LVM cache in six easy steps

This article shows how to quickly create LVM cache in six quick steps, and how to remove it properly.

In the simple case, let’s assume there is a (slow) HDD and (fast) NVMe SSD and the first primary partition on the slow device (sda1) should be backed by cache created on the first secondary partition of the fast device (nvme0n1p5). This is actually my workstation setup, I keep my root filesystem, swap, boot and EFI partition as primary without any caching or LVM.

PV_SLOW=/dev/sda1
PV_FAST=/dev/nvme0n1p5
VG=vg_home
LV_SLOW=lv_home
LV_FAST=lv_home_cache

Here are the six steps. Physical volumes are created, volume group consisting both volumes, then logical backing volume, then logical cache volume (pool) and the final logical volume is converted (renamed) and the cache is built:

pvcreate $PV_SLOW
pvcreate $PV_FAST
vgcreate $VG $PV_SLOW $PV_FAST
lvcreate -l 100%PVS -n $LV_SLOW $VG $PV_SLOW
lvcreate --type cache-pool -l 100%PVS -n $LV_FAST $VG $PV_FAST
lvconvert --type cache --cachepool $LV_FAST $VG/$LV_SLOW

Few extra (hidden) logical volumes are created with suffixes _cdata, _cmeta and _corig. I will not go into details but important thing to understand is that the volume that shall be used for mounting and working is named $LV_SLOW (lv_home in my example). So to create a filesystem do something like:

mkfs.ext4 $LV_SLOW

To remove LVM cache and go back to single origin (normal) logical volume, just do the following:

lvconvert --uncache $VG/$LV_SLOW

This will actually do the job of flushing the cache (if writeback was set) and renaming logical volume back. Now you can access your data using the very same logical volume ($LV_SLOW) which is cool.

This is why LVM cache is better than bcache in my opinion - easy way to opt-out if something goes wrong (SSD dies). And today, after my Fedora 30 upgrade over the weekend, it went really bad due to fatal bug in GCC9/bcache/kernel: https://bugzilla.redhat.com/show_bug.cgi?id=1708315

Lesson learned? Always read release notes prior upgrading, this was a known issue. Honestly, I’d probably vote for delaying the release if I could or implement some safety net for bcache users.

At least I know how simple it is to recover files with duplicity backup tool.

20 May 2019 | linux | fedora
twitter.com linkedin.com
google.com/+ facebook.com
flickr.com youtube.com