LVM SSD Caching
Nov 23, 2017
LVM has a cool feature that allows you to cache slow LVs on faster media. It has some limitations like not being able to cache an entire volume group but it does work. I had an extra Intel 520 240GB SSD that I was not using so I decided to add it to one of my servers to speed up an mdadm array.
I can probably say that using Gitlab is a bit snappier through the web interface after implementing the SSD cache.
- vg0
- Slow array (4 x 4TB spinning disks)
- /dev/mapper/ssd
- LUKS encrypted fast SSD
I will be adding a cache to existing LVs.
- Create the PV on the SSD
pvcreate /dev/mapper/ssd
- Extend the original volume group
vgextend vg0 /dev/mapper/ssd
- Create LV caches
-
lvcreate -L 40G -n cache0 vg0 /dev/mapper/ssd lvcreate -L 40G -n cache1 vg0 /dev/mapper/ssd lvcreate -L 40G -n cache2 vg0 /dev/mapper/ssd
-
- Add the cache to the existing logical volumes
-
lvconvert --type cache --cachepool vg0/cache0 vg0/lv0 lvconvert --type cache --cachepool vg0/cache1 vg0/lv1 lvconvert --type cache --cachepool vg0/cache2 vg0/lv2
-
That’s pretty much it. You can see utilization with this command: lvs -a -o +devices. I wouldn’t recommend using writeback caching unless your data is easily replaceable.
There is also bcache but I believe I’d have to wipe the array and start over. It’s probably not worth doing when a simpler solution exists.