1. Identify main LV that needs caching
The main LV may already exist, and is located on larger, slower
devices. A main LV would be created with a command like:
# lvcreate -n main -L Size vg /dev/slow_hhd
2. Identify fast LV to use as the cache
A fast LV is created using one or more fast devices, like an SSD.
This special LV will be used to hold the cache:
# lvcreate -n fast -L Size vg /dev/fast_ssd
# lvs -a
LV Attr Type Devices
fast -wi------- linear /dev/fast_ssd
main -wi------- linear /dev/slow_hhd
3. Start caching the main LV
To start caching the main LV, convert the main LV to the desired
caching type, and specify the fast LV to use as the cache:
using dm-cache (with cachepool):
# lvconvert --type cache --cachepool fast vg/main
using dm-cache (with cachevol):
# lvconvert --type cache --cachevol fast vg/main
using dm-writecache (with cachevol):
# lvconvert --type writecache --cachevol fast vg/main
For more alteratives see:
dm-cache command shortcut
dm-cache with separate data and metadata LVs
4. Display LVs
Once the fast LV has been attached to the main LV, lvm reports
the main LV type as either cache or writecache depending on the
type used. While attached, the fast LV is hidden, and renamed
with a _cvol or _cpool suffix. It is displayed by lvs -a. The
_corig or _wcorig LV represents the original LV without the
cache.
using dm-cache (with cachepool):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cpool] cache main_corig(0)
[fast_cpool] cache-pool fast_pool_cdata(0)
[fast_cpool_cdata] linear /dev/fast_ssd
[fast_cpool_cmeta] linear /dev/fast_ssd
[main_corig] linear /dev/slow_hhd
using dm-cache (with cachevol):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cvol] cache main_corig(0)
[fast_cvol] linear /dev/fast_ssd
[main_corig] linear /dev/slow_hhd
using dm-writecache (with cachevol):
# lvs -ao+devices
LV Pool Type Devices
main [fast_cvol] writecache main_wcorig(0)
[fast_cvol] linear /dev/fast_ssd
[main_wcorig] linear /dev/slow_hhd
5. Use the main LV
Use the LV until the cache is no longer wanted, or needs to be
changed.
6. Stop caching
To stop caching the main LV and also remove unneeded cache pool,
use the --uncache:
# lvconvert --uncache vg/main
# lvs -a
LV VG Attr Type Devices
main vg -wi------- linear /dev/slow_hhd
To stop caching the main LV, separate the fast LV from the main
LV. This changes the type of the main LV back to what it was
before the cache was attached.
# lvconvert --splitcache vg/main
# lvs -a
LV VG Attr Type Devices
fast vg -wi------- linear /dev/fast_ssd
main vg -wi------- linear /dev/slow_hhd
7. Create a new LV with caching
A new LV can be created with caching attached at the time of
creation using the following command:
# lvcreate --type cache|writecache -n Name -L Size
--cachedevice /dev/fast_ssd vg /dev/slow_hhd
The main LV is created with the specified Name and Size from the
slow_hhd. A hidden fast LV is created on the fast_ssd and is
then attached to the new main LV. If the fast_ssd is unused, the
entire disk will be used as the cache unless the --cachesize
option is used to specify a size for the fast LV. The
--cachedevice option can be repeated to use multiple disks for
the fast LV.
