The entire database is designed for performance with message-passing thread-per-core async architecture. XFS is the only filesystem that has good async support.
Yes, exactly. Scylla needs proper AIO/DIO support at the filesystem level and XFS has so far been the one that implements it best. There are fixes to Scylla to make it also run well on ext4 (by working around some of its limitations), but that's not part of any release yet.
