Stupid Recipe for Creating LVM2 SSD Cache

Silly recipe for creating a SSD cache with LVM without wasting space on the SSD device:


# pvcreate /dev/slowdisk
# pvcreate /dev/fastdisk
# vgcreate plum_disk_cache /dev/slowdisk /dev/fastdisk
# lvcreate -n origin plum_disk_cache -l $size_of_slowdisk /dev/slowdisk
# lvcreate -n cache plum_disk_cache -l $size_of_fastdisk /dev/fastdisk
# lvconvert --type cache --cachemode writeback --cachepool plum_disk_cache/cache plum_disk_cache/origin --chunksize 512k -v --poolmetadataspare n
# lvreduce plum_disk_cache/cache -l -6 (or however many it claims to need)
# lvconvert --type cache --cachemode writeback --cachepool plum_disk_cache/cache plum_disk_cache/origin --chunksize 512k -v --poolmetadataspare n


A somewhat clumsy alternative method is this:

(Create VG, allocate "origin" LV across slow disks, add fastdisk to VG...)

# vgs plum_disk_cache -o vg_free_count
Free
67236
# lvcreate --type cache -n cache plum_disk_cache/origin /dev/fastdisk -l 67236 --cachemode writeback --chunksize 512k --poolmetadataspare n; done
# lvchange --cachepolicy smq plum_disk_cache/origin


(Replace 67236 with successive values until it succeeds, being careful to remove any LVs it erroneously creates while trying to set up the cache.)