This particular XPS is packing a removable 128 GB Samsung PM851 M.2 form factor SSD.

2 x Samsung 431 K9CHGY8S5M-CCK0 64 GB TLC NAND flash

Samsung 428 K4P2G324ED-FGC2 512 MB LPDDR2 DRAM

Samsung S4LN045X01-8030 MDX controller

This is a well-established form factor for SATA 3 SSDs, so finding an upgrade or replacement will be as easy as something that's really easy.

But wait, there's more! This SSD comes complete with an unremarkable underside.