Add the SSD to the LVM as Cache

pvcreate /dev/sdb
vgextend pve /dev/sdb

lvcreate -L 360G -n CacheDataLV pve /dev/sdb
lvcreate -L 5G -n CacheMetaLV pve /dev/sdb

lvconvert --type cache-pool --poolmetadata pve/CacheMetaLV pve/CacheDataLV

lvconvert --type cache --cachepool pve/CacheDataLV --cachemode writeback pve/data
/dev/sdb is the SSD

Update for Proxmox 6

It is necessary to add the following modules to the initramfs.

echo "dm_cache" >> /etc/initramfs-tools/modules
echo "dm_cache_mq" >> /etc/initramfs-tools/modules
echo "dm_persistent_data" >> /etc/initramfs-tools/modules
echo "dm_bufio" >> /etc/initramfs-tools/modules

update-initramfs -u

Disable LVM Cache

To reverse the above procedure please run the following command.

lvconvert --uncache pve/data