The highlevel organizational units of a filesystem are block
groups of three types: data, metadata and system.
DATA
store data blocks and nothing else
METADATA
store internal metadata in b-trees, can store file data if
they fit into the inline limit
SYSTEM
store structures that describe the mapping between the
physical devices and the linear logical space representing
the filesystem
Other terms commonly used:
block group, chunk
a logical range of space of a given profile, stores data,
metadata or both; sometimes the terms are used
interchangeably
A typical size of metadata block group is 256MiB (filesystem
smaller than 50GiB) and 1GiB (larger than 50GiB), for data
it's 1GiB. The system block group size is a few megabytes.
RAID
a block group profile type that utilizes RAID-like features
on multiple devices: striping, mirroring, parity
profile
when used in connection with block groups refers to the
allocation strategy and constraints, see the section PROFILES
for more details
