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GEOM is the main storage framework for the FreeBSD operating system. It is available in FreeBSD 5.0 and later releases, and provides a standardized way to access storage layers. GEOM is modular and allows for geom modules to connect to the framework. For example, the geom_mirror module provides RAID1 or mirroring functionality to the system. A number of modules are provided as part of FreeBSD and others have been developed independently and are distributed via (e.g.) GitHub.

GEOM was developed for the FreeBSD Project by Poul-Henning Kamp and NAI Labs, the Security Research Division of Network Associates, Inc. under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the DARPA CHATS research program. The name symbolizes its impact on disk geometry.

Stacked design[edit]

Because of geom's modular design, modules can be 'stacked' together to form a chain of geom layers. For example, on top of the geom_mirror module an encryption module can be added, such as geom_eli to provide a mirrored and encrypted volume. Each module has both consumers and providers. A provider is the 'source' of the geom module, often a physical hard drive but sometimes a virtualized disk such as a memory disk. The geom module in turn provides an 'output' device. Other geom modules, called consumers, can use this provider to create a chain of modules connected to each other.

Source → geom module → Output

is referred to as:

Provider → geom module → Consumer(s)

For example, the geom_mirror module may use (as a consumer) the following providers: /dev/ada0, /dev/ada1, while it creates (as a provider) a new device called /dev/mirror/gm0. At the end of the geom chain, often a filesystem is applied to actually use the geom provider for something useful. The provider created by geom modules behaves just like a physical hard drive and as such can contain filesystems such as FreeBSD's native Unix File System (UFS).

Available modules[edit]

Storage modules[1]

  • geom_stripe (RAID0)
  • geom_mirror (RAID1)
  • geom_raid (Supports RAID functionality on "software raid" controllers)
  • geom_raid3 (RAID3)
  • geom_raid5 (RAID5, not present in -CURRENT yet)
  • geom_concat (concatenating, also called spanning or JBOD)
  • geom_vinum (legacy volume manager with RAID0/1/4/5 support)
  • geom_ccd (legacy volume manager with RAID0 and rudimentary RAID1 support)

Encryption and compression modules

Filesystem modules

  • geom_label (allows providers to have their own name labeled for easy partitioning)
  • geom_journal (adds journaling support to the Unix File System (UFS))
  • geom_cache (adds caching support for increased performance using RAM as buffercache)

Disk partitioning


  • geom_mountver (enables mount verification)
  • geom_multipath (support Multipath I/O to underlying disks)
  • geom_nop (creates a transparent module used for debugging and testing)
  • geom_gate (creates a virtual disk using network disk back-end)
  • geom_virstor (allows overloading a geom provider by creating a provider larger than its consumer)
  • geom_linux_lvm (reads Linux LVM2 volumes)
  • geom_zero (creates a dummy provider that absorbs all writes and returns zeros on reads)

External links[edit]

  • Rhodes, Tom. "GEOM: Modular Disk Transformation Framework". FreeBSD Handbook.


  1. ^ Pawel Jakub Dawidek. "GEOM(8)". FreeBSD System Manager's Manual. Retrieved 2021-10-31.