document how colchicifolium's cache was rebuilt (team#40535)

# Caching

WARNING: those instructions are deprecated. There's a newer, simpler

way of setting up the cache that doesn't require two logical volumes,

see the [rebuild instructions](#rebuilding-the-cache-after-removal) for instructions that need to be

adapted here. See also the [lvmcache(7) manual page](https://manpages.debian.org/lvmcache.7) for further

instructions.

Create the VG consisting of 2 block devices (a slow and a fast)

    apt install lvm2 &&

    vg="vg_$(hostname)" &&

    cat /proc/partitions &&

    vg="vg_$(hostname)_cache" &&

    lsblk &&

    echo -n 'slow disk: ' && read slow &&

    echo -n 'fast disk: ' && read fast &&

    vgcreate "$vg" "$slow" "$fast"

    lvconvert --uncache vg_colchicifolium/srv

## Rebuilding the cache after removal

If you've just `--uncached` a volume, for example to resize it, you

might want to re-establish the cache. For this, you can't follow the

same procedure above, as that requires recreating a VG from

scratch. Instead, you need to extend the VG and then create new

volumes for the cache. It should look something like this:

 1. extend the VG with the fast storage:

        VG=vg_$(hostname)

        FAST=/dev/sdc

        vgextend $VG $FAST

 2. create a LV for the cache:

        lvcreate -n cache -l '100%FREE' $VG $FAST

 3. add the cache to the existing LV to be cached:

        lvconvert --type cache --cachevol cache $VG

Example run:

    root@colchicifolium:~# vgextend vg_colchicifolium /dev/sdc

      Volume group "vg_colchicifolium" successfully extended

    root@colchicifolium:~# lvcreate -n cache -l '100%FREE' vg_colchicifolium /dev/sdc 

      Logical volume "cache" created.

    root@colchicifolium:~# lvconvert --type cache --cachevol cache vg_colchicifolium

    Erase all existing data on vg_colchicifolium/cache? [y/n]: y

      Logical volume vg_colchicifolium/srv is now cached.

      Command on LV vg_colchicifolium/cache_cvol requires LV with properties: lv_is_visible .

You can see the cache in action with the `lvs` command:

    root@colchicifolium:~# lvs

      LV   VG                Attr       LSize  Pool         Origin      Data%  Meta%  Move Log Cpy%Sync Convert

      srv  vg_colchicifolium Cwi-aoC--- <1.68t [cache_cvol] [srv_corig] 0.01   13.03           0.00

You might get a `modprobe` error on the last command:

        root@colchicifolium:~# lvconvert --type cache --cachevol cache vg_colchicifolium

        Erase all existing data on vg_colchicifolium/cache? [y/n]: y

        modprobe: ERROR: could not insert 'dm_cache_smq': Operation not permitted

          /sbin/modprobe failed: 1

        modprobe: ERROR: could not insert 'dm_cache_smq': Operation not permitted

          /sbin/modprobe failed: 1

          device-mapper: reload ioctl on  (254:0) failed: Invalid argument

          Failed to suspend logical volume vg_colchicifolium/srv.

          Command on LV vg_colchicifolium/cache_cvol requires LV with properties: lv_is_visible .

That's because the kernel module can't be loaded. Reboot and try

again.

See also the [lvmcache(7) manual page](https://manpages.debian.org/lvmcache.7) for further instructions.

# Resizing

Assume we want to grow this partition to take the available free space