Increase the specific resource
How can I increase the storage of my existing server? I can also resize the server but extra CPU cores and ram will be useless for me. Does Linode provide custom resources configuration? I will also need to increase custom CPU cores as well.
As far as storage goes, you can add block storage very easily for a reasonable price ($1/mo per 10GiB). Keep in mind that block storage is slower than the storage that comes with your plan, so you'd probably only want to use it for less-frequently-used files: git(1) archives; backups; low-traffic web sites; etc.
Block storage is pretty easy to arrange into a RAID array as well.