geom, glabel, and related terminology

[Pieter de Goeje]

On Tuesday 27 April 2010 13:49:02 Eitan Adler wrote:
> I'm a little bit confused about some of the file system terminology.
> 
> What exactly is a GEOM label?  
A geom label is a name (hence label) for a GEOM provider. This label is 
interpreted/read by glabel(8). The source of the label can be the volume label 
or id of a filesystem, or a custom label created by glabel(8).

> What does it mean to have one or for
> one to be stopped?

The underlying provider was mounted which causes the label to disappear 
(become unavailable). For example if we have a filesystem labeled "data" on 
/dev/ad1p7 it shows up as /dev/label/data in addition to /dev/ad1p7. When the 
user mounts /dev/ad1p7 the label disappears.

> What is a GEOM provider?

A GEOM provider is a piece of software/hardware which implements the common 
GEOM provider interface: it manifests itself as a device which supports 
read/write operations and a number of other common operations and transforms. 
Because all GEOM providers implement the same interface, a file system for 
example only needs to support the GEOM interface to support all storage 
hardware supported by FreeBSD. Because some providers are also consumers, GEOM 
classes can be stacked together to create advanced storage architectures. For 
example, two disks, both GEOM providers, can be consumed by a mirror GEOM 
class (gmirror(8)), which itself is just another provider which can be 
consumed by a filesystem or another GEOM consumer .

> What is the difference between a bsd label and a geom label?
Bsdlabel is the name of FreeBSD's old school partitioning system and tool 
(superseeded by gpart(8)). It is a layer above DOS partitions (slices in 
FreeBSD lingo).  bsdlabel(8) supports 8 partitions per slice. A geom label is 
just another name for the underlying GEOM provider.

> 
> If you could provide a high level overview the terminology it would
> help me immensely.

I hope this helps,

- Pieter