Earthly makes many needless copies of data in its cache, so unless you have truly excessive amounts of storage you will need to perform planet builds on a system with deduplication. There are two main options for that that I'm aware of: LVM VDO and ZFS. I've only attempted to use ZFS for this purpose.
With deduplication enabled the disk space requirement for a full-planet build is fairly low, expect to need around 1TB of fast storage. You'll also need to disable BuildKit parallelism in earthly and set cache_size_mb to 10000000 to trick it into using more disk space than you technically have available. I've observed deduplication ratios as high as 15x, with an average of 8-10x for full-planet work.
Expect to need 64GB of RAM
TODO