тонкое предоставление LVM (LVM thin provisioning)
Описание (Description)
Blocks in a standard lvm(8) Logical Volume (LV) are allocated
when the LV is created, but blocks in a thin provisioned LV are
allocated as they are written. Because of this, a thin
provisioned LV is given a virtual size, and can then be much
larger than physically available storage. The amount of physical
storage provided for thin provisioned LVs can be increased later
as the need arises.
Blocks in a standard LV are allocated (during creation) from the
Volume Group (VG), but blocks in a thin LV are allocated (during
use) from a special "thin pool LV". The thin pool LV contains
blocks of physical storage, and blocks in thin LVs just reference
blocks in the thin pool LV.
A thin pool LV must be created before thin LVs can be created
within it. A thin pool LV is created by combining two standard
LVs: a large data LV that will hold blocks for thin LVs, and a
metadata LV that will hold metadata. The metadata tracks which
data blocks belong to each thin LV.
Snapshots of thin LVs are efficient because the data blocks
common to a thin LV and any of its snapshots are shared.
Snapshots may be taken of thin LVs or of other thin snapshots.
Blocks common to recursive snapshots are also shared in the thin
pool. There is no limit to or degradation from sequences of
snapshots.
As thin LVs or snapshot LVs are written to, they consume data
blocks in the thin pool. As free data blocks in the pool
decrease, more free blocks may need to be supplied. This is done
by extending the thin pool data LV with additional physical space
from the VG. Removing thin LVs or snapshots from the thin pool
can also free blocks in the thin pool. However, removing LVs is
not always an effective way of freeing space in a thin pool
because the amount is limited to the number of blocks not shared
with other LVs in the pool.
Incremental block allocation from thin pools can cause thin LVs
to become fragmented. Standard LVs generally avoid this problem
by allocating all the blocks at once during creation.
