A shared network attached and varies from vendor to vendor and is called the storage. Here both regular magnetic disks and SSD storage can be used for the backup of the database (HANA runs in memory remember, so disk storage is just for backup, and later, for data ageing). If the operator require 2x storage that you have RAM, which is 2x the database size - i.e. storage size = 4x database size. Anyways in most cases there is additional ultra-high speed SSD storage for log files.