Lazy answer but we'll blog about this in the next month.

On a high level it's variable-sized blocks packed into 1GB extents, which are then aggregated into volumes and erasure coded across a set of disks on different machines/racks/rows/etc. We also replicate cross-country in addition to this. Live writes are written into non-erasure-coded volumes and encoded in the background.

The volume metadata on the disks contains enough information to be self-describing (as a safety precaution), but we also have a two-level index that maps blocks to volumes and volumes to disks.

More info to come later.