of changes between snapshots and transmits only them; asymptotically faster than rsync)
• Integrated RAID with support for N-way mirrors and up to three levels of parity-based RAID (RAID-Z) similar to RAID 5, RAID 6 and an additional level beyond that.
• Variable stripe (no RAID write-hole)
• Abstraction of all storage into a vdev (virtual device) tree.
• Scaling of IOPS across top level vdevs in a pool
  • e.g. If two RAID-Z2 vdevs are in a pool, objects are written to one or another, such that IOPS are more intelligently distributed than traditional striped storage.
• ARC page replacement algorithm
  • Higher hit rate than commonly used LRU page replacement algorithm increases IOPS performance
• ZFS Intent Log (ZIL)
  • Sequentially write intent records of pending small synchronous writes to safely reduce latencies to levels of asynchronous IO
• Tiered storage
  • L2ARC devices that act as an extension of the system's main memory.
    • Supports LZ4 compression for increased cache as of ZFSOnLinux 0.6.2
  • SLOG devices that permit ZIL to be written to a dedicated hardware.
• Data deduplication
• Data compression with zle (zero-length encoding — fast, but only compresses sequences of zeros), LZJB or its replacement LZ4, or gzip (higher compression, but slower)
• Endian Independence (different machine designs do not prevent ZFS formatted disks from being read)
• Easy disk format upgrades
• Persistent pool settings
  • Failure mode configuration
  • bootfs (used by GRUB2 to find binaries)
  • Ability to annotate pool with a comment
  • Other miscellaneous settings, especially readonly settings
• Persistent configuration for mountpoints (datasets) and block devices (zvols)
  • Uses inheritance in the hierarchical namespace
  • Dataset Specific Options
    • NFS/SMB sharing
      • Automates configuration of NFS and SMB servers, but original manual way is optional
    • Case sensitivity/insensitivity
    • Unicode normalization
    • Quotas (limit to how much storage can be allocated from the pool)
    • Multiple copies of data (transparent)
    • Atime Updates
    • Xattr
    • setuid
    • Mountpoints
      • Depreciates fstab (although still an option with mountpoint=legacy)
      • Makes it easy to have thousands of datasets, including one per home directory
    • Control visibility of .zfs directory
    • Recordsize (tunable that controls internal CoW granularity)
  • Zvol Specific options
    • volblocksize (tunable that controls internal CoW granularity)
    • volsize (allows online resizing of zvol)
    • Control visibility of snapshots in /dev (Linux-specific)
  • Options common to both
    • Compression (already mentioned)
    • Deduplication (already mentioned)
    • Cache control (great for software that implements its own cache in userland)
    • Control of synchronous IO (whether ZIL is used)
    • Reservations (storage reserved for use by a zvol/dataset)
      • This permits thin provisioning on zvols
    • Many other miscellaneous settings, especially readonly settings
    • User defined settings (for use by scripts)

Features in Illumos/Solaris that have yet to be implemented in ZFSOnLinux are:

• On-access virus scanner integration (ClamAV)
• iSCSI integration
• NFSv4 ACLs
  • Does not prevent NFS from being used with ZFS
• Delegated administration
  • Allows system administrator to give ownership of datasets to users (e.g. their home directories) so that they can manage snapshots, configure compression, etcetera.